BIOLOGY LIBRARY G MENDELISM THE MACMILLAN COMPANY NEW YORK BOSTON CHICAGO SAN FRANCISCO MACMILLAN & CO., LIMITED LONDON BOMBAY CALCUTTA MELBOURNE THE MACMILLAN CO. OF CANADA, LTD. TORONTO Gregor Mendel M E N D E L I S M R. C. PUNNETT FELLOW OF GONVILLE AND CAIUS COLLEGE PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF CAMBRIDGE THIRD EDITION ENTIRELY REWRITTEN AND MUCH ENLARGED gorfe THE MACMILLAN COMPANY 191 1 ^411 rights reserved I BlOLOGt G COPYRIGHT, tgii, BY THE MACMILLAN COMPANY. Set up and electrotyped. Published May, 1911. Nnrfoooft \3rtss J. 8. Gushing Co. Berwick & Smith Co. Norwood, Mass., U.S.A. PREFACE A FEW years ago I published a short sketch of Mendel's discovery in heredity, and of some of the recent experiments which had arisen from it. Since then progress in these studies has been rapid, and the present account, though bearing the same title, has been completely rewritten. A number of illustrations have been added, and here I may acknowledge my indebted- ness to Miss Wheldale for the two coloured plates of sweet peas, to the Hon. Walter Rothschild for the but- terflies figured on Plate VI., to Professor Wood for photographs of sheep, and to Dr. Drinkwater for the figures of human hands. To my former publishers also, Messrs. Bowes and Bowes, I wish to express my thanks for the courtesy with which they acquiesced in my desire that the present edition should be published elsewhere. As the book is intended to appeal to a wide audience, I have not attempted to give more experimental in- stances than were necessary to illustrate the story, nor have I burdened it with bibliographical reference. The reader who desires further information may be referred to Mr. Bateson's indispensable volume on Mendel's 225259 vi MENDELISM Principles of Heredity (Cambridge, 1909), where a full account of these matters is readily accessible. Neither have I alluded to recent cytological work in so far as it may bear upon our problems. Many of the facts con- nected with the division of the chromosomes are striking and suggestive, but while so much difference of opinion exists as to their interpretation they are hardly suited for popular treatment. In choosing typical examples to illustrate the growth of our ideas it was natural that I should give the prefer- ence to those with which I was most familiar. For this reason the book is in some measure a record of the work accomplished by the Cambridge School of Genetics, and it is not unfair to say that under the leadership of William Bateson the contributions of this school have been second to none. But it should not be forgotten that workers in other European countries, and especially in America, have amassed a large and valuable body of evidence with which it is impossible to deal in a small volume of this scope. It is not long since the English language was" -enriched by two new words Eugenics and Genetics and their similarity of origin has sometimes led to confusion be- tween them on the part of those who are innocent of Greek. Genetics is the term applied to the experi- mental study of heredity and variation in animals and plants, and the main concern of its students is the establishing of law and order among the phenomena PREFACE vii there encountered. Eugenics, on the other hand, deals with the improvement of the human race under existing conditions of law and sentiment. The Eugenist has to take into account the religious and social beliefs and prejudices of mankind. Other issues are involved be- sides the purely biological one, though as time goes on it is coming to be more clearly recognised that the Eugenic ideal is sharply circumscribed by the facts of heredity and variation, and by the laws which govern the transmission of qualities in living things. What these facts, what these laws are, in so far as we at present know them, I have endeavoured to indicate in the following pages; for I feel convinced that if the Eugenist is to achieve anything solid it is upon them that he must primarily build. Little enough material, it is true, exists at present, but that we now see to be largely a question of time and means. Whatever be the outcome, whatever the form of the structure which is eventually to emerge, we owe it first of all to Mendel that the foundations can be well and truly laid. R. C. P. CAMBRIDGE, March, 1911. CONTENTS CHAPTER I THE PROBLEM CHAPTER II HISTORICAL ... .... CHAPTER III MENDEL'S WORK CHAPTER IV. THE PRESENCE AND ABSENCE THEORY CHAPTER V INTERACTION OF FACTORS CHAPTER VI REVERSION * * 59 DOMINANCE . CHAPTER VII 68 x MENDELISM CHAPTER VIII PAGE WILD FORMS AND DOMESTIC VARIETIES ... 79 CHAPTER IX REPULSION AND COUPLING OF FACTORS .... 88 CHAPTER X SEX . . . 99 CHAPTER XI SEX (continued) -115 CHAPTER XII INTERMEDIATES . . . . . . ... 125 CHAPTER XIII VARIATION AND EVOLUTION . . . . . . . 135 CHAPTER XIV ECONOMICAL :..... 153 CHAPTER XV MAN .170 APPENDIX . . . . . . . ' . .187 INDEX 191 ILLUSTRATIONS PLATES PLATK PAGE Gregor Mendel Frontispiece I. Rabbits To face 60 II. Sweet Peas . "64 III. Sheep "78 IV. Sweet Peas . 80 V. Fowls "107 VI. Butterflies "146 FIGURES IN TEXT FIG. 1. Scheme of Inheritance in simple Mendelian Case . . 21 2. Feathers of Silky and Common Fowl . . 30 3. Single and Double Primulas '31 4. Fowls 1 Combs ......... 32 5. Diagram of Inheritance of Fowls 1 Combs . 37 6. Fowls 1 Combs 40 7. Diagram of F 2 Generation resulting from Cross between two White Sweet Peas 46 8. Diagram illustrating 9 : 3 : 4 Ratio in Mice 5 2 9. Sections of Primulas 55 xii MENDELISM FIG. PACK 10. Small and Large-eyed Primulas ..... 56 11. Diagram illustrating Reversion in Pigeons ... 67 12. Primula sinensis x Primula stellata ... 69 13. Diagram illustrating Cross between Dominant and Re- cessive White Fowls 74 14. Bearded and Beardless Wheat 75 15. Feet of Fowls 77 16. Scheme of Inheritance of Horns in Sheep ... 78 17. Abraxas grossulariata and var. laeticolor . . . 100 18. Scheme of Inheritance in Abraxas . . . . . 102 19. Scheme of Inheritance of Silky Hen x Brown Leghorn Cock 106 20. Scheme of Inheritance of Brown Leghorn Hen x Silky Cock . . . . 106 2 1 . Scheme of Fj (ex Brown Leghorn x Silky Cock) crossed with pure Brown Leghorn . . . . .' .107 22. Scheme for. Silky Hen x Brown Leghorn Cock . . 109 23. Scheme for Brown Leghorn Hen x Silky Cock . .no 24. Diagram illustrating Nature of Offspring from Brown Leg- horn Hen x Fj Cock . . .". . . .in 25. Scheme to illustrate Heterozygous Nature of Brown Leg- horn Hen 112 26. Scheme of Inheritance of Colour-blindness . . . 117 27. Single and Double Stocks . . . . . 123 28. F 2 Generation ex Silky Hen x Brown Leghorn Cock . 127 29. Pedigree of Eurasian Family * . 130 30. Curve illustrating Influence of Selection . . . . 159 ILLUSTRATIONS xiii FIG. PAGE 31. Curve illustrating Conception of pure Lines . . . 161 32. Brachydactylous and Normal Hands % . . .171 33. Radiograph of Brachydactylous Hand . . . .172 34. Pedigree of Brachydactylous Family . . . 173 35. Pedigree of Haemophilic Family 175 For although it be a more new and diffi- cult way, to find out the nature of things, by the things themfelves ; then by reading of Books, to take our knowledge upon truft from the opinions of Philofophers : yet muft it needs be confeffed, that the former is much more open, and leffe fraudulent, efpe- cially in the Secrets relating to Natural Philofophy. WILLIAM HARVEY, Anatomical Exercitations, 1653. xiv CHAPTER I THE PROBLEM A CURIOUS thing in the history of human thought so far as literature reveals it to us is the strange lack of interest shown in one of the most interesting of all human relationships. Few if any of the more primitive peoples seem to have attempted to define the part played by either parent in the formation of the offspring, or to have assigned peculiar powers of transmission to them, even in the vaguest way. For ages man must have been more or less consciously improving his domesticated races of animals and plants, yet it is not until the time of Aris- totle that we have clear evidence of any hypothesis to account for these phenomena of heredity. The pro- duction of offspring by man was then held to be similar to the production of a crop from seed. The seed came from the man, the woman provided the soil. ' This re- mained the generally accepted view for many centuries, and it was not until the recognition of woman as more than a passive agent that the physical basis of heredity became establishec^fcliat recognition was effected by the microscope, for only with its advent was actual ob- 2 . , , . , MENDELISM CHA P. servation of the minute sexual cells made possible. After more than a hundred years of conflict lasting until the end of the eighteenth century, scientific men settled down to the view that each of the sexes makes a definite material contribution to the offspring produced by their joint efforts. Among animals the female contributes the ovum and the male the spermatozoon ; among plants the cor- responding cells are the ovules and pollen grains. As a general rule it may be stated that the reproductive cells produced by the female are relatively large and without the power of independent movement. In addi- tion to the actual living substance which is to take part in the formation of a new individual, the ova are more or less heavily loaded with the yolk substance that is to pro- vide for the nutrition of the developing embryo during the early stages of its existence. The size of the ova varies enormously in different animals. In birds and reptiles where the contents of the egg form the sole resources of the developing young they are very large in comparison with the size of the animal which lays them. In mam- mals, on the other hand, where the young are parasitic upon the mother during the earlier stages of their growth, the eggs are minute and only contain the small amount of yolk that enables them to reach the stage at which they develop the processes for attaching themselves to the wall of the maternal uterus. But whatever the differences in the size and appearance of the ova produced by different i THE PROBLEM 3 animals, they are all comparable in that each is a distinct and separate sexual cell which, as a rule, is unable to de- velop into a new individual of its species unless it is fertilised by union with a sexual cell produced by the male. The male sexual cells are always of microscopic size and are produced in the generative gland or testis in exceedingly large numbers. In addition to their minuter size they differ from the ova in their power of active move- ment. Animals present various mechanisms by which the sexual elements may be brought into juxtaposition, but in all cases some distance must be traversed in a fluid or semifluid medium (frequently within the body of the female parent) before the necessary fusion can occur. To accomplish this latter end of its journey the spermatozoon is endowed with some form of motile apparatus, and this frequently takes the form of a long flagellum, or whip-like process, by the lashing of which the little creature propels itself much as a tadpole with its tail. In plants as in animals the female cells or ovules are larger than the pollen grains, though the disparity in size is not nearly so marked. Still they are always relatively minute cells since the circumstances of their develop- ment as parasites upon the mother plant render it unnec- essary for them to possess any great supply of food yolk. The ovules are found surrounded by maternal tissue in the ovary, but through the stigma and down the pistil a po- 4 MENDELISM CHAP. tential passage is left for the male cell. The majority of flowers are hermaphrodite, and in many cases they are also self-fertilising. The anthers burst and the contained pollen grains are then shed upon the stigma. When this happens, the pollen cell slips through a little hole in its coat and bores its way down the pistil to reach an ovule in the ovary. Complete fusion occurs, and the minute embryo of a new plant immediately results. But for some time it is incapable of leading a separate existence, and, like the embryo mammal, it lives as a parasite upon its parent. By the parent it is provided with a protective wrapping, the seed coat, and beneath this the little em- bryo swells until it reaches a certain size, when as a ripe seed it severs its connection with the maternal organism. It is important to realise that the seed of a plant is not a sexual cell but a young individual which, except for the coat that it wears, belongs entirely to the next generation. It is with annual plants in some respects as with many butterflies. During one summer they are initiated by the union of two sexual cells and pass through certain stages of larval development the butterfly as a caterpillar, the plant as a parasite upon its mother. As the summer draws to a close each passes into a resting-stage against the winter cold the butterfly as a pupa arid the plant as a seed, with the difference that while the caterpillar provides its own coat, that of the plant is provided by its mother. With the advent of spring both butterfly and I THE PROBLEM 5 plant emerge, become mature, and themselves ripen germ cells which give rise to a new generation. Whatever the details of development, one cardinal fact is clear. Except for the relatively rare instances of parthenogenesis a new individual, whether plant or animal, arises as the joint product of two sexual cells derived from individuals of different sexes. Such sexual cells, whether ovules or ova, spermatozoa or pollen grains, are known by the general term of gametes, or marrying cells, and the individual formed by the fusion or yoking together of two gametes is spoken of as a zygote. Since a zygote arises from the yoking together of two separate gametes, the individual so formed must be regarded throughout its life as a double structure in which the components brought in by each of the gametes remain in- timately fused in a form of partnership. But when the zygote in its turn comes to form gametes, the partnership is broken and the process is reversed. The component parts of the dual structure are resolved, with the formation of a set of single structures, the gametes. The life cycle of a species from among the higher plants or animals may be regarded as falling into three periods : (i) a period of isolation in the form of gametes, each a liv- ing unit incapable of further development without inti- mate association with another produced by the opposite sex; (2) a period of association in which two gametes become yoked together into a zygote and react upon one 6 MENDELISM CHAP. another to give rise by a process of cell division to what we ordinarily term an individual with all its various attri- butes and properties; and (3) a period of dissociation when the single structured gametes separate out from that portion of the double structured zygote which constitutes its generative gland. What is the relation between gamete and zygote, between zygote and gamete ? how are the properties of the zygote represented in the gamete, and in what manner are they distributed from the one to the other ? these are questions which serve to indicate the nature of the problem underlying the process of heredity. Owing to their peculiar power of growth and the rela- tively large size to which they attain, many of the proper- ties of zygotes are appreciable by observation. The col- our of an animal or of a flower, the shape of a seep!, or the pattern on the wings of a moth are all zygotic properties, and all capable of direct estimation. It is otherwise with the properties of gametes. While the difference be- tween a black and a white fowl is sufficiently obvious, no one by inspection can tell the difference between the egg that will hatch into a black and that which will hatch into a white. Nor from a mass of pollen grains can any one to-day pick out those that will produce white -from those that will produce coloured flowers. Nevertheless, we know that in spite of apparent similarity there must exist fundamental differences among the gametes, even I THE PROBLEM 7 among those that spring from the same individual. At present our only way of appreciating those differences is to observe the properties of the zygotes which they form. And as it takes two gametes to form a zygote, we are in the position of attempting to decide the proper- ties of two unknowns from one known. Fortunately the problem is not entirely one of simple mathematics. It can be attacflM. by tte experimental method, and with what measure of success will appear in the following pages. CHAPTER II HISTORICAL To Gregor Mendel, monk and abbot, belongs the credit of founding the modern science of heredity. Through him there was brought into these problems an entirely new idea, an entirely fresh conception of the nature of living things. Born in 1822 of Austro-Silesian parentage, he early entered the monastery of Briinn, and there in the seclusion of the cloister garden he carried out with the common pea the series of experiments which has since become so famous. In 1865 after eight years' work he published the results of his experiments in the Proceedings of the Natural History Society of Briinn, in a brief paper of some forty pages. But brief as it is the importance of the results and the lucidity of the exposition will always give it high rank among the classics of biological litera- ture. For thirty -five years Mendel's paper remained unknown, and it was not until 1900 that it was simulta- neously discovered by several distinguished botanists. The causes of this curious neglect are not altogether with- out interest. Hybridisation experiments before Mendel there had been in plenty. The classificatory work of Lin- CHAP, ii HISTORICAL 9 naeus in the latter half of the eighteenth century had given a definite significance to the word species, and scientific men began to turn their attention to attempting to discover how species were related to one another. And one ob- vious way of attacking the problem was to cross different species together and see what happened. This was largely done during the earlier half of the nineteenth cen- tury, though such work was almost entirely confined to the botanists. Apart from the fact that plants lend them- selves to hybridisation work more readily than animals, there was probably another reason why zoologists neg- lected this form of investigation. The field of zoology is a wider one than that of botany, presenting a far greater variety of type and structure. Partly owing to their im- portance in the study of medicine, and partly owing to their smaller numbers, the anatomy of the vegetable was far better known than that of the animal kingdom. It is, therefore, not surprising that the earlier part qf^ the nineteenth century found the zoologists, under the in- fluence of Cuvier and his pupils, devoting their entire energies to describing the anatomy of the new forms of animal life which careful search at home and fresh voyages of discovery abroad were continually bringing to light. During this period the zoologist had little inclination or inducement to carry on those investigations in hybridisa- tion which were occupying the attention of some botan- ists. Nor did the efforts of the botanists afford much io MENDELISM CHAP. encouragement to such work, for in spite of the labour devoted to these experiments, the results offered but a confused tangle of facts, contributing in no apparent way to the solution of the problem for which they had been undertaken. After half a century of experimental hy- bridisation the determination of the relation of species and varieties to one another seemed as remote as ever. Then in 1859 came the Origin of Species, in which Darwin presented to the world a consistent theory to account for the manner in which one species might have arisen from another by a process of gradual evolution. Briefly put, that theory was as follows : In any species of plant or animal the reproductive capacity tends to outrun the available food supply, and the resulting competition leads to an inevitable struggle for existence. Of all the individ- uals born, only a portion, and that often a very small one, can survive to produce offspring. According to Darwin's theory, the nature of the surviving portion is not deter- mined by chance alone. No two individuals of a species are precisely alike, and among the variations that occur some enable their possessors to cope more successfully with the competitive conditions under which they exist. In comparison with their less favoured brethren they have a better chance of surviving in the struggle for existence and consequently of leaving offspring. The argument is completed by the further assumption of a principle of heredity, in virtue of which offspring tend to ii HISTORICAL ii resemble their parents more than other members of the species. Parents possessing a favourable variation tend to transmit that variation to their offspring, to some in greater, to others in less degree. Those possessing it in greater degree will again have a better chance of survival, and will transmit the favourable variation in even greater degree to some of their offspring. A competitive struggle for existence working in combination with certain princi- ples of variation and heredity results in a slow and con- tinuous transformation of species through the operation of a process which Darwin termed natural selection. The coherence and simplicity of the theory, sup- ported as it was by the great array of facts which Darwin had patiently marshalled together, rapidly gained the enthusiastic support of the great majority of biologists. The problem of the relation of species at last appeared to be solved, and for the next forty years zoologists and botanists were busily engaged in classifying by the light of Darwin's theory the great masses of anatomical facts which had already accumulated and in adding and classi- fying fresh ones. The study of comparative anatomy and embryology received a new stimulus, for with the ac- ceptance of the theory of descent with modification it be- came incumbent upon the biologist to demonstrate the manner in which animals and plants differing widely in structure* and appearance could be conceivably related to one another. Thenceforward the energies of both . 12 MENDELISM CHAP. botanists and zoologists have been devoted to the con- struction of hypothetical pedigrees suggesting the various tracks of evolution by which one group of animals or plants may have arisen from another through a long con- tinued process of natural selection. The result of such work on the whole may be said to have shown that the diverse forms under which living things exist to-day, and have existed in the past so far as palaeontology can tell us, are consistent with the view that they are all related by the community of descent which the accepted theory of evolution demands, though as to the exact course of descent for any particular group of animals there is often considerable diversity of opinion. It is obvious that all this work has little or nothing to do with the manner in which species are formed. Indeed, the effect of Darwin's Origin of Species was to divert attention from the way in which species originate. At the time that it was put forward his explanation appeared so satisfying that bi- ologists accepted the notions of variation and heredity there set forth and ceased to take any further interest in the work of the hybridisers. Had Mendel's paper ap- peared a dozen years earlier it is difficult to believe that it could have failed to attract the attention it deserved. Coming as it did a few years after the publication of Dar- win's great work, it found men's minds set at rest on the problems that he raised and their thoughts and energies directed to other matters. ii HISTORICAL 13 Nevertheless one interesting and noteworthy attempt to give greater precision to the term heredity was made about this time. Francis Galton, a cousin of Darwin, working upon data relating to the breeding of Basset hounds, found that he could express on a definite statistical scheme the proportion in which the different colours ap- peared in successive generations. Every individual was conceived of as possessing a definite heritage which might be expressed as unity. Of this, i was on the average derived from the two parents (i.e. \ from each parent), } from the four grandparents, | from the eight great- grandparents, and so on. The Law of Ancestral Heredity, as it was termed, expresses with fair accuracy some of the statistical phenomena relating to the transmission of char- acters in a mixed population. But the problem of the rway in which characters are distributed from gamete to zygote and from zygote to gamete remained as before. Heredity is essentially a physiological problem, and though statistics may be suggestive in the initiation of experiment, it is upon the basis of experimental fact that progress must ultimately rest. For this reason, in spite of its ingenuity and originality, Galton's theory and the subsequent statistical work that has been founded upon it failed to give us any deeper insight into the nature of the hereditary process. While Galton was working in England the German zool- ogist August Weismann was elaborating the complicated i 4 MENDELISM CHAP. theory of heredity which eventually appeared in his work on The Germplasm (1885), a book which will be remem- bered for one notable contribution to the subject. Until the publication of Weismann's work it had been generally accepted that the modifications brought about in the individual during its lifetime, through the varying conditions of nutrition and environment, could be trans- mitted to the offspring. In this biologists were but fol- lowing Darwin, who held that the changes in the parent resulting from increased use or disuse of any part or or- gan were passed on to the children. Weismann's theory involved the conception of a sharp cleavage between the general body tissues or somatoplasm and the reproductive glands or germplasm. The individual was merely a car- rier for the essential germplasm whose properties had been determined long before he was capable of leading a separate existence. As this conception ran counter to the possibility of the inheritance of " acquired charac- ters," Weismann challenged the evidence upon which it rested and showed that it broke down wherever it was critically examined. By thus compelling biologists to revise their ideas as to the inherited effects of use and dis- use, Weismann rendered a valuable service to the study of genetics and did much to clear the way for subsequent research. A further important step was taken in 1895, when Bate- son once more drew attention to the problem of the origin ii HISTORICAL 15 of species, and questioned whether the accepted ideas of variation and heredity were after all in consonance with the facts. Speaking generally, species do not grade grad- ually from one to the other, but the differences between them are sharp and specific. Whence comes this preva- lence of discontinuity if the process by which they have arisen is one of accumulation of minute and almost imper- ceptible differences? Why are not intermediates of all sorts more abundantly produced in nature than is actually known to be the case ? Bateson saw that if we are ever to answer this question we must have more definite know- ledge of the nature of variation and of the nature of the hereditary process by which these variations are trans- mitted. And the best way to obtain that knowledge was to let the dead alone and to return to the study of the liv- ing. It was true that the past record of experimental breeding had been mainly one of disappointment. It was true also that there was no tangible clue by which experiments might be directed in the present. Neverthe- less in this kind of work alone there seemed any promise of ultimate success. A few years later appeared the first volume of de Vries' remarkable book on The Mutation Theory. From a pro- longed study of the evening primrose (Oenotherd) de Vries concluded that new varieties suddenly arose from older ones by sudden sharp steps or mutations, and not by any process involving the gradual accumulation of minute 16 MENDELISM CHAP, n differences. The number of striking cases from among widely different plants which he was able to bring forward went far to convincing biologists that discontinuity in variation was a more widespread phenomenon than had hitherto been suspected, and not a few began to question whether the account of the mode of evolution so generally accepted for forty years was after all the true account. Such in brief was the outlook in the central problem of biology at the time of the rediscovery of Mendel's work. CHAPTER III THE task that Mendel set before himself was to gain some clear conception of the manner in which the definite and fixed varieties found within a species are related to one another, and he realised at the outset that the best chance of success lay in working with material of such a nature as to reduce the problem to its simplest terms. He decided that the plant with which he was to work must be normally self-fertilising and unlikely to be crossed through the interference of insects, while at the same time it must possess definite fixed varieties which bred true to type. In the common pea (Pisum sativum) he found the plant he sought. A hardy annual, prolific, easily worked, Pisum has a further advantage in that the insects which normally visit flowers are unable to gather pollen from it and so to bring about cross fertilisation. At the same time it exists in a number of strains presenting* well- marked and fixed differences. The flowers may be purple, or red, or white ; the plants may be tall or dwarf ; the ripe seeds may be yellow or green, round or wrinkled - such are a few of the characters in which the various races of peas differ from one another, c 17 i8 MENDELISM CHAP. In planning his crossing experiments Mendel adopted an attitude which marked him off sharply from the earlier hybridisers. He realised that their failure to elucidate any general principle of heredity from the results of cross fertilisation was due to their not having concentrated upon particular characters or traced them carefully through a sequence of generations. That source of fail- ure he was careful to avoid, and throughout his experi- ments he crossed plants presenting sharply contrasted characters, and devoted his efforts to observing the be- '^haviour of these characters in successive generations. Thus in one series of experiments he concentrated his at- tention on the transmission of the characters tallness and dwarfness, neglecting in so far as these experiments were concerned any other characters in which the parent plants might differ from one another. For this purpose he chose two strains of peas, one of about 6 feet in height, and another of about ij feet. Previous testing had shown that each strain bred true to its peculiar height. These two strains were artificially crossed l with one an- other, and it was found to make no difference which was used as the pollen parent and which was used as the ovule parent. In either case the result was the same. The result of crossing tall with dwarf was in every case nothing but tails, as tall or even a little taller than the tall parent. For this reason Mendel termed tallness the dominant and 1 Cf. note on p. 171. in MENDEL'S WORK 19 dwarfness the recessive character. The next stage was to collect and sow the seeds of these tall hybrids. Such seeds in the following year gave rise to a mixed genera- tion consisting of tails and dwarfs but no intermediates. By raising a considerable number of such plants Mendel was able to establish the fact that the jmmhe.r oLtails which occurred in this generation was almost exactly three times as great as the number of the dwarfs. As in the previous year, seed were carefully collected from this, the second hybrid generation, and in every case the seeds from each individual plant were harvested separately and separately sown in the following year. By this respect for the individuality of the different plants, however closely they resembled one another, Mendel found the clue that had eluded the efforts of all his predecessors. The seeds collected from the dwarf recessives bred true, giving noth- ing but dwarfs. And this was true for every dwarf tested. But with the tails it was quite otherwise. Although in- distinguishable in appearance, some | of them bred truQ v T(D) F t while others be.- l I l haved like the. T. T^D) ,' T(D> Pr--f* original tall hy- brids, giving.. ' 1&W&KWW**' generation con^,. T D F* sisting 'of tails and dwarfs in the proportion of three of $0 MENDELISM CHAP. the former to one of the latter. Counting showed that -- . * ... . ** the number of the tails which gave dwarfs was double that of the tails which bred true, If we denote a dwarf plant as D, a true breeding tall plant as T, and a tall which gives both tails and dwarfs in the ratio 3 : i as T (D), the result of these experiments may be briefly summarised in the foregoing scheme. 1 Mendel experimented with other pairs of contrasted characters and found that in every instance they followed the same scheme of inheritance. Thus coloured flowers were dominant to white, in the ripe seeds yellow was dom- inant to green, and round shape was dominant to wrin- kled, and so on. In every case where the inheritance of an alternative pair of characters was concerned the effect of the cross in successive generations was to produce three and only three different sorts of individuals, viz. domi- nants which bred true, dominants which gave both dominant and recessive offspring in the ratio 3:1. and recessives which always bred true. Having determined a general scheme of inheritance which experiment showed to hold good for each of the seven pairs of alternative char- acters with which he worked. Mendel set himself to pro- viding a theoretical interpretation of this scheme which, as he clearly realised, must be in terms of germ cells. He 1 It has been found convenient to denote the various generations resulting from a cross by the signs FI. F : . F-.. etc. FI on this system denotes the first filial generation. F- the second filial generation pro- duced by t\vo parents belonging to the FI generation, and so on. in MENDEL'S WORK 21 conceived of the gametes as bearers of something capable of giving rise to the characters of the plant, but he re- garded any individual gamete as being able to carry one and one only of any alternative pair of characters. A given gamete could carry tallness or dwarfness, but not both. The two were mutually exclusive so far as the gamete was concerned. It must be pure for one or the other of such a pair^and this conception of the purity of the gametes'is the most. essential part of Mendel's theory. We may now proceecl with the help of the accompany- ing scheme (Fig. i) to deduce the results that should flow from Mendel's con- ception of the nature of the gametes, and to & ametes see how far they are in accordance with the facts. Since the original tall plant belonged to a strain which bred true, t all the gametes produced by it must bear the tall character. Similarly all the gametes of the original dwarf plant must bear the dwarf character. A cross between these two means the union of \ CL Q gametei F, to F 2 generation FIG. i. Scheme of inheritance in the cross of tall with dwarf pea. Gametes represented by small and zygotes by larger circles. 22 MENDELISM CHAP. a gamete containing tallness with one bearing dwarfness. Owing to the completely dominant nature of the tall character, such a plant is in appearance indistinguishable from the pure tall, but it differs markedly from it in the nature of the gametes to which it gives rise. When the formation of the gametes occurs, the elements represent- ing dwarfness and tallness segregate from one another, so that half of the gametes produced contain the one, and half contain the other of these two elements. For on hypothesis every gamete must be pure for one or other of these two characters. And this is true for the ovules as well as for the pollen grains. Such hybrid Y l plants, there- fore, must produce a series of ovules consisting of those bearing tallness and those bearing dwarfness, and must produce them in equal numbers. And similarly for the pollen grains. We may now calculate what should hap- pen when such a series of pollen grains meets such a series of ovules, i.e. the nature of the generation, that should be produced when the hybrid is allowed to fertilise itself. Let us suppose that there are 4 x ovules so that 2 x are "tall" and 2 x are "dwarf." These are brought in con tact with a mass of pollen grains of which half are "tall" and half are "dwarf." It is obvious that a " tall" ovule has an equal chance of being fertilised by a "tall" or a "dwarf" pollen grain. Hence of our 2x "tall" ovules, x will be fertilised by " tall " pollen grains and x will be fertilised by " dwarf " pollen grains. The former must give rise to tall in MENDEL'S WORK 23 plants, and since the dwarf character has been entirely eliminated from them they must in the future breed true. The latter must also give rise to tall plants, but since they carry also the recessive dwarf character they must when bred from produce both tails and dwarfs^ Each of the 2 x dwarf ovules, again, has an equal chance of being fertil- ised by a "tall" or by a " dwarf" pollen grain. Hence x will give rise to tall plants carrying the recessive dwarf character, while x will produce plants from which the tall character has been eliminated, i.e. to pure recessive dwarfs. Consequently from the 4% ovules of the self- fertilised hybrid we ought to obtain 3 x tall and x dwarf plants. And of the 3 x tails x should breed true to tallness, while the remaining 2 x, having been formed like the original hybrid by the union of a "tall" and a "dwarf" gamete, ought to behave like it when bred from and give tails and dwarfs in the ratio 3:1. Now this is precisely the result actually obtained by experiment (cf. p. 17), and the close accord of the experimental results with those deduced on the assumption of the purity of the gametes as enun- ciated by Mendel affords the strongest of arguments for regarding the nature of the gametes and their relation to the characters of the zygotes in the way that he has done. It is possible to put the theory to a further test. The explanation of the 3 : i ratio of dominants and recessives in the F 2 generation is regarded as due to the Fj individ- uals producing equal numbers of gametes bearing the 24 MENDELISM CHAP. dominant and recessive elements respectively. If now the F! plant be crossed with the pure recessive, we are bringing together a series of gametes consisting of equal numbers of dominants and recessives with a series con- sisting solely of recessives. We ought from such a cross to obtain equal numbers of dominant and recessive in- dividuals, and further, the dominants so produced ought all to give both dominants and recessives in the ratio 3 : i when they themselves are bred from. Both of these ex- pectations were amply confirmed by experiment, and cross- ing with the recessive is now a recognised way of testing whether a plant or animal bearing a dominant character is a pure dominant, or an impure dominant which is carry- ing the recessive character. In the former case the off- spring will be all of the dominant form, while in the latter they will consist on the average of equal numbers of dominants and recessives^ So far we have been concerned with the results ob- tained when two individuals differing in a single pair of characters are crossed together and with the interpreta- tion of those results. But Mendel also used plants which differed in more than a single pair of differentiating characters. In such cases' he found that each pair of characters followed the same definite rule, but that the inheritance of each pair was absolutely independent of the other. Thus, for example, when a tall plant bear- ing coloured flowers was crossed with a dwarf plant in MENDEL'S WORK 25 bearing white flowers the resulting hybrid was a tall plant with coloured flowers. For coloured flowers are dominant to white, and tallness is dominant to dwarf- ness. ' In the succeeding generation there are plants with coloured flowers and plants with white flowers in the pro- portion of 3:1, and at the same time tall plants and dwarf plants in the same proportion. Hence the chances that a tall plant will have coloured flowers are three times as great as its chance of having white flowers. And this is also true for the dwarf plants. As the result of this cross, therefore, we should expect an F 2 genera- tion consisting of four classes, viz. coloured tails, white tails, coloured dwarfs, and white dwarfs, and we should further expecjLjLhese four forms to appear in the ratio of 9 coloured jtalls,. 3 white tails, 3 coloured dwarfs, and i white dwarf. For this is the only ratio which satisfies the conditions that the tails should be to the dwarfs as 3:1, and at the same time the coloured should be to the whites" as 3 : i . And these are the proportions that Mendel found to obtain actually in his experiments. Put in a more general form, it may be stated that when two individuals are crossed which differ in two pairs of differentiating characters the hybrids (Fj) are all of the same form, exhibiting the dominant character of each of the two pairs r while the F 2 generation produced by such hybrids consists on the average of 9 showing both dominants, 3 showing one dominant and one recessive, 26 MKNDELISM CHAP. 3 showing the other dominant and the other recessive, and i showing both recessive characters. And, as Men- del pointed out, the principle may be extended in- definitely. If, for example, the parents differ in three pair of characters A, B, and C, respectively dominant to a, b, and c, the Y 1 individuals will be all of the form ABC, while the F 2 generation will consists of 27 ABC, 9 A Be; 9 AbC, 9 aBC, 3 Abe, 3 aBc, 3 abC, and_ i abc. When individuals differing in a number of alternative characters are crossed together, the hybrid generation, provided that the original parents were of pure strains, consists of plants of the same form ; but when these are bred from a redistribution of the various characters occurs. That redistribution follows the same definite rule for each character, and if the constitution of the original parents be known, the nature of the F 2 genera- tion, i.e. the number of possible forms and the propor- tions in which they occur, can be readily calculated. Moreover, as Mendel showed, we can calculate also the chances of any given form breeding true. To this point, however, we shall return later. Of Mendel's experiments with beans it is sufficient to say here that they corroborated his more ample work with peas. He is also known to have made experiments with many other plants, and a few of his results are incidentally given in his series of letters to Nageli the botanist. To the breeding and crossing of bees he also devoted much in MENDEL'S WORK 27 time and attention, but unhappily the record of these experiments appears to have been lost. The only other published work that we possess dealing with heredity is a brief paper on some crossing experiments with the hawk- weeds (Hieracium), a genus that he chose for working with because of the enormous number of forms under which it naturally exists. By crossing together the more distinct varieties, he evidently hoped to produce some of these numerous wild forms, and so throw light upon their origin and nature. In this hope he was disappointed. Owing in part to the great technical difficulties attending the cross fertilisation of these flowers he succeeded in ob- taining very few hybrids. Moreover, the behaviour of those which he did obtain was quite contrary to what he had found in t^he peas. Instead of giving a variety of forms in the F 2 generation, they bred true and continued to do so as long as they were kept under observation. More recent research has shown that this is due ta a pe- culiar form of parthenogenesis (cf. p. 135), and not to any failure of the characters to separate clearly from one an- other in the gametes. Mendel, however, could not have known of this, and his inability to discover in Hieracium any indication of the rule which he had found to hold good for both peas and beans must have been a source of considerable disappointment. Whether for this reason, or owing to the utter neglect of his work by the scientific world, Mendel gave up his experimental 28 MEXDELISM CHAP, m researches during the latter part of his life. His closing years were shadowed with ill-health and em- bittered by a controversy with the Government on a question of the rights of his monastery. He died of B right's disease in 1884. Note. Shortly after the discovery of Mendel's paper a need was felt for terms of a general nature to express the constitution of individuals in respect ot-inherited characters, and Bateson ac- cordingly proposedkhe words homozygote and heterozygote. An individual is said to be homozygous for a given character when it has been formed by two gametes each bearing the character, and all the gametes of a homozygote bear the character in respect of which it is homozygous. When, however, the zygote is formed by two gametes of which one bears the given character while the other does not, it is said to be heterozygous for the character in question, and only half the gametes produced by such a heterozygote bear the character. An individual may be homozygous for one or more characters, and at the same time may be heterozygous for others. CHAPTER IV THE PRESENCE AND ABSENCE THEORY IT was fortunate for the development of biological science that the rediscovery of Mendel's work found a small group of biologists deeply interested in the problems of heredity, and themselves engaged in experimental breeding. To these men the extraordinary significance of the discovery was at once apparent. From their experi- ments, undertaken in ignorance of Mendel's paper, de ^Vries, Correns, and Tschermak were able to confirm his results in peas and other plants, while Bateson was the first to demonstrate their application to animals. Thence- forward the record has been one of steady progress, and the result of ten years' work has been to establish more and more firmly the fundamental nature of MendeFs discovery. The scheme of inheritance, which he was the first to enunciate, has been found to hold good for such diverse things as height, hairiness, and flower colour and flower form in plants, the shape of pollen grains, and the structure of fruits ; while among animals the coat colour of mammals, the form of the feathers and of the comb in poultry, the waltzing habit of Japanese mice, and eye 29 30 MENDELISM CHAP. colour in man are but a few examples of the diversity of characters which all follow the same law of transmission. And as time went on many cases which at first seemed to fall without the scheme have been gradually brought into line in the light of fuller knowledge. Some of these will be FIG. 2. A wing feather and a contour feather of an ordinary and a silky fowl. The peculiar ragged appearance of the silky feathers is due to the absence of the little hooks or barbules which hold the barbs together. The silky condition is recessive. dealt with in the succeeding chapters of this book. Mean- while we may concern ourselves with the single modifica- tion of Mendel's original views which has arisen out of more ample knowledge. As we have already seen, Mendel considered that in the gamete there was either a definite something correspond- iv PRESENCE AND ABSENCE THEORY 31 ing to the dominant character or a definite something corresponding to the recessive character, and that these somethings whatever they were could not coexist in any single gamete. For these somethings we shall in future use the term factor. The factor, then, is what corre- sponds in the gamete to the unit-character that appears in some shape or other in the development of the zygote. Talmess in the pea is a unit-character, and the gametes in FIG. 3. Two double and an ordinary single primula flower. This form of double is recessive to the single. which it is represented are said to contain the factor for tallness. Beyond their existence in the gamete and their mode of transmission we make no suggestion as to the nature of these factors. MENDELISM CHAP. On Mendel's view there was a factor corresponding to the dominant character and another factor corresponding to the recessive character of each alternative pair of unit- characters, and the characters were alternative because no B FIG. 4. Fowls' combs. A, pea ; B, rose ; C, single ; D, walnut. gamete could carry more than one of the two factors belonging to the alternative pair. On the other hand, Mendel supposed that it always carried either one or the other of such a pair. As experimental work proceeded, iv PRESENCE AND ABSENCE THEORY 33 it soon became clear that there were cases which could not be expressed in terms of this conception. The na- ture of the difficulty and the way in which it was met will perhaps be best understood by considering a set of experiments in which it occurred. Many of the different breeds of poultry are characterised by a particular form of comb, and in certain cases the inheritance of these has been carefully worked out. It was shown that the ro^e comb (Fig. 4, B) with its flattened papillated upper sur- face and backwardly projecting pike was dominant in the ordinary way to the deeply serrated high single comb (Fig. 4, C) which is characteristic of the Mediterranean races. Experiment also showed that the rjea comb (Fig. 4, A), a form with a low central and two well-developed i lateral ridges, such as^is found in Indian game, behaves as a simple dominant to the single -comb. The inter- esting question arose as to what would happen when the rose and the pea, two forms each dominant to the same third form, were mated together. It seemed reasonable to suppose that things which were alternative to the same thing would be alternative to one another that either rose or pea would dominate in the hybrids, and that the F 2 generation would consist of dominants and recessives in the ratio 3:1. The result of the ex- periment was, however, very different. The cross rose x pea led to the production of a comb quite unlike either of them. This, the so-called walnut comb (Fig. 4, D), 34 MENDELISM CHAP. from its resemblance to the half of a walnut, is a type of comb which is normally characteristic of the Malay fowl. Moreover, when these F x birds were bred together, a further unlooked-for result was obtained. As was ex- pected, there appeared in the F 2 generation the three forms walnut, rose, and pea. But there also appeared a definite proportion of single-combed birds, and among many hundreds of chickens bred in this way the propor- tions in which the four forms walnut, rose, pea, and single appeared was 9:3:3:1. Now this, as Mendel showed, is the ratio found in an F 2 generation when the original par- ents differ in two pairs of alternative characters, and from the proportions in which the different forms of comb occur Q j ij we mus t m ^ er that the wal- Rose X Pea tf nut contains both domi- i i*"* nants, the rose and the pea Walnut x Walnut one dominant each, while the single is pure for both Walnut Rose Pea Single recessive characters. This (9) (3) (3) ( J ) accorded with subsequent breeding experiments, for the singles bred perfectly true as soon as they had once made their appearance. So far the case is clear. The difficulty comes when we attempt to define these two pairs of characters. How are we to express. the fact that while single behaves as a simple recessive to either pure rose, or to pure pea, it can yet appear in F 2 from a cross iv PRESENCE AND ABSENCE THEORY 35 between these two pure forms, though neither of them should, on Mendel's view, contain the single? An ex- planation which covers the facts in a simple way is that which has been termed the "Presence and Absence" theory. On this theory the dominant character. of an alternative pair owes its dominance to the presence oi SL factor which is absent in the recessive. The tall pea is tall owing to the presence in it .of the factor for tallness, but in the absence of this factor the pea remains a dwarf. All peas are dwarf , but the tall is a dwarf plus a factor which turns it into a tall. Instead of the characters of an al- ternative pair being due to two separate factors, we now regard them as the expression of the only two possible states of a single factor, viz. its presence or its absence. The conception will probably become clearer if we follow its application in detail to the case of the fowl's combs. In this case we are concerned with the transmission of the two factors, rose (R) and pea (P), the presence of each of which is alternative to its absence. The rose-combed bird contains the factor for rqse^but not that for pea, and the pea-combed bird contains the factor for pea but not that for rose. When both factors are present in a bird, as in the hybrid made by crossing rose with pea, the result is a walnut. For convenience of argument we may de- note the presence of a given factor by a capital letter and its absence by the corresponding small letter. The use of the small letter is merely a symbolic way of intimating 36 MENDELISM CHAP. that a particular factor is absent in a gamete or zygote. Represented thus the.zygotic constitution of a pure rose- combed bird is RRpp; for it has been formed by the union of two gametes both of which contained R but not P. Similarly we may denote the pure pea-combed bird as rrPP. On crossing the rose with the pea union occurs between a gamete Rp and a gamete rP, resulting in the formation of a heterozygote of the constitution RrPp. " t The use of the small letters here informs us that such a zygote contains only a single dose of each of the factors R and Pj although, of course, it is possible for a zygote, if made in a suitable way, to have a double dose of any factor. Now when such a bird comes to form gametes a separation takes place between the part of the zygotic cell containing R and the part which does not contain it (r). Half of its gametes, therefore, will contain R and the other half will be without it (r). Similarly half of its gametes will contain P and the other half will be without it (p). It is obvious that the chances of R being distributed to a gamete with or without P are equal. Hence the gametes containing R will be of two sorts, PR and Rp, and these will be produced in equal numbers. Similarly the gam- etes without R will also be of two sorts, rP and rp, and these, again, will be produced in equal numbers. Each of the hybrid walnut-combed birds, therefore, gives rise to a series consisting of equal numbers of gametes of the four different types RP, Rp, rP, and rp ; and the breeding to- IV PRESENCE AND ABSENCE THEORY 37 gether of such F x birds means the bringing together of two such series of gametes. When this happens an ovum of any one of the four types has an equal chance of being fertilised by a spermatozoon of any one of the four types. A convenient and simple method of demonstrating what happens under such circumstances is the method some- times termed the " chessboard" method. For two series each consisting of four different types of gamete we re- quire a square divided up into 16 parts. The four terms of the gametic series are first written horizontally across the four sets of four squares, so that the series is repeated four times. It is then written vertically four times, care being taken to keep to the same order. In this simple mechanical way all the possible combinations are 'rep- resented and in their proper proportions. Fig. 5 shows the re- sult of applying this method to our series RP, Rp, rP, RP. RP RP Rp RP rP RP H> Walnut Walnut Walnut Walnut fr Rp Rp RP rP Rp r P Walnut Rose Walnut Rose rP; RP rP Rp rP rP rP rp Walnut Walnut Pea Pea n>~ RP 2 Rp rp T> Walnut Rose Pea Single FIG. 5. Diagram to illustrate the nature of the F 2 generation and from the cross of rose comb x pea comb. the 1 6 squares represent the different kinds of zygotes formed and the proportions in which they occur. As 38 MENDELISM CHAP. the figure shows, 9 zygotes contain both R and P, having a double or a single dose of either or both of these factors. Such birds must be all walnut combed. Three out of the 16 zygotes contain R but not P, and these must be rose-combed birds. Three, again, contain P but not R and must be pea-combed birds. Finally one out of the 16 contains neither R nor P. It cannot be rose it cannot be pea. It must, therefore, be some- thing else. As a matter of fact it is single. Why it should be single and not something else follows from what we already know about the behaviour of these various forms of comb. For rose is dominant to single ; therefore on the Presence and Absence theory a rose is a single plus a factor which turns the single into a rose. If we could remove the "rose" factor from a rose-combed bird the underlying single would come into view. Similarly a pea comb is a single plus a factor which turns the single into a pea, and a walnut is a single which possesses two addi- tional modifying factors. Singleness, in fact, underlies all these combs, and if we write their zygotic constitution in full we must denote a walnut as RRPPSS, a rose as RRppSS, a pea as rrPPSS, and a single as rrppSS. The crossing of rose with pea results in a reshuffling of the factors concerned, and in accordance with the principle of segregation some zygotes are formed in which neither of the modifying factors R and P are present, and the single character can then become manifest. iv PRESENCE AND ABSENCE THEORY 39 The Presence and Absence theory is to-day generally accepted by students of these matters. Not only does it afford a simple explanation of the remarkable fact that in all cases of Mendelian inheritance we should be able to express our unit-characters in terms of alternative pairs, but, as we shall have occasion to refer to later, it suggests a clue as to the course by which the various domesticated varieties of plants and animals have arisen from their wild prototypes. Before leaving this topic we may draw attention to some experiments which offer a pretty confirmation of the view that the rose comb is a single to which a modifying factor for roseness has been added. It was argued that if we could find a type of comb in which the factor for single- ness was absent, then on crossing such a comb with a rose we ought, if singleness really underlies rose, to obtain some single combs in F 2 from such a cross. Such a comb we had the good fortune to find in the Breda fowl, a breed largely used in Holland. This fowl is usually spoken of as conibless, for the place of the comb is taken by a covering of short bristlelike feathers (Fig. 6, D). In reality it pos- sesses the vestige of a comb in the form of two minute lateral knobs of comb tissue. Characteristic also of this breed is the high development of the horny nostrils, a feature probably correlated with the almost complete absence of comb. The first step in the experiment was to prove the absence of the factor for singleness in the Breda. MENDELISM CHAP. On crossing Breda with single the Y l birds exhibit a large comb of the form of a double single comb in which the two portions are united anteriorly, but diverge from one an- other towards the back of the head (Fig. 6, C). The FIG. 6. Fowls' combs. A and B, F, hen from rose x Breda; C, an F t cock from the cross of single x Breda; D, head of Breda cock. Breda contains an element of duplicity which is dominant to the simplicity of the ordinary single comb. But it can- not contain the factor for the single comb, because as soon as that is put into it by crossing with a single the comb iv PRESENCE AND ABSENCE THEORY 41 assumes a large size, and is totally distinct in appearance from its almost complete absence in the pure Breda. Now when the Breda is crossed with the rose duplicity is dominant to simplicity, and rose is dominant to lack of comb, and the F 1 generation consists of birds possessing duplex rose combs (Fig. 6, A and B). On breeding such birds together we obtain a generation consisting of Bredas, duplex roses, roses, duplex singles, and singles. From our previous experiment we know that the singles Rose X Breda I . Duplex x Duplex Rose Rose r r Duplex Rose Duplex Single Breda Rose Single (Duplex and Simplex) could not have come from the Breda, since a Breda comb to which the factor for single has been added no longer remains a Breda. Therefore it must have come from the rose, thus confirming our view that the rose is in reality a single comb which contains in addition a dominant modi- fying factor (R) whose presence turns it into a rose. We shall take it, therefore, that there is good experimental evidence for the Presence and Absence theory, and we shall express in terms of it the various cases which come up for discussion in succeeding chapters. CHAPTER V INTERACTION OF FACTORS WE have now reached a point at which it is possible to formulate a definite conception of the living organism. A plant or animal is a living entity whose properties may in large measure be expressed in terms of unit-characters, and it is the possession of a greater or lesser number of such unit-characters renders it possible for us to draw **harp distinctions between one individual and another. These unit-characters are represented by definite factors in the gamete which in the process of heredity behave as indivisible entities, and are distributed according to a definite scheme. The factor for this or that unit-char- acter is either present in the gamete or it is riot present. It must be there in its entirety or completely absent. Such at any rate is the view to which recent experiment has led us. But as to the nature of these factors, the conditions under which they exist in the gamete, and the manner in which they produce their specific 'effects in* the zygote, we are at present almost completely in the 4ark. The case of the fowls' combs opens up the important question of the extent to which the various factojs can 42 V INTERACTION OF FACTORS 43 influence one another in the zygote. The rose and the pea factors are separate entities, and each when present alone produces a perfectly distinct and characteristic effect upon the single comb, turning it into a rose or a pea as the case may be. But when both are present in the same zygote their combined effect is to produce the wal- nut comb, a comb which is quite distinct from either and in no sense intermediate between them. The question of the influence of factors upon one another did not pre- sent itself to Mendel because he worked with characters which affected different parts of the plant. It was un- likely that the factor which led to the production of colour in the flower would affect the shape of the pod, or that the height of the plant would be influenced by presence or absence of the factor that determined the shape of the ripe seed. But when several factors can modify the same structure it is reasonable to suppose that they will influence one another in the effects which their simultaneous presence has upon the zygote. By them- selves the pea and the rose factors each produce a definite modification of the single comb, but when both are pres- ent in the zygote, whether as a single or double dose, the modification that results is quite different to that pro- duced by either when present alone. Thus we are led to the conception of characters which depend for their manifestation on more than one factor in the zygote, and in the present chapter we may consider a few of the 44 MENDELISM CHAP. phenomena which result from such interaction between separate and distinct factors. One of the most interesting and instructive cases in which the interaction between separate factors has been demonstrated is a case in the sweet pea. All white sweet peas breed true to whiteness. And generally speaking the result of crossing different whites is to produce noth- ing but whites, whether in F l or in succeeding generations. But there are certain strains of ., white sweet peas which when crossed together produce only coloured flowers. The colour may be different in different cases, though for our present purpose we may take a case in which the colour is red. When such reds are allowed to self- fertilise themselves in the normal way and the seeds White x White sown, the resulting F 2 genera- | tion consists of reds and whites, Red Fj the former being rather more i ' 1 numerous than the latter in the White- F 5 proportion of g . ?> The rais . ing of a further generation from the seeds of these F 2 plants shows that the whites always breed true to whiteness, but that different reds may behave differently. Some breed true, others give reds and whites in the ratio 3:1, while others, again, give reds and whites in the ratio 9:7. As in the case of the fowls' combs, this case may be interpreted in terms of the presence and absence of two factors. Red in the v INTERACTION OF FACTORS 45 sweet pea results from the interaction of two factors, and unless these are both present the red colour cannot appear. Each of the white parents carried one of the two factors whose interaction is necessary for the production of the red colour, and as a cross between them brings these two complementary factors together the F x plants must all be red. As this case is of considerable importance for the proper understanding of much that is to follow, and as it has been completely worked out, we shall consider it in some detail. Denoting these two colour factors by A and B respectively we may proceed to follow out the conse- quences of this cross. Since all the F x plants were red the constitution of the parental whites must have been A Abb and aaBB respectively, and their gametes conse- quently Ab and aB. White White The constitution AAbb . aaBB of the F! plants / \ must, therefore, be Ab Ab aB AaBb. Such a plant _ 7T being heterozygous A B for two factors pro- ^ / \ w duces a series of g AB AB \ e 1 f ,&tu Ab Ab cjJtoT gametes of the four yz aB aB *-g kinds AB, Ab, aB, 1 ab a6 J ab, and produces them in equal numbers (cf. p. 36). To obtain the various types of zygotes which are produced when such MENDELISM CHAP. Ab Ab Ab ab a series of pollen grains meets a similar series of ovules we may make use of the same "chessboard" system which we have already adopted in the case of the fowls' combs. An examination of this figure (Fig. 7) shows that 9 out of the 16 squares contain both A and B, while 7 contain either A or B .alone, or neither. In other words, on this view of the nature of the two white sweet peas we should in the F 2 generation look for the appearance of coloured and white flowers in the ratio 9:7. And this, as we have already seen, is what was actually found by experiment. Further examination of the figure shows that the coloured plants are not all of the same constitution, but are of four kinds with respect to their zygotic constitution, coloured F t . viz. AABB, AABb, AaBB, and AaBb. Since AABB is homozygous for both A and B, all the gametes which it produces must contain both of these factors, and such a plant must therefore breed true to the red colour. A plant of the ab Ab aB aB ab aB aB ab ab ab FIG. 7. Diagram to illustrate the nature of the F 2 generation from the two white sweet peas which give a v INTERACTION OF FACTORS 47 constitution AABb is homozygous for the factor A, but heterozygous for B. All of its gametes will contain A , but only one-half of them will contain B, i.e. it produces equal numbers of gametes AB and Ab. Two such series of gametes coming together must give a generation consist- ing of x AABB, 2x AABb, and x AAbb, that is, reds and whites in the ratio 3:1. Lastly the red zygotes of the constitution A aBb have the same constitution as the original red made from the two whites, and must there- fore when bred from give reds and whites in the ratio 9:7. The existence of all these three sorts of reds was demonstrated by experiment, and the proportions in which they were met with tallied with the theoretical explanation. The theory was further tested by an examination into the properties of the various F 2 whites which come from a coloured plant that has itself been produced by the mating of two whites. As Fig. 7 shows, these are, in respect of their constitution, of five different kinds, viz. A Abb, Aabb, aaBB, aaBb, and aabb. Since none of them produce any- thing but whites on self-fertilisation it was found neces- sary to test their properties in another way, and the method adopted was that of crossing them together. It is obvious that when this is done we should expect differ- ent results in different cases. Thus the cross between two whites of the constitution A Abb and aaBB should give nothing but coloured plants ; for these two whites are of 48 MENDELISM CHAP. the same constitution as the original two whites from which the experiment started. On the other hand, the cross between a white of the constitution aabb and any other white can never give anything but whites. For no white contains both A and B, or it would not be white, and a plant of the constitution aabb cannot supply the complementary factor necessary for the production of colour. Again, two whites of the constitution Aabb and aaBb when crossed should give both coloured and white flowers, the latter being three times as numerous as the former. Without going into further detail it may be stated that the results of a long series of crosses between the various F 2 whites accorded closely with the theoretical explanation. From the evidence afforded by this exhaustive set of experiments it is impossible to resist the deduction that the appearance of colour in the sweet pea depends upon the interaction of two factors which are independently transmitted according to the ordinary scheme of Mende- lian inheritance. What these factors are is'still an open question. Recent evidence of a chemical nature in- dicates that colour in a flower is due to the interaction of two definitive substances : (i) a colourless "chromogen," or colour basis; and (2) a ferment which behaves as an activator of the chromogen, and by inducing some process of oxidation, leads to the formation of a coloured substance. But whether these two bodies exist as such v INTERACTION OF FACTORS 49 in the gametes or whether in some other form we have as yet no means of deciding. Since the elucidation of the nature of colour in the sweet pea phenomena of a similar kind have been wit- nessed in other plants, notably in stocks, snapdragons, and orchids. Nor is this class of phenomena confined to plants. In the course of a series of experiments upon the plumage colour of poultry, indications were obtained that different white breeds did not always owe their white- ness to the same cause. Crosses were accordingly made between the white Silky fowl and a pure white strain derived from the white Dorking. Each of these had been previously shown to behave as a simple recessive to colour. When the two were crossed only fully coloured birds resulted. From analogy with the case of the sweet pea it was anticipated that such Fj coloured birds when bred together would produce an F 2 generation consisting of coloured and white birds in the ratio 9:7, and when the experiment was made this was actually shown to be the case. With the growth of knowledge it is probable that further striking parallels of this nature between the plant and animal worlds will be met with. Before quitting the subject of these experiments atten- tion may be drawn to the fact that the 9 : 7 ratio is in reality a 9:3:3:1 ratio in which the last three terms are indistinguishable owing to the special circumstances that neither factor can produce a visible effect without 50 MENDELISM CHAP. the co-operation of the other. And we may further em- phasise the fact that although the two factors thus inter- act upon one another they are nevertheless transmitted quite independently and in accordance with the ordinary Mendelian scheme. One of the earliest sets of experiments demonstrating the interaction of separate factors was that made by the French zoologist Cuenot on Agouti X Albino I the coat colours of mice. . I ! 3 IT 1 t It was shown that in cer- Agouti X Agouti tain cases agouti, which I 1 1 is the colour of the A?outi Black Albino ,. .*, , } (3) (4) ordinary wild grey mouse, behaves as a dominant to the albino variety, i.e. the F2 generation from such a cross consists of agoutis and albinos in the ratio 3:1. But in other cases the cross between albino and agouti gave a different result. In the FI generation appeared only agoutis as before, but the F 2 generation consisted of three distinct types, viz. agoutis, albinos, and blacks. Whence the sudden appearance of the new type ? The answer is a simple one. The albino parent was really a black. But it lacked the factor without which the colour is unable to develop, and consequently it remained an albino. If we denote this factor by C, then the constitution of an albino must be cc, while that of a coloured animal may be CC or Cc, according as to whether it breeds true to colour or can v INTERACTION OF FACTORS 51 throw albinos. Agouti was previously known to be a sim- ple dominant to black, i.e. an agouti is a black rabbit plus an additional greying factor which modifies the black into agouti. This factor we will denote by G, and we will use B 'for the black factor. Our original agouti and albino parents we may therefore regard as in constitution GGCCBB and ggccBB respectively. Both of the parents are homozygous for black. The gametes produced by the two parents are GCB, and gcB, and the constitution of the F! animals must be GgCcBB. Being heterozygous for two factors they will produce four kinds of gametes in equal numbers, viz. GCB, GcB, gCB, and gcB. The results of the mating of two such similar series of gametes when the FI animals are bred together we may determine by the usual " chessboard" method (Fig. 8). Out of the 1 6 squares 9 contain both^and G in addition to B. Such animals must be agoutis. Three squares contain C but not G. Such animals must be coloured, but as they do not contain the modifying agouti factor their colour will be black. The remaining four squares do not contain C, and in the absence of this colour-developing factor they must all be albinos. Theory demands that the three classes agouti, black, and albino should appear in F 2 in the ratio 9:3:4; experiment has shown that these are the only classes that appear, and that the proportions in which they are produced accord closely with the theoret- ical expectation. Put briefly, then, the explanation MENDELISM CHAP. of this case is that all the animals are black, and that we are dealing with the presence and absence of two factors, a colour devel- oper (C), and a colour modifier (G), both act- as it were, upon substratum of black. The F 2 generation really consists of the four classes agoutis, blacks, albino agoutis, and albino blacks in the ratio 9:3:3: i. But since in the absence of the colour Diagram to illustrate the nature of the F 2 generation which may arise from the mating of agouti with developer C the Colour albino in mice or rabbits. modifier G can pro- duce no visible result, the last two classes of the ratio are indistinguishable, and our F 2 generation conjes to consist of three classes in the ratio 9:3:4, instead of four classes in the ratio 9:3:3:1. This explanation was further tested by experiments with the albinos. In an F 2 family of this nature there ought to be three kinds, viz. albinos homozygous for G (GGccBB), albinos heterozygous for G (GgccBB), and albinos \&J;hout G (ggccBB}. These albinos are, as it were, like photographic plates exposed but undeveloped. v INTERACTION OF FACTORS 53 Their potentialities may be quite different, although they all look alike, but this can only be tested by treating them with a colour developer. In the case of the mice and rab- bits the potentiality for which we wish to test is the pres- ence or absence of the factor G, and in order to develop the colour we must introduce the factor C. Our de- veloper, therefore, must contain C but not G. In other words, it must be a homozygous black mouse or rabbit, ggCCBB. Since such an animal is pure for C it must, when mated with any of the albinos, produce only col- oured offspring. And since it does not contain G the ap- pearance of agoutis among its offspring must be attrib- uted to the presence of G in the albino. Tested in this way the F 2 albinos were proved, as was expected, to be of three kinds : (i) those which gave only agouti, i.e. which were homozygous for G; (2) those which gave agoutis and blacks in approximately equal numbers, i.e. which were heterozygous for G ; and (3) those which gave only blacks, and therefore did not contain G. Though albinos, whether mice, rabbits, rats, or other animals, breed true to albinism, and though albinism be- haves as a simple recessive to colour, yet albinos may be of many different sorts. There are in fact just as many kinds of albinos as there are coloured forms neither more nor less. And all these different kinds of albinos may breed together, transmitting the various colour fac- tors according to the Mendelian scheme of inheritance, 54 MENDELISM CHAP. and yet the visible result will be nothing but albinos. Under the mask of albinism is all the while occurring that segregation of the different colour factors which would result in all the varieties of coloured forms, if only the essential factor for colour development were present. But put in the developer by crossing with a pure coloured form and their variety of constitution can then at last become manifest. So far we have dealt with cases in which the production of a character is dependent upon the interaction of two factors. But it may be that some characters require the simultaneous presence of a greater number of factors for their manifestation, and the experiments of Miss Saunders have shown that there is a character in stocks which is un- able to appear except through the interaction of three distinct factors. Coloured stocks may be either hoary, with the leaves and stem covered by small hairs, or they may lack the hairy covering, in which case they are termed glabrous. Hoariness is dominant to glabrousness ; that is to say, there is a definite factor which can turn the glabrous into a hoary plant when it is present. But in families where coloured and white stocks occur the white are always glabrous, while the coloured plants may or may not be hoary. Now colour in the stock as in the sweet pea has been proved to be dependent upon the interaction of two separate factors. Hence hoariness depends upon three separate factors, and a stock cannot be hoary unless INTERACTION OF FACTORS 55 it contains the hoary factor in addition to the two colour factors. It requires the presence of all these three factors to produce the hoary character, though how this comes about we have not at present the least idea. A somewhat different and less usual form of inter- action between factors may be illustrated by a case in primulas recently worked out by Bateson and Gregory. Like the common primrose, the primula exhibits both pin-eyed and thrum-eyed varieties. In the former the style is long, and the centre of the eye is formed by the end of the stigma which more or less plugs up the opening of the corolla (cf. Fig. 9, A) ; in the latter the style is short FIG. 9. Sections of primula flowers. The anthers are shown as black. A, " pin " form with long style and anthers set low down ; B, " thrum " form with short style and anthers set higher up ; C, homostyle form with anthers set low down as in " pin," but with short style. This form only occurs with the large eye. and hidden by the four anthers which spring from higher up in the corolla and form the centre of the eye (cf. Fig. 9, B). The greater part of the "eye" is formed by the greenish-yellow patches on each petal just at the opening 56 MENDELISM CHAP. of the corolla. In most primulas the eye is small, but there are some in which it is large and extends as a flush over a considerable part of the petals (Fig. 10). Experi- ments showed that these two pairs of characters behave in simple Mendelian fashion, short style (= "thrum") being dominant to long style (= "pin") and small eye dominant to large. Besides the normal long and short styled forms, there occurs a third form, which has been termed homostyle. In this form the anthers are placed low down in the corolla tube as they are in the long- styled form, but the style remains short instead of reach- ing up to the corolla opening (Fig. 9, C). In the course FIG. 10. Two primula flowers showing the extent of the small and of the large eye. of their experiments Bateson and Gregory crossed a large- eyed homostyle plant with a small-eyed thrum (= short style). The F l plants were all short styled with small v INTERACTION OF FACTORS 57 eyes. On self- fertilisation these gave an F 2 generation consisting of four types, viz. short styled with small eyes, short styled with large eyes, long styled with small eyes, and homostyled with large eyes. The notable feature of this generation is the appearance of long-styled plants, which, however, occur only in association with the small eye. The proportions in which these four types appeared shows that the presence or absence of but two factors is concerned, and at the same time provides the key to the nature of the homostyled plants. These are poten- tially long styled, and the position of the anthers is that of normal long-styled plants, but owing to some interac- tion between the factors the style itself is unable to reach its full development unless the factor for the small eye is present. For this reason long-styled plants with the Short style \ f Homo style small eye j \ large eye Short style small eye Short style Short style Long style Homo style small eye large eye ("pin") large eye . (9) (3) (3) (i) large eye are always of the homostyle form. What the connecting-link between these apparently unrelated structures may be we cannot yet picture to ourselves, any more than we can picture the relation between flower 58 MENDELISM CHAP, v colour and hairiness in stocks. It is evident, however, that the conception of the interaction of factors, besides clearing up much that is paradoxical in heredity, prom- ises to indicate lines of research which may lead to valu- able extensions in our knowledge of the way in which the various parts of the living organism are related to one another. CHAPTER VI REVERSION As soon as the idea was grasped that characters in plants and animals might be due to the interaction of complementary factors, it became evident that this threw clear light upon the hitherto puzzling phenomenon of reversion. We have already seen that in certain cases the cross between a black mouse or rabbit and an albino, each belonging to true breeding strains, might produce nothing but agoutis. In other words, the cross between the black and the white in certain instances results in a complete reversion to the wild grey form. Expressed in Mendelian terms, the production of the agouti was the necessary consequence of the meeting of the factors C and G in the same zygote. As soon as they are brought to- gether, no matter in what way, the reversion is bound to occur, ^ej/ersion, therefore, in such cases we may regard as the bringing together of complementary factors which had somehow in the course of evolution become separated from one another. In the simplest cases, such as that of the black and the white rabbit, only two factors are con- cerned, and one of them is brought in from each of the 59 60 MENDELISM CHAP. two parents. But in other cases the nature of the rever- sion may be more complicated owing to a larger number of factors being concerned, though the general princi- ple remains the same. Careful breeding from the rever- sions will enable us in each case to determine the number and nature of the factors concerned, and in illustration of this we may take another example from rabbits. The Himalayan rabbit is a well-known breed. In appearance it is a white rabbit with pink eyes, but the ears, paws, and nose are black (PL I., 2). The Dutch rabbit is another well-known breed. Generally speaking, the anterior por- tion of the body is white, and the posterior part coloured. Anteriorly, however, the eyes are surrounded by coloured patches extending up to the ears, which are entirely col- oured. At the same time the hind paws are white (cf. PL I., i). Dutch rabbits exist in many varieties of colour, though in each one of these the distribution of colour and white shows the same relations. In the ex- periments about to be described a yellow Diitch rabbit was crossed with a Himalaya. The resuk was a reversion to the wild agouti colour (PL I., 3). Some of the F] in- dividuals showed white patches, while others were self- coloured. On breeding from the FI animals a series of coloured forms resulted in F 2 . These were agoutis, blacks, yellows, and sooty yellows, the so-called tortoise shells of the fancy (PL L, 4-7). In addition to these appeared Himalayans with either black points or with lighter brown- PLATE I. vi REVERSION 61 ish ones, and the proportions in which they came showed the Himalayan character to be a simple recessive. A cer- tain number of the coloured forms exhibited the Dutch marking to a greater or less extent, but as its inheritance in this set of experiments is complicated and has not yet been worked out, we may for the present neglect it and confine our attention to the coloured types and to the Himalayans. The proportion in which the four col- oured types appeared in F 2 was very nearly 9 agoutis, 3 blacks, 3 yellows, and i tortoiseshell. Evidently we are here dealing with two factors: (i) the grey factor (G), which modifies black into agouti, or tortoiseshell into yel- low; and (2) an intensifying factor (7), which intensifies yellow into agouti and tortoiseshell into black. It may Yellow X Himalayan Agouti X Agouti Agouti Yellow Black Tortoise Himalayan (27) (9) (9) S (sf (16) be mentioned here that other experiments confirmed the view that the yellow rabbit is a dilute agouti, and the tortoiseshell a dilute black. The Himalayan pattern be- haves as a recessive to self-colour. It is a self-coloured black rabbit lacking a factor that allows the colour to develop except in the points. That factor we may denote 62 MENDELISM CHAP. by X, and as far as it is concerned the Himalayan is con- stitutionally xx. The Himalayan contains the intensi- fying factor, for such pigment as it possesses in the points is full coloured. At the same time it is black, i.e. lacking in the factor G. With regard to these three factors, there- fore, the constitution of the Himalayan is ggllxx. The last character which we have to consider in this cross is the Dutch character. This was found by Hurst to be- have as a recessive to self-colour (S), and for our present purpose we will regard it as differing from a self-coloured rabbit in the lack of this factor. 1 The Himalayan is really a self-coloured animal, which, however, is unable to show itself as a full black owing to its not possessing the fac- tor X. The results of breeding experiments then sug- gest that we may denote the Himalayan by the formula ggllxxSS and the yellow Dutch by GGiiXXss. Each lacks two of the factors, upon the full complement of which the agouti colour depends. By crossing them the complete series GIXS is brought into the same,zygote, and the result is a reversion to the colour of the wild rabbit. Most of the instances of reversion yet worked out are those in which colour characters are concerned. The sweet pea, however, supplies us with a good example of reversion in structural characters. A dwarf variety known as the "Cupid" has been extensively grown for 1 Hurst's original cross was between a Belgian hare and an albina Angora, which turned to out be a masked Dutch. vi REVERSION 63 some years. In these little plants the internodes are very short and the stems are few in number, and attain to a length of only 9-10 inches. In course of growth they diverge from one another, and come to lie prostrate on the ground (PI. II., 2). Curiously enough, although the whole plant is dwarfed in other respects, this does not seem to affect the size of the flower, which is that of a normal sweet pea. Another though less-known variety is the "Bush" sweet pea. Its name is derived from its habit of growth. The numerous stems do not diverge from one another, but all grow up side by side, giving the plant the appearance of a compact bush (PI. II., i). Under ordinary conditions it attains a height of 3^-4 feet. A number of crosses were made between the Bush Bush X Cupid Tall F, Tall Bush Cupid Cupid F 2 (procumbent) (erect) (9) (3) (3) (i) and Cupid varieties, with the somewhat unexpected result that in every instance the FI plants showed complete reversion to the size and habit of the ordinary tall sweet pea (PI. II., 3), which is the form of the wild plant as it occurs in Sicily to-day. The F 2 generation from these reversionary tails consisted of four different types, viz. 64 MENDELISM CHAP. tails, bushes, Cupids of the procumbent type like the orig- inal Cupid parent, and Cupids with the compact upright Bush habit (PI. II., 4). These four types appeared in the ratio 9:3:3:1, and this, of course, provided the clue to the nature of the case. The characters concerned are (i) long internode of stem between the leaves which is domi- nant to short internode, and (2) the creeping procum- bent habit which is dominant to the erect bush-like habit. Of these characters length of internode was carried by the Bush, and the procumbent habit by the original Cupid parent. The bringing of them together by the cross resulted in a procumbent plant with long internodes. This is the ordinary tall sweet pea of the wild Sicilian type, reversion here, again, being due to the bringing together of two complementary factors which had somehow be- come separated in the course of evolution. To this interpretation it may be objected that the or- dinary sweet pea is a plant of upright habit. This, how- ever, is not true. It only appears so because,the conven- tional way of growing it is to train it up sticks. In reality it is of procumbent habit, with divergent stems like the ordinary Cupid, a fact which can easily be observed by anyone who will watch them grow without the artificial aid of prepared supports. The cases of reversion with which we have so far dealt have been cases in which the reversion occurs as an im- mediate result of a cross, i.e. in the FI generation. This is PLATE II. i, Bush Sweet Pea; 2, Cupid Sweet Pea; 3, Fj reversionary Tall; 4, Erect Cupid Sweet Pea ; 5, Purple Invincible ; 6, Painted Lady ; 7, Duke of Westminster (hooded standard). vi REVERSION 65 perhaps the commonest mode of reversion, but instances are known in which the reversion that occurs when two pure types are crossed does not appear until the F 2 generation. Such a case we have already met with in the fowls' combs. It will be remembered that the cross be- tween pure pea and pure rose gave walnut combs in FI, while in the F 2 generation a definite proportion, i in 16, of single combs appeared (cf. p. 32). Now the single comb is the form that is found in the wild jungle fowl, which is generally regarded as the ancestor of the domestic breeds. If this is so, we have a case of reversion in F 2 ; and this in the absence of the two factors brought together by the rose-comb and pea-comb parents. Instead of the reversion being due to the bringing together of two com- plementary factors, we must regard it here as due to the association of two complementary absences. To this question, however, we shall revert later in discussing the origin of domesticated varieties. There is one other instance of reversion to which we must allude. This is Darwin's famous case of the oc- casional appearance of pigeons reverting to the wild blue rock (Columba lima), when certain domesticated races are crossed together. As is well known, Darwin made use of this as an argument for regarding all the domesticated va- rieties as having arisen from the same wild species. The original experiment is somewhat complicated, and is shown in the accompanying scheme. Essentially it lay in 66 MENDELISM CHAP. following the results flowing from crosses between blacks and whites. Experiments recently made by Staples- Browne have shown that this case of reversion also can be readily interpreted in Mendelian terms. In these ex- Black Barb x White Fantail Black Barb x Spot l I I Dark X Dark Among the offspring one very similar to the wild blue rock. periments the cross was made between black barbs and white fantails. The FI birds were all black with some white splashes, evidently due to a separate factor intro- duced by the fan tail. On breeding these blacks to- gether they gave an F 2 generation, consisting of blacks Black Y White Barb i Fantail Black Y Black (White Splashed) (White Splashed) Black Black Blue Blue White White Splashed) (White Splashed) ~~$T IsP (4) (with or without white splashes) , blues (with or without white splashes), and whites in the ratio 9:3:4. The factors concerned are colour (C), in the absence of 1 This is an almost white bird, the colour being confined to the tail and the characteristic spot on the head. VI REVERSION 67 which a bird is white, and a black modifier (), in the absence of which a coloured bird is blue/ The original black barb contained "both of these factors, being in constitution CCBB. The fantail, however, contained neither, and was con- stitutionally ccbb. The FI birds produced by crossing were in constitution CcBb, and being heterozy- gous for two factors produced in equal numbers the four sorts of gametes CB, Cb, cB, cb. The results of two such series of gametes being brought together are shown in the usual way in Fig. ii. A blue is a bird containing the colour factor but lacking the black modifier, i.e. of the constitution CCbb, or Ccbb, and such birds as the figure shows appear in the F 2 generation on the average three times out of sixteen. Reversion here comes about in F 2 , when the redistribution of the factors leads to the formation of zygotes containing one of the two factors but not the other. FIG. ii. Diagram to illustrate the appearance of the rever- sionary blue pigeon in F 2 from the cross of black with white. CHAPTER VII DOMINANCE IN the cases which we have hitherto considered the presence of a factor produces its full effect whether it is introduced by both of the gametes which go to form the zygote, or by one of them alone. The heterozygous tall pea or the heterozygous rose-combed fowl cannot be dis- tinguished from the homozygous form by mere inspection, however close. Breeding tests alone can decide which is the heterozygous and which the homozygous form. Though this is true for the majority of characters yet investigated, there are cases known in which the hetero- zygous form differs in appearance from either parent. Among plants such a case has been met with in the prim- ula. The ordinary Chinese primula (P .' sinensis) (Fig. 12) has large rather wavy petals much crenated at the edges. In the Star Primula (P. stellata) the flowers are much smaller, while the petals are flat and present only a terminal notch instead of the numerous crenations of P. sinensis. The heterozygote produced by crossing these forms is intermediate in size and appearance. When self- fertilised such plants behave in simple Mendelian fashion, 68 CHAP. VII DOMINANCE giving a generation consisting of sinensis, intermediates, and stellata in the ratio 1:2:1. Subsequent breeding from these plants showed that both the sinensis and stel- lata which appeared in the F 2 generation bred true, while OOOO FIG. 12. Primula flowers to illustrate the intermediate nature of the F t flower when sinensis is crossed with stellata. the intermediates always gave all three forms again in the same proportion. But though there is no dominance of the character of either parent in such a case as this, the Mendelian principle of segregation could hardly have a better illustration. 70 MENDELISM CHAP. - Among birds a case of similar nature is that of the Blue Andalusian fowl. Fanciers have long recognised the Sinensis X Stellata I Intermediate F 1 Sinensis Inter. Inter. Stellata F 2 Sinensis sin. int. int. StelL Stellata F 3 Sinensis Stellata- F 4 difficulty of getting this variety to breed true. Of a slaty blue colour itself with darker hackles and with black lacing on the feathers of the breast, it always throws Blue X Blue Black Blue X Blue White I _ Black Black Blue Blue White White Black X White Blue (all) "wasters" of two kinds, viz. blacks, and whites splashed with black. Careful breeding from the blues shows that the three sorts are always produced in the same definite vii DOMINANCE 71 proportions, viz., one black, two blues, one splashed white. This at once suggests that the black and the splashed white are the two homozygous forms, and that the blues are heterozygous, i.e., producing equal numbers of "black" and "white splashed" gametes. The view was tested by breeding the " wasters" together black with black, and splashed white with splashed white - and it was found that each bred true to its respective type. But when the black and the splashed white were crossed they gave, as was expected, nothing but blues. In other words, we have the seeming paradox of the black and the splashed white producing twice as many blues as do the blues when bred together. The black and the splashed white " wasters" are in reality the pure breeds, while the "pure" Blue Andalusian is a mongrel which no amount of selection will ever be able to fix. In such cases as this it is obvious that we cannot speak of dominance. And with the disappearance of this phenomenon we lose one criterion for determining which of the two .parent forms possesses the additional factor. Are we, for example, to regard the black Andalusian as a splashed white to which has been added a double dose of a colour-intensifying factor, or are we to consider the white splashed bird as a black which is unable to show its true pigmentation owing to the possession of some in- hibiting factor which prevents the manifestation of the black. Either interpretation fits the facts equally well, 72 MENDELISM CHAP. and until further experiments have been devised and car- ried out it is not possible to decide which is the correct view. Besides these comparatively rare cases where the heterozygote cannot be said to bear a closer resemblance to one parent more than to the other, there are cases in which it is often possible to draw a visible distinction be- tween the heterozygote and the pure dominant. There are certain white breeds of poultry, notably the White Leghorn, in which the white behaves as a dominant to colour. But the heterozygous whites made by crossing the dominant white birds with a pure coloured form (such as the Brown Leghorn) almost invariably show a few col- oured feathers or ''ticks" in their plumage. The domi- nance of white is not quite complete, and renders it pos- sible to distinguish the pure from the impure dominant without recourse to breeding experiments. This case of the dominant white fowl opens up another interesting problem in connection with dominance. By accepting the " Presence and Absence " hypothesis we are committed to the view that the dominant form possesses an extra factor as compared with the recessive. The natural way of looking at this case of the fowl is to regard white as the absence of colour. But were this so, colour should be dominant to white, which is not the case. We are therefore forced to suppose that the absence of colour in this instance is due to the presence of a factor whose vii DOMINANCE 73 property is to inhibit the production of colour in what would otherwise be a pure coloured bird. On this view the dominant white fowl is a coloured bird plus a factor which inhibits the development of the colour. The view can be put to the test of experiment. We have already seen that there are other white fowls in which white is recessive to colour, and that the whiteness of such birds is due to the fact that they lack a factor for the develop- ment of colour. If we denote this factor by C and our postulated inhibitor factor in the dominant white bird by /, then we must write the constitution of the recessive white as ccii, and the dominant white as CCII. We may now work out the results we ought to obtain when a cross is made between these two pure white breeds. The con- stitution of the FI bird must be Ccii. Such birds being heterozygous for the inhibitor factor, should be whites showing some coloured " ticks.' 7 Being heterozygous for both of the two factors C and /, they will produce in equal numbers the four different sorts of gametes C/, Ci, ci, ci. The result of bringing two such similar series of gametes together is shown in Fig. 13. Out of the sixteen squares, twelve contain / ; these will be white birds either with or without a few coloured ticks. Three contain C but not / : these must be coloured birds. One contains neither C nor / ; this must be a white. From such a mating we ought, therefore, to obtain both white and coloured birds in the ratio 13 : 3. The results thus theoretically 74 MENDELISM CHAP. CI CI CI Ci CI cl CI ci Ci CI Ci ci ci CI ci Ci ci ci Cl CI Cl cl deduced were found to accord with the actual facts of experiment. The FI birds were all " ticked " whites, and in the F 2 generation came white and coloured birds in the expected ratio. There seems, therefore, little reason to doubt that the dominant white is a coloured bird in which the absence of colour is due to the action of a colour- inhibiting factor, though as to the nature of that factor we can at present make no surmise. It is probable that other facts, which at first sight do not appear to be in agreement with the " Presence and Absence " hypothesis, will eventually be brought into line through the action of inhibitor factors. Such a case, for instance, is that of bearded and beardless wheats. Though the beard is obviously the additional character, the bearded condition is recessive to the beardless. Prob- ably we ought to regard the beardless as a bearded wheat in which there is an inhibitor that stops the beard from growing. It is not unlikely that as time goes on we shall FIG. 13. Diagram to illustrate the nature of the F 2 generation from the cross between dominant white and recessive white fowls, VII DOMINANCE 75 find many more such cases of the action of inhibitor factors, and we must be prepared to find that the same visible effect may be produced either by the addition or FIG. 14. Ears of beardless and bearded wheat, The beardless condition is dominant to the bearded. by the omission of a factor. The dominant and recessive white poultry are indistinguishable in appearance. Yet the one contains a factor more and the other a factor less than the coloured bird. 76 MENDELISM CHAP. A phenomenon sometimes termed irregularity of domi- nance has been investigated in a few cases. In certain breeds of poultry such as Dorkings there occurs an extra toe directed backwards like the hallux (cf. Fig. 15). In some families this character behaves as an ordinary dominant to the normal, giving the expected 3 : i ratio in F 2 . But in other families similarly bred the pro- portions of birds with and without the extra toe appear to be unusual. It has been shown that in such a family some of the birds without the extra toe may nevertheless transmit the peculiarity when mated with birds be- longing to strains in which the extra toe never occurs. Though the external appearance of the bird generally affords some indication of the nature of the gametes which it is carrying, this is not always the case. Nevertheless we have reason to suppose that the character segregates in the gametes, though the nature of these can- not always be decided from the appearance of the bird which bears them. There are cases in which an apparent .irregularity of dominance has been shown to depend upon another character, as in the experiments with sheep carried out by Professor Wood. In these experiments two breeds were crossed, of which one, the Dorset, is horned in both sexes, while the other, the Suffolk, is without horns in either sex. Whichever way the cross was made the resulting FI generation was similar; the rams were horned, and vii DOMINANCE 77 the ewes were hornless. In the F 2 generation raised from these FI animals both horned and hornless types ap- peared in both sexes but in very different proportions. FIG. 15. Fowls' feet. On the right a normal and on the left one with an extra toe. While the horned rams were about three times as nu- merous as the hornless, this relation was reversed among the females, in which the horned formed only about one- quarter of the total. The simplest explanation of this interesting case is to suppose that the dominance of the horned character depends upon the sex of the animal that it is dominant in the male but recessive in the female. A pretty experiment was devised for putting this view to the test. If it is true, equal numbers of gametes with and without the horned factor must be produced by the F 1 ewes, while the factor should be lack- ing in all the gametes of the hornless F 2 rams. A horn- 78 MENDELISM CHAP, vn less ram, therefore, put to a flock of F! ewes should give rise to equal numbers of zygotes which are heterozygous for the horned character, and of zygotes in which it is Dorset* Suffolk Suffolk Dorset Ram Ewe Ram Ewe I X Q rfxf -F, ? 9 FIG. 16. Scheme to illustrate the inheritance of horns in sheep. Heterozygous males shown dark with a white spot, heterozygous females light with a dark spot in the centre. completely absent. And since the heterozygous males are horned, while the heterozgyous females are hornless, we should expect from this mating equal numbers of horned and hornless rams, but only hornless ewes. The result of the experiment confirmed this expectation. Of the ram lambs 9 were horned and 8 were hornless, while all the 1 1 ewe lambs were completely destitute of horns. - CHAPTER VIII WILD FORMS AND DOMESTIC VARIETIES IN discussing the phenomena of reversion we have seen that in most cases such reversion occurs when the two varieties which are crossed each contain certain factors lacking in the other, of which the full complement is necessary for the production of the reversionary wild form. This at once suggests the idea that the various domestic forms of animals and plants have arisen by the omission from time to time of this factor or of that. In some cases we have clear evidence that this is the most natural interpretation of the relation between the culti- vated and the wild forms. Probably the species in which it is most evident is the sweet pea (Lathyrus odora- tus). We have already seen reason to suppose that as regards certain structural features the Bush variety is a wild lacking the factor for the procumbent habit, that the Cupid is a wild without the factor for the long inter- node, and that the Bush Cupid is a wild minus both these factors. Nor is the evidence less clear for the many colour varieties. In illustration we may consider in more de- tail a case in which the cross between two whites resulted 79 8o MENDELISM CHAP. in a complete reversion to the purple colour characteris- tic of the wild Sicilian form (PL IV.)- In this particular instance subsequent breeding from the purples resulted in the production of six different colour forms in addi- tion to whites. The proportion of the coloured forms to the whites was 9 : 7 (cf. p. 44), but it is with the relation of the six coloured forms that we are concerned here. Of these six forms three were purples and three were reds. The three purple forms were (i) the wild bicolor purple with blue wings known in cultivation as the Purple Invincible (PL IV., 4); (2) a deep purple with purple wings (PL IV., 5) ; and (3) a very dilute purple known as the Pico tee (PL IV., 6). Corresponding to these three purple forms were three reds: (i) a bicolor red known as Painted Lady (PL IV., 7) ; (2) a deep red with red wings known as Miss Hunt (PL IV., 8) ; and (3) a very pale red which we have termed Tinged White 1 (PL IV., 9). In the F 2 generation the total number of purples bore to the total number of reds the ratio 3:1, and this ratio was maintained for each of the corresponding classes. Purple, therefore, is dominant to red, and each of the three classes of red differs from its corresponding purple in not possess- ing the blue factor (B) which turns it into purple. Again, 1 The reader who searches florists' catalogues for these varieties will probably experience disappointment. The sweet pea has been much "improved" in the past few years, and it is unlikely that the modern seedsman would list such unfashionable forms. PLATE IV. i, 2, Emily Henderson; 3, F! reversionary Purple; 4-10, Various F. forms : 4, Purple ; 5, Deep Purple ; 6, Picotee ; 7, Painted Lady 8, Miss Hunt ; 9, Tinged White ; 10, White. vm WILD AND DOMESTIC VARIETIES 81 the proportion in which the three classes of purples ap- peared was 9 bicolors, 3 deep purples, 4 picotees. We are, therefore, concerned here with the operation of two factors : (i) a light wing factor, which renders the bicolor dominant to the dark winged form; and (2) a factor for intense colour, which occurs in the bicolor and in the deep purple, but is lacking in the dilute picotee. And here it should be mentioned that these conclusions rest upon an exhaustive set of experiments involving the breeding of many thousands of plants. In this cross, therefore, we are concerned with the presence or absence of five factors, which we may denote as follows : A colour base, R. A colour developer, C. A purple factor, B. A light wing factor, L. A factor for intense colour, 7. On this notation our six coloured forms are : (1) Purple bicolor . . . CRBLI. 1 (2) Deep purple . . . CRBII. (3) Picotee .... CRBLi or CRBH. (4) Red bicolor (= Painted Lady) CRbLI. (5) Deep red ( = Miss Hunt) . CRbll. (6) Tinged white . . . CRbLi or CRbli. It will be noticed in this series that the various coloured 1 It is to be understood that wherever a given factor is present the plant may be homozygous or heterozygous for it without alteration in its colour. G 82 MENDELISM CHAP. forms can be expressed by the omission of one or more factors from the purple bicolor of the wild type. With the complete omission of each factor a new colour type results, and it is difficult to resist the inference that the various cultivated forms of the sweet pea have arisen from the wild by some process of this kind. Such a view tallies with what we know of the behaviour of the wild form when crossed by any of the garden varieties. Wherever such crossing has been made the form of the hybrid has been that of the wild, thus supporting the view that the wild contains a complete set of all the dif- ferentiating factors which are to be found in the sweet pea. Moreover, this view is in harmony with such historical evidence as is to be gleaned from botanical literature, and from old seedsmen's catalogues. The wild sweet pea first reached England in 1699, having been sent from Sicily by the monk Franciscus Cupani as a present to a certain Dr. Uvedale in the county of Middlesex. Some- what later we hear of two new varieties, the red bicolor, or Painted Lady, and the white, each of which may be regarded as having "sported" from the wild purple by the omission of the purple factor, or of one of the two colour factors. In 1793 we find a seedsman offering also what he called black and scarlet varieties. ' It is probable that these were our deep purple and Miss Hunt varieties, and that somewhere about this time the factor for the viii WILD AND DOMESTIC VARIETIES 83 light wing (L) was dropped out in certain plants. In 1860 we have evidence that the pale purple or Pico tee, and with it doubtless the Tinged White, had come into existence. This time it was the factor for intense colour which had dropped out. And so the story goes on until the present day, and it is now possible to express by the same simple method the relation of the modern shades, of purple and reds, of blues and pinks, of hooded and wavy standards, to one another and to the original wild form. The constitution of many of these has now been worked out, and to-day it would be a simple though per- haps tedious task to denote all the different varieties by a series of letters indicating the factors which they con- tain, instead of by the present system of calling them after kings and queens, and famous generals, and ladies more or less well known. From what we know of the history of the various strains of sweet peas one thing stands out clearly. The new character does not arise from a pre-existing variety by any process of gradual selection, conscious or otherwise. It turns up suddenly complete in itself, and thereafter it can be associated by crossing with other existing characters to produce a gamut of new varieties. If, for example, the character of hooding in the standard (cf. PL II., 7) suddenly turned up in such a family as that shown on Plate IV. we should be able to get a hooded form corresponding to each of the forms with the erect 84 MENDELISM CHAP. standard; in other words, the arrival of the new form would give us the possibility of fourteen varieties instead of seven. As we know, the hooded character already exists. It is recessive to the erect standard, and we have reason to suppose that it arose as a sudden sport by the omission of the factor in whose presence the standard assumes the erect shape characteristic of the wild flower. It is largely by keeping his eyes open and seizing upon such sports for crossing purposes that the horticulturist " improves" the plants with which he deals. How these sports or mutations come about we can now sur- mise. They must owe their origin to a disturbance in the processes of cell division through which the gametes originate. At some stage or other the normal equal distribution of the various factors is upset, and some of the gametes receive a factor less than others. From the union of two such gametes, provided that they are still capable of fertilisation, comes the zygote which in course of growth develops the new character. Why these mutations arise : what leads to the sur- mised unequal division of the gametes: of this we know practically nothing. Nor until we can induce the pro- duction of mutations at will are we likely to understand the conditions which govern their formation. Never- theless there are already hints scattered about the recent literature of experimental biology which lead us to hope that we may know more of these matters in the future. vm WILD AND DOMESTIC VARIETIES 85 In respect of the evolution of its now multitudinous varieties, the story of the sweet pea is clear and straight- forward. These have all arisen from the wild by a pro- cess of continuous loss. Everything was there in the beginning, and as the wild plant parted with factor after factor there came into being the long series of derived forms. Exquisite as are the results of civilization, it is by the degradation of the wild that they have been brought about. How far are we justified in regarding this as a picture of the manner in which evolution works ? There are certainly other species in which we must suppose that this is the way that the various domesti- cated forms have arisen. Such, for example, is the case in the rabbit, where most of the colour varieties are re- cessive to the wild agouti form. Such also is the case in the rat, where the black and albino varieties and the various pattern forms are also recessive to the wild agouti type. And with the exception of a certain yellow variety to which we shall refer later, such is also the case with the many fancy varieties of mice. Nevertheless there are other cases in which we must suppose evolution to have proceeded by the interpola- tion of characters. In discussing reversion on crossing, we have already seen that this may not occur until the F 2 generation, as, for example, in the instance of the fowls' combs (cf. p. 65). The reversion to the single comb occurred as the result of the removal of the two factors 86 MENDELISM CHAP. for rose and pea. These two domesticated varieties must be regarded as each possessing an additional factor in comparison with the wild single-combed bird. Dur- ing the evolution of the fowl, these two factors must be conceived of as having been interpolated in some way. And the same holds good for the inhibitory factor on which, as we have seen, the dominant white character of certain poultry depends. In pigeons, too, if we regard the blue rock as the ancestor of the domesticated breeds, we must suppose that an additional melanic factor has arisen at some stage. For we have already seen that black is dominant to blue, and the characters of FI, together with the greater number of blacks than blues in F 2 , negatives the possibility that we are here dealing with an inhibitory factor. The hornless or polled condition of cattle, again, is dominant to the horned condition, and if, as seems reasonable, we regard the original ancestors of domestic cattle as having been horned, we have here again the interpolation of an inhibitory factor-somewhere in the course of evolution. On the whole, therefore, we must be prepared to admit that the evolution of domestic varieties may come about by a process of addition of factors in some cases and of subtraction in others. It may be that what we term additional factors fall into distinct categories from the rest. So far, experiment seems to show that they are either of the nature of melanic factors, or of inhibitory vm WILD AND DOMESTIC VARIETIES 87 factors, or of reduplication factors as in the case of the fowls' combs. But while the data remain so scanty, speculation in these matters is too hazardous to be profitable. CHAPTER IX REPULSION AND COUPLING OF FACTORS ALTHOUGH different factors may act together to pro- duce specific results in the zygote through their inter- action, yet in all the cases we have hitherto considered the heredity of each of the different factors is entirely independent. The interaction of the factors affects the characters of the zygote, but makes no difference to the distribution of the separate factors, which is always in strict accordance with the ordinary Mendelian scheme. Each factor in this respect behaves as though the other were not present. A few cases have been worked out in which the dis- tribution of the different factors to the gametes is af- fected by their simultaneous presence in -.the zygote. And the influence which they are able to exert upon one another in such cases is of two kinds. They may repel one another, refusing, as it were, to enter into the same zygote, or they may attract one another, and, becoming linked together, pass into the same gamete, as it were by preference. For the moment we may consider these two sets of phenomena apart. 88 CHAP, ix REPULSION AND COUPLING 89 One of the best illustrations of repulsion between factors occurs in the sweet pea. We have already seen that the loss of the blue or purple factor (B) from the wild bicolor results in the formation of the red bicolor known as Painted Lady (PL IV., 7). Further, we have seen that the hooded standard is recessive to the ordinary erect standard. The omission of the factor for the erect standard (E) from the purple bicolor (PL II., 5) results in a hooded purple known as Duke of Westminster (PL II., 7). And here it should be mentioned that in the corresponding hooded forms the difference in colour be- tween the wings and standard is not nearly so marked as in the forms with the erect standard, but the difference in structure appears to affect the colour, which becomes nearly uniform. This may be readily seen by comparing the picture of the purple bicolor on Plate II. with that of the Duke of Westminster flower. Now when a Duke of Westminster is mated with a Painted Lady the factor for erect standard (E) is brought in by the red, and that for blue (B) by the Duke, and the offspring are consequently all purple bicolors. Purples so formed are all heterozygous for these two factors, and were the case a simple one, such as those which have already been discussed, we should expect the F 2 genera- tion to consist of the four forms : erect purple, hooded purple, erect red, and hooded red in the ratio 9:3: 3:1. Such, however, is not the case. The F 2 generation go MENDELISM CHAP. actually consists of only three forms, viz. erect red, erect purple, and hooded purple, and the ratio in which these three forms occur is 1:2:1. No hooded red has been known to occur in such a family. Moreover further breeding shows that while the erect reds and the hooded purples always breed true, the erect purples Painted Lady X Duke of Westminster (erect red) (hooded purple) Purple Invincible (srect purple) Painted Purple Invincible Duke of Lady Westminster (I) (2) (I) in such families never breed true, but always behave like the original FI plant, giving the three forms again in the ratio 1:2:1. Yet we know that there is no diffi- culty in getting purple bicolors to breed true. from other families ; and we know also that hooded red, sweet peas exist in other strains. On the assumption that there exists a repulsion be- tween the factors for erect standard and blue in a plant which is heterozygous for both, this peculiar case receives a simple explanation. The constitutions of the erect red and the hooded purple are EEbb and eeBB respectively and that of the FI erect purple is EeBb. Now let us suppose that in such a zygote there exists a repulsion IX REPULSION AND COUPLING between E and B, such that when the plant forms gametes these two factors will not go into the same gamete. On this view it can only form two kinds of gametes, viz. Eb and eB, and these, of course, will be formed in equal numbers. Such a plant on self-fertilisation must give EEbb eeBB Parents 9 gametes of 1 Eb Eb eB eB - EEbb EeBb EeBb eeBB 111 v ^ Eb" eB Eb eB . generation the zygotic series EEbb + 2 EeBb + eeBB, i.e. i erect red, 2 erect purples, and i hooded purple. And because the erect reds and the hooded purples are respectively homo- zygous for E and B, they must thenceforward breed true. The erect purples, on the other hand, being always formed by the union of a gamete Eb with a gamete eB, are always heterozygous for both of these factors. They can, consequently, never breed true, but must always give erect reds, erect purples, and hooded purples in the ratio 1:2:1. The experimental facts are readily ex- plained on the assumption of repulsion between the two 9 2 MENDELISM CHAP. factors B and E during the formation of the gametes in a plant which is heterozygous for both. Other similar cases of factorial repulsion have been demonstrated in the sweet pea, and two of these are also concerned with the two factors with which we have just been dealing. Two distinct varieties of pollen grains occur in this species, viz. the ordinary oblong form and a rather smaller rounded grain. The former is dominant to the latter. 1 When a cross is made between a purple with round pollen and a red with long pollen the FI plant is a long pollened purple. But the F 2 generation con- sists of purples with round pollen, purples with long pollen, and reds with long pollen in the ratio 1:2:1. No red with round pollen appears in F 2 owing to repul- sion between the factors for purple (B) and for long pollen (L). Similarly plants produced by crossing a red hooded long with a red round having an erect standard give in FI long pollened reds with an erect standard, and these in F 2 produce the three types, round pollened erect, long pollened erect, and long pollened hooded, in the ratio 1:2:1. The repulsion here is between the long pollen factor (L) and the factor for the erect standard 1 It should be mentioned that as the shape of the pollen coat, like that of the seed coat, is a maternal character, all the grains of any given plant are either long or else round. The two kinds do not occur together on the same plant. ix REPULSION AND COUPLING 93 Yet another similar case is known in which we are con- cerned with quite different factors. In some sweet peas the axils whence the leaves and flower stalks spring from the main stem are of a deep red colour. In others they are green. The dark pigmented axil is dominant to the light one. Again, in some sweet peas the anthers are sterile, setting no pollen, and this condition is recessive to the ordinary fertile condition. When a sterile plant with a dark axil is crossed by a fertile plant with a light axil, the FI plants are all fertile with dark axils. But such plants in F 2 give fer tiles with light axils, fer tiles with dark axils, and steriles with dark axils in the ratio 1:2:1. No light axilled steriles appear from such a cross owing to the repulsion between the factor for dark axil (D) and that for the fertile anther (F). These four cases have already been found in the sweet pea, and similar phenomena have been met with by Gregory in primulas. To certain seemingly analogous cases in animals where sex is concerned we shall refer later. Now all of these four cases present a common feature which probably has not escaped the attention of the reader. In all of them the original cross was such as to introduce one of the repelling factors with each of the two parents. If we denote our two factors by A and B, the crosses have always been of the nature AAbbxaaBB. Let us now consider what happens when both of the 94 MENDELISM CHAP. factors, which in these cases repel one another, are in- troduced by one of the parents, and neither by the other parent. And in particular we will take the case in which we are concerned with purple and red flower colour, and with long and round pollen, i.e. with the factors B and L. When a purple long (BBLL) is crossed with a red round (bbll) the FI (BbLl) is a purple with long pollen, identical in appearance with that produced by crossing the long pollened red with the round pollened purple. But the nature of the F 2 generation is in some respects very dif- ferent. The ratio of purples to reds and of longs to rounds is in each case 3 : i, as before. But instead of an association between the red and the long pollen characters the reverse is the case. The long pollen character is now associated with purple and the round pollen with red. The association, however, is not quite complete, and the examination of a large quantity of similarly bred material shows that the purple longs are about twelve times as numerous as the purple rounds, while the red rounds are rather more than three times as many as the red longs. Now this peculiar result could be brought about if the gametic series produced by the FI plant consisted of jBL+iBl+ibL + jbl out of every 16 gametes. Fer- tilization between two such similar series of 16 gametes would result in 256 plants, of which 177 would be purple longs, 15 purple rounds, 15 red longs, and 49 red rounds a proportion of the four different kinds very close to ix REPULSION AN'D COUPLING 95 that actually found by experiment. It will be noticed that in the whole family the purples are to the reds as 3:1, and the longs are also three times as numerous as the rounds. The peculiarity of the case lies in the dis- tribution of these two characters with regard to one an- other. In some way or other the factors for blue and for long pollen become linked together in the cell divisions that give rise to the gametes, but the linking is not com- plete. This holds good for all the four cases in which repulsion between the factors occurs when one of the two factors is introduced by each of the parents. When both of the factors are brought into the cross by the same parent we get coupling between them instead of repulsion. The phenomena of repulsion and coupling between separate factors are intimately related, though hitherto we have not been able to suggest why this should be so. Nor for the present can we suggest why certain factors should be linked together in the peculiar way that we have reason to suppose that they are during the process of the formation of the gametes. Nevertheless the phenomena are very definite, and it is not unlikely that a further study of them may throw important light on the architecture of the living cell. APPENDIX TO CHAPTER IX As it is possible that some readers may care, in spite of its com- plexity, to enter rather more fully into the peculiar phenomenon 96 MENDELISM CHAP. of the coupling of characters, I have brought together some further data in this Appendix. In the case we have already considered, where the factors for blue colour and long pollen are concerned, we have been led to suppose that the gametes produced by the hetero- zygous plant are of the nature 7 BL : i Bl: i bL : 7 bl. Such a series of ovules fertilised by a similar series of pollen grains will give a generation of the following composition : 49 BBLL + 7 BBLl + 7 BbLL + 49 BID, + 7 BBLl + 7 BbLL + BbLl + BbLl -f 49 BbLl 177 purple, long + BBll + 7 BUI 4- bbLL + 7 bbU + 49 bbtt + 7 BUI + 7 bbLl 15 purple, 15 red, round long and as this theoretical result fits closely with the actual figures obtained by experiment we have reason for supposing that the heterozygous plant produces a series of gametes in which the factors are coupled in this way. The intensity of the coupling, however, varies in different cases. Where we are dealing with another, viz. fertility (F) and the dark axil (/?), the experimental numbers accord with the view that the gametic series is here 1 5 FD : i Fd : i jD : 15 fd. The coupling is in this instance more intense. In the case of the erect standard (E) and blueness (B) the coupling is even more intense, and the experimental evidence available at present points to the gametic series here being 63 Eb : i EB : i eB:6$ eb. There is evidence also for supposing that the intensity of the coupling may vary in different families for the same pair of factors. The coupling between blue and long pollen is generally on the 7 : i : i : 7 IX REPULSION AND COUPLING 97 basis, but in some cases it may be on the 15 : i : i : 15 basis. But though the intensity of the coupling may vary it varies in an orderly way. If A and B are the two factors concerned, the results ob- tained in F 2 are explicable on the assumption that the ratio of the four sorts of gametes produced is a term of the series sab 7 ab $ ab, etc., etc. In such a series the number of gametes containing A is equal to the number lacking A , and the same is true for B. Consequently the number of zygotes formed containing A is three times as great as the number of zygotes which do not contain A ; and similarly for B. The proportion of dominants to recessives in each case is 3 : i. It is only in the distribution of the characters with relation to one another that these cases differ from a simple Mendelian case. As the study of these series presents another feature of some interest, we may consider it in a little more detail. In the accom- panying table are set out the results produced by these different series of gametes. The series marked by an asterisk have already been demonstrated experimentally. The first term in the series, 13 iS Distribution of ^ 1R that f the female ' fm /fertilisations But Qn flfo v j ew & further SUp- position is necessary. If each of the two kinds of spermatozoa were capable of fertilis- ing each of the two kinds of ova, we should get individuals of the constitution MmFf and mmff, as well as the normal males and females, Mmff and Ffmm. As the facts of or- dinary bisexual reproduction afford us no grounds for assuming the existence of these two classes of individuals, whatever they may be, we must suppose that fertilisation is productive only between the spermatozoa carrying M and the ova without F, or between the spermatozoa xi SEX 117 without M and the ova containing F. In other words we must on this view suppose that fertilisations between certain forms of gametes, even if they can occur, are in- capable of giving rise to zygotes with the capacity for further development. If we admit this supposition, the scheme just given will cover such cases as those of the currant moth and the fowl, equally as well as that of the pomace fly. In the former there is repulsion between either the grossulariata factor and F, or else between the pigment inhibitor factor and F, while in the latter there is repulsion between the factor for red eye and M . Whatever the merits or demerits of such a scheme it certainly does offer an explanation of a peculiar form of sex limited inheritance in man. * * It has long been a matter of com- | mon knowledge that colour-blind- 9 # ? x <* ness is much more common among 7"^ ' ' ' ' men than among women, and also FIG ^ that Unaffected WOmen Can tranS- Scheme to illustrate the probable . . . . . .. mode of inheritance of colour- mit it tO their SOnS. At first blindness. The dark signs re- ... . , M present affected individuals. Sight the Case IS not Unlike A black dot in the centre de- , , notes an unaffected female who that OI the Sheep, Where the i s capable of transmitting the . . , . , , condition to her sons. horned character is apparently dominant in the male but recessive in the female. The hypothesis that the colour-blind condition is due to the presence of an extra factor as compared with the normal, and that a single dose of it will produce n8 MENDELISM CHAP. colour-blindness in the male but not in the female, will cover a good many of the observed facts (cf. Fig. 26). Moreover, it serves to explain the remarkable fact that all the sons of colour-blind women are also colour-blind. For a woman cannot be colour-blind unless she is homozygous for the colour-blind factor, in which case all her children must get a single dose of it even if she marries a normal male. And this is sufficient to produce colour-blindness in the male, though not in the female. But there is one notable difference in this case as com- pared with that of the sheep. When crossed with pure hornless ewes the heterozygous horned ram transmits the horned character to half his male offspring (cf. p. 71). But the heterozygous colour-blind man does not behave altogether like a sheep, for he apparently does not trans- mit the colour-blind condition to any of his male offspring. If, however, we suppose that the colour-blind factor is repelled by the factor for maleness, the amended scheme will cover the observed facts. For, denoting the colour- blind factor by X, the gametes produced by the colour- blind male are of two sorts only, viz. Mfx and m/X. If he marries a normal woman (Ffmmxx) , the spermatozoa Mfx unite with ova fmx to give normal males, while the spermatozoa mfX unite with ova Fmx to give females which are heterozygous for the colourblind factor. These daughters are themselves normal, but transmit the condition to about half their sons. xi SEX 119 The attempt to discover a simple explanation of the nature of sex has led us to assume that certain combina- tions between gametes are incapable of giving rise to zy- gotes which can develop further. In the various cases hitherto considered there is no reason to suppose that anything of the sort occurs, or that the different gametes are otherwise than completely fertile one with another. One peculiar case, however, has been known for several years in which some of the gametes are apparently incapa- ble of uniting to produce offspring. Yellow in the mouse is dominant to agouti, but hitherto a homozygous yellow has never been met with. The yellows from families where only yellows and agoutis occur produce, when bred together, yellows and agoutis in the ratio 2:1. If it were an ordinary Mendelian case the ratio should be 3 : i , and one out of every three yellows so bred should be homo- zygous and give only yellows when crossed with agouti. But Cuenot and others have shown that all of the yellows are heterozygous, and when crossed with agoutis give both yellows and agoutis. We are led, therefore, to sup- pose that an ovum carrying the yellow factor is unpro- ductive if fertilised by a spermatozoon which also bears this factor. In this way alone does it seem possible to explain the deficiency of yellows and the absence of homo- zygous ones in the families arising from the mating of yellows together. At present, however, it remains the only definite instance among animals in which we have 120 MENDELISM CHAP. grounds for assuming that anything in the nature of unproductive fertilisation takes place. 1 If we turn from animals to plants we find a more com- plicated state of affairs. Generally speaking, the higher plants are hermaphrodite, both ovules and pollen grains occurring on the same flower. Some plants, however, like most animals, are of separate sexes, a single plant bearing only male or female flowers. In other plants the separate flowers are either male or female, though both are borne on the same individual. In others, again, the conditions are even more complex, for the same plant may bear flowers of three kinds, viz. male, female, and hermaphrodite. Or it may be that these three forms occur in the same species but in different individuals -female and hermaphrodites in one species; males, females, and hermaphrodites in another. One case, however, must be mentioned as it suggests a possibility which we have not hitherto encountered. In the com- mon English bryony (Bryonia dioica} the sexes are sep- arate, some plants having only male and others only fe- male flowers. In another European species, B. alba, both male and female flowers occur on the same plant. Correns crossed these two species reciprocally, and also fertilised B. dioica by its own male with the following results : 1 For the most recent discussion of this peculiar case the reader is re- ferred to Professor Castle's paper in Science, December 16, 1910. xi SEX 121 dioica x dioica $ gave 9 9 and $ $ x alba $ ,, $ cross but a few generations back, and it is possible that they often oust the older kinds not because they started as some- thing intrinsically better, but because the latter had gradually deteriorated through continuous self-fertilisa- tion. Most breeders are fully alive to the beneficial re- sults of a cross so far as vigour is concerned, but they often hesitate to embark upon it owing to what was held 1 64 MENDELISM CHAP. to be the inevitably lengthy and laborious business of re- covering the original variety and refixing it, even if in the process it was not altogether lost. That danger Mendel- ism has removed, and we now know that by working on these lines it is possible in three or four generations to recover the original variety in a fixed state with all the superadded vigour that follows from a cross. Nor is the problem one that concerns self -fertilised plants only. Plants that are reproduced asexually often appear to deteriorate after a few generations unless a sex- ual generation is introduced. New varieties of potato, for example, are frequently put upon the market, and their excellent qualities give them a considerable vogue. Much is expected of them, but time after time they de- teriorate in a disappointing way and are lost to sight. It is not improbable that we are here concerned with a case in which the plants lose their vigour after a few asexual generations of reproduction from tubers, and can only recover it with the stimulus that results from the inter- polation of a sexual generation. Unfortunately this generally means that the variety is lost, for owing to the haphazard way in which new kinds of potatoes are repro- duced it is probable that most cultivated varieties are complex heterozygotes. Were the potato plant subjected to careful analysis and the various factors determined upon which its variations depend, we should be in a posi- tion to remake continually any good potato without xiv ECONOMICAL 165 running the risk of losing it altogether, as is now so often the case. The application of Mendelian principles is likely to prove of more immediate service for plants than animals, for owing to the large numbers which can be rapidly raised from a single individual and the prevalence of self- fertilisation, the process of analysis is greatly simplified. Even apart from the circumstance that the two sexes may sometimes differ in their powers of transmission, the mere fact of their separation renders the analysis of their properties more difficult. And as the constitution of the individual is determined by the nature and quality of its offspring, it is not easy to obtain this knowledge where the offspring, as in most animals, are relatively few. Still, as has been abundantly shown, the same principles hold good here also, and there is no reason why the pro- cess of analysis, though more troublesome, should not be effectively carried out. At the same time, it affords the breeder a rational basis for some familiar but puzzling phenomena. The fact, for instance, that certain characters often " skip a generation " is simply the effect of dominance in F! and the reappearance of the recessive character in the following generation. " Reversion" and " atavism," again, are phenomena which are no longer mysterious, but can be simply expressed in Mendelian terms as we have already suggested in Chap. VI. The occasional appearance of a sport in a supposedly pure strain is i66 MENDELISM CHAP. often due to the reappearance of a recessive character. Thus even in the most highly pedigreed strains of polled cattle such as the Aberdeen Angus, occasional individuals with horns appear. The polled character is dominant to the horned, and the occasional reappearance of the horned animal is due to the fact that some of the polled herd are heterozygous in this character. When two such indi- viduals are mated, the chances are i in 4 that the offspring will be horned. Though the heterozygous individuals may be indistinguishable in appearance from the pure domi- nant, they can be readily separated by the breeding test. For when crossed by the recessive, in this case horned ani- mals, the pure dominant gives only polled beasts, while the heterozygous individual gives equal numbers of polled and horned ones. In this particular instance it would probably be impracticable to test all the cows by crossing with a horned bull. For in each case it would be necessary to have several polled calves from each before they could with reasonable certainty be regarded as pure dominants. But to ensure that no horned calves should come, it is enough to use a bull which is pure for that character. This can easily be tested by crossing him with a dozen or so horned cows. If he gets no horned calves out of these he may be regarded as a pure dominant and thenceforward put to his own cows, whether horned or polled, with the certainty that all his calves will be polled. xiv ECONOMICAL 167 Or, again, suppose that a breeder has a chestnut mare and wishes to make certain of a bay foal from her. We know that bay is dominant to chestnut, and that if a homozygous bay stallion is used a bay foal must result. In his choice of a sire, therefore, the breeder must be guided by the previous record of the animal, and select one that has never given anything but bays when put to either bay or chestnut mares. In this way he will assure himself of a bay foal from his chestnut mare, whereas if the record of the sire shows that he has given chestnuts he will be heterozygous, and the chances of his getting a bay or a chestnut out of a chestnut mare are equal. It is not impossible that the breeder may be unwilling to test his animals by crossing them with a different breed through fear that their purity may be thereby impaired, and that the influence of the previous cross may show itself in succeeding generations. He might hesitate, for instance, to test his polled cows by crossing them with a horned bull for fear of getting horned calves when the cows were afterwards put to a polled bull of their own breed. The belief in the power of a sire to influence sub- sequent generations, or telegony as it is sometimes called, is not uncommon even to-day. Nevertheless, carefully conducted experiments by more than one competent observer have failed to elicit a single shred of unequiv- ocal evidence in favour of the view. Until we have evidence based upon experiments which are capable of i68 MENDELISM CHAP. repetition, we may safely ignore telegony as a factor in heredity. Heterozygous forms play a greater part in the breeding of animals than of plants, for many of the qualities sought after by the breeder are of this nature. Such is the blue of the Andalusian fowl, and, according to Professor Wil- son, the roan of the Shorthorn is similar, being the hetero- zygous form produced by mating red with white. The characters of certain breeds of canaries and pigeons again appear to depend upon their heterozygous nature. Such forms cannot, of course, ever be bred true, and where sev- eral factors are concerned they may when bred together produce but a small proportion of offspring like them- selves. As soon, however, as their constitution has been analysed and expressed in terms of Mendelian factors, pure strains can be built up which when crossed will give nothing but offspring of the desired heterozygous form. The points with which the breeder is concerned are often fine ones, not very evident except to the, practised eye. Between an ordinary Dutch rabbit and a winner, or between the comb of a Hamburgh that is fit to show and one that is not, the differences are not very apparent to the uninitiated. Whether Mendelism will assist the breeder in the production of these finer points is at present doubtful. It may be that these small differences are heritable, such as those that form the basis of Johann- sen's pure lines. In this case the breeder's outlook is xiv ECONOMICAL 169 hopeful. But it may be that the variations which he seeks to perpetuate are of the nature of fluctuations, de- pendent upon the earlier life conditions of the individual, and not upon the constitution of the gametes by which it was formed. If such is the case, he will get no help from the science of heredity, for we know of no evidence which might lead us to suppose that variations of this sort can ever become fixed and heritable. CHAPTER XV MAN THOUGH the interest attaching to heredity in man is more widespread than in other animals, it is far more difficult to obtain evidence that is both complete and ac- curate. The species is one in which the differentiating characters separating individual from individual are very numerous, while the number of the offspring is compara- tively few, and the generations are far between. For these reasons, even if it were possible, direct experimen- tal work with man would be likely to prove both tedious and expensive. There is, however, another method be- sides the direct one from which something can be learned. This consists in collecting all the evidence possible, ar- ranging it in the form of pedigrees, and comparing it with standard cases already worked out in animals and plants. In this way it has been possible to demonstrate in man the existence of several characters showing simple Men- delian inheritance. As few besides medical men have hitherto been concerned practically with heredity, such records as exist are, for the most part, records of deform- ity or of disease. So it happens that most of the pedi- 170 xv MAN 171 grees at present available deal with characters which are usually classed as abnormal. In some of these the in- heritance is clearly Mendelian. One of the cases which FIG. 32. Normal and brachydactylous hands placed together for comparison. (From Drink water.) has been most fully worked out is that of a deformity known as brachydactyly. In brachydactylous people the 172 MENDELISM CHAP. whole of the body is much stunted, and the fingers and toes appear to have two joints only instead of three (cf. Figs. 32 and 33). The inheritance of this peculiarity has been carefully investigated by Dr. Drinkwater, who col- lected all the data he was able to find among the members of a large family in which it occurred. The result is the FIG. 33. Radiograph of a brachydactylous hand. ->. pedigree shown on p. 173. It is assumed that all who are recorded as having offspring were married to normals. Examination of the pedigree brings out the facts (i) that all affected individuals have an affected parent ; (2) that none of the unaffected individuals, though sprung from the affected, ever have descendants who are affected, and (3) that in families where both affected and unaffected XV MAN 173 occur, the numbers of the two classes are, on the average, equal. (The sum of such families in the com- plete pedigree is thirty-nine affected and thirty-six nor- mals.) It is obvious that these are the conditions which are fulfilled in a simple Mendelian case, and there is nothing in this pedigree to contradict the assertion that brachydactyly, what- ever it may be due to, behaves as a simple dominant to the nor- mal form, i.e. that it ^ depends upon a factor which the normal does not contain. The re- cessive normals can- not transmit the affected condition whatever their an- 174 MENDELISM CHAP. cestry. Once free they are always free, and can marry other normals with full confidence that none of their children will show the deformity. The evidence available from pedigrees has revealed the simplest form of Mendelian inheritance in several human defects and diseases, among which may be men- tioned presenile cataract of the eyes, an abnormal form of skin thickening in the palms of the hands and soles of the feet, known as tylosis, and epidermolysis bullosa, a disease in which the skin rises up into numerous burst- ing blisters. Among the most interesting of all human pedigrees is, one recently built up by Mr. Nettleship from the records of a night-blind family living near Monpelier in the south of France. In night-blind people the retina is insensitive to light which falls below a certain intensity, and such people are consequently blind in failing daylight or in moonlight. As the Monpelier case had excited interest for some time, the records are unusually complete. They commence with a certain Jean Nougaret, who was born in 1637, and suffered from night-blindness, and they end for the present with children who are to-day but a few years of age. Particulars are known of over 2000 of the de- scendants of Jean Nougaret. Through ten generations and nearly three centuries the affection has behaved as a Mendelian dominant, and there is no sign that long- continued marriage with folk of normal vision has pro- duced any amelioration of the night-blind state. xv MAN 175 Besides cases such as these where a simple form of Mendelian inheritance is obviously indicated, there are others which are more difficult to read. Of some it may be said that on the whole the peculiarity behaves as though it were an ordinary dominant ; but that exceptions occur in which affected children are born to unaffected parents. It is not impossible that the condition may, like colour in the sweet pea, depend upon the presence or rf i ^9 9 9 9 9 -rf * 9 9 9 ^ 1 i ) i ill r~i i Hi FIG. 35- Pedigree of a haemophilic family. Affected (all males) represented by black, and normals of both sexes by light circles. (From Stahel.) absence of more than one factor. In none of these cases, however, are the data sufficient for determining with cer- tainty whether this is so or not. A group of cases of exceptional interest is that in which the incidence of disease is largely, if not absolutely, re- stricted to one sex, and so far as is hitherto known the burden is invariably borne by the male. In the inheri- tance of colour-blindness (p. 117) we have already dis- cussed an instance in which the defect is rare, though not 176 MENDELISM CHAP. unknown, in the female. Sex-limited inheritance of a similar nature is known for one or two ocular defects, and for several diseases of the nervous system. In the pe- culiarly male disease known as haemophilia the blood re- fuses to clot when shed, and there is nothing to prevent great loss from even a superficial scratch. In its general trend the inheritance of haemophilia is not unlike that of horns among sheep, and it is possible that we are here again dealing with a character which is dominant in one sex and recessive in the other. But the evidence so far collected points to a difference somewhere, for in haemo- philic families the affected males, instead of being equal in number to the unaffected, show a considerable prepon- derance. The unfortunate nature of the defect, however, forces us to rely for our interpretation almost entirely upon the families produced by the unaffected females who can transmit it. Our knowledge of the offspring of " bleeding "males is as yet far too scanty, and until it is improved, or until we can find some parallel case in ani- mals or plants, the precise scheme of inheritance for haemophilia must remain undecided. Though by far the greater part of the human evidence relates to abnormal or diseased conditions, a start has been made in obtaining pedigrees of normal characters. From the ease with which it can be observed, it was natural that eye-colour should be early selected as a subject of investigation, and the work of Hurst and others xv MAN 177 has clearly demonstrated the existence of one Mendelian factor in operation here. Eyes are of many colours, and the colour depends upon the pigment in the iris. Some eyes have pigment on both sides of the iris on the side that faces the retina as well as on the side that looks out upon the world. Other eyes have pigment on the retinal side only. To this class belong the blues and clear greys ; while the eyes with pigment in front of the iris also are brown, hazel, or green in various shades according to the amount of pigment present. In albino animals the pig- ment is entirely absent, and as the little blood-vessels are not obscured the iris takes on its characteristic pinkish- ^ red appearance. The condition in which pigment is , present in front of the iris is dominant to that in which .^A it is absent. Greens, browns, or hazels mated together may, if heterozygous, give the recessive blue, but no in- dividuals of the brown class are to be looked for among the offspring of blues mated together. The blues, how- ever, may carry factors which are capable of modifying the brown. Just as the pale pink- tinged sweet pea (PI. IV., 9) when mated with a suitable white gives only deep purples, so an eye with very little brown pigment mated with cer- tain blues produces progeny of a deep brown, far darker than either parent. The blue may carry a factor which brings about intensification of the brown pigment. There are doubtless other factors which modify the brown when present, but we do not yet know enough of the in- 178 MENDELISM CHAP. heritance of the various shades to justify any statement other than that the heredity of the pigment in front of the iris behaves as though it were due to a Mendelian factor. Even this fact is of considerable importance, for it at once suggests that the present systems of classification of eye-colours, to which some anthropologists attach con- siderable weight, are founded on a purely empirical and unsatisfactory basis. Intensity of colour is the criterion at present in vogue, and it is customary to arrange the eye-colours in a scale of increasing depth of shade, start- ing with pale greys and ending with the deepest browns. On this system the lighter greens are placed among the blues. But we now know that blues may differ from the deep browns in the absence of only a single factor, while, on the other hand, the difference between a blue and a green may be a difference dependent upon more than one factor. To what extent eye-colour may be valuable as a criterion of race it is at present impossible to say, but if it is ever to become so, it will only be after a searching Men- delian analysis has disclosed the factors upon which the numerous varieties depend. A discussion of eye-colour suggests reflections of another kind. It is difficult to believe that the markedly different states of pigmentation which occur in the same species are not associated with deep-seated chemical differences influencing the character and bent of the individual. xv MAN 179 May not these differences in pigmentation be coupled with and so become in some measure a guide to mental and temperamental characteristics? In the National Portrait Gallery in London the pictures of celebrated men and women are largely grouped according to the vocations in which they have succeeded. The observant will probably have noticed that there is a tendency for a given type of eye-colour to predominate in some of the larger groups. It is rare to find anything but a blue among the soldiers and sailors, while among the actors, preachers, and orators the dark eye is predominant, although for the population as a whole it is far scarcer than the light. The facts are suggestive, and it is not impossible that future research may reveal an intimate connection between pecu- liarities of pigmentation and peculiarities of mind. The inheritance of mental characters is often elusive, for it is frequently difficult to appraise the effects of early environment in determining a man's bent. That ability can be transmitted there is no doubt, for this is borne out by general experience, as well as by the numerous cases of able families brought together by Galton and others. But when we come to inquire more precisely what it is that is transmitted we are baffled. A distinguished son follows in the footsteps of a distinguished father. Is this due to the inheritance of a particular mental aptitude, or is it an instance of general mental ability displayed in a field rendered attractive by early association ? We have i8o MENDELISM CHAP. at present very little definite evidence for supposing that what appear to be special forms of ability may be due to specific factors. Hurst, indeed, has brought forward some facts which suggest that musical sense sometimes be- haves as a recessive character, and it is likely that the study of some clean-cut faculty such as the mathematical one would yield interesting results. The analysis of mental characters will no doubt be very difficult, and possibly the best line of attack is to search for cases where they are associated with some physical feature such as pigmentation. If an association of this kind be found, and the pigmentation factors be determined, it is evident that we should thereby obtain an insight into the nature of the units upon which mental conditions depend. Nor must it be forgotten that men- tal qualities, such as quickness, generosity, instability, etc., qualities which we are accustomed to regard as convenient units in classifying the different minds with which we are daily brought into contact, are not necessarily qualities that correspond to heritable units. Effective mental ability is largely a matter of tempera- ment, and this in turn is quite possibly dependent upon the various secretions produced by the different tissues of the body. Similar nervous systems associated with different livers might conceivably result in individuals upon whose mental ability the world would pass a very different judgment. Indeed, it is not at all impossible xv MAN 181 that a particular form of mental ability may depend for its manifestation, not so much upon an essential difference in the structure of the nervous system, as upon the pro- duction by another tissue of some specific poison which causes the nervous system to react in a definite way. We have mentioned these possibilities merely to indicate how complex the problem may turn out to be. Though there is no doubt that mental ability is inherited, what it is that is transmitted, whether factors involving the quality and structure of the nervous system itself, or factors involving the production of specific poisons by other tissues, or both together, is at present uncertain. Little as is known to-day of heredity in man, that little is of extraordinary significance. The qualities of men and women, physical and mental, depend primarily upon the inherent properties of the gametes which went to their making. Within limits these qualities are elastic, and can be modified to a greater or lesser extent by in- fluences brought to bear upon the growing zygote, pro- vided always that the necessary basis is present upon which these influences can work. If the mathematical faculty has been carried in by the gamete, the education of the zygote will enable him to make the most of it. But if the basis is not there, no amount of education can transform that zygote into a mathematician. This is a matter of common experience. Neither is there any reason for supposing that the superior education of a 182 MENDELISM CHAP. mathematical zygote will thereby increase the mathe- matical propensities of the gametes which live within him. For the gamete recks little of quaternions. It is true that there is progress of a kind in the world, and that this progress is largely due to improvements in education and hygiene. The people of to-day are better fitted to cope with their material surroundings than were the people of even a few thousand years ago. And as time goes on they are able more and more to control the workings of the world around them. But there is no rea- son for supposing that this is because the effects of educa- tion are inherited. Man stores knowledge as a bee stores honey or a squirrel stores nuts. With man, however, the hoard is of a more lasting nature. Each generation in using it sifts, adds, and rejects, and passes it on to the next a little better and a little fuller. When we speak of progress we generally mean that the hoard has been im- proved, and is of more service to man in his attempts to control his surroundings. Sometimes this hoarded know- ledge is spoken of as the inheritance which a generation receives from those who have gone before. This is mis- leading. The handing on of such knowledge has nothing more to do with heredity in the biological sense than has the handing on from parent to offspring of a picture, or a title, or a pair of boots. All these things are but the transfer from zygote to zygote of something extrinsic to the species. Heredity, on the other hand, deals with the xv MAN 183 transmission of something intrinsic from gamete to zygote and from zygote to gamete. It is the participation of the gamete in the process that is our criterion of what is and what is not heredity. Better hygiene and better education, then, are good for the zygote, because they help him to make the fullest use of his inherent qualities. But the qualities them- selves remain unchanged in so far as the gamete is con- cerned, since the gamete pays no heed to the intellectual development of the zygote in whom he happens to dwell. Nevertheless, upon the gamete depend those inherent faculties which enable the zygote to profit by his oppor- tunities, and, unless the zygote has received them from the gamete, the advantages of education are of little worth. If we are bent upon producing a permanent bet- terment that shall be independent of external circum- stances, if we wish the national stock-to become inherently more vigorous in mind and body, more free from con- genital physical defect and feeble mentality, better able to assimilate and act upon the stores of knowledge which have been accumulated through the centuries, then it is the gamete that we must consult. The saving grace is with the gamete, and with the gamete alone. People generally look upon the human species as having two kinds of individuals, males and females, and it is for them that the sociologists and legislators frame their schemes. This, however, is but an imperfect view to i8 4 MENDELISM CHAP. take of ourselves. In reality we are of four kinds, male zygotes and female zygotes, large gametes and small gametes, and heredity is the link that binds us together. If our lives were like those of the starfish or the sea-urchin, we should probably have realised this sooner. For the gametes of these animals live freely, and contract their marriages in the waters of the sea. With us it is different, because half of us must live within the other half or perish. Parasites upon the rest, levying a daily toll of nutriment upon their hosts, they are yet in some meas- ure the arbiters of the destiny of those within whom they dwell. At the moment of union of two gametes is de- cided the character of another zygote, as well as the nature of the population of gametes which must make its home within him. The union once affected the inevitable sequence takes its course, and whether it be good, or whether it be evil, we, the zygotes, have no longer power to alter it. We are in the hands of the gamete ; yet not entirely. For though we cannot influence their behaviour we can nevertheless control their unions if we choose to do so. By regulating their marriages, by encouraging the desirable to come together, and by keeping the undesir- able apart we could go far towards ridding the world of the squalor and the misery that come through disease and weakness and vice. But before we can be prepared to act, except, perhaps, in the simples cases, we must learn far more about them. At present we are woefully ignorant xv MAN 185 of much, though we do know that full knowledge is largely a matter of time and means. One day we shall have it, and the day may be nearer than most suspect. Whether we make use of it will depend in great measure upon whether we are prepared to recognise facts, and to modify or even destroy some of the conventions which we have become accustomed to regard as the foundations of our social life. Whatever be the outcome, there can be little doubt that the future of our civilisation, perhaps even the possibility of a future at all, is wrapped up with the recognition we accord to those who live unseen and in- articulate within us the fateful race of gametes so irrevocably bound to us by that closest of all ties, heredity. APPENDIX As some readers may possibly care to repeat Mendel's experiments for themselves, a few words on the methods used in crossing may not be superfluous. The flower of the pea with its standard, wings, and median keel is too familiar to need description. Like most flowers it is hermaphrodite. Both male and female organs occur on the same flower, and are covered by the keel. The an- thers, ten in number, are arranged in a circle round the pistil. As soon as they are ripe they burst and shed their pollen on the style. The pollen tubes then pene- trate the stigma, pass down the style, and eventually reach the ovules in the lower part of the pistil. Fertilisa- tion occurs here. Each ovule, which is reached by a pollen tube, swells up and becomes a seed. At the same time the fused carpels enclosing the ovules enlarge to form the pod. When this, the normal mode of fertilisa- tion, takes place, the flower is said to be selfed. In crossing, it is necessary to emasculate a flower on the plant chosen to be the female parent. For this pur- pose a young flower must be taken in which the anthers have not yet burst. The keel is depressed, and the sta- mens bearing the anthers are removed at their base by a 187 i88 MENDELISM pair of fine forceps. It will probably be found necessary to tear the keel slightly in order to do this. The pistil is then covered up again with the keel, and the flower is enclosed in a bag of waxed paper until the following day. The stigma is then again exposed and dusted with ripe pollen from a flower of the plant selected as the male parent. This done, the keel is replaced, and the flower again enclosed in its bag to protect it from the possible attentions of insects until it has set seed. The bag may be removed in about a week after fertilisation. It is perhaps hardly necessary to add that strict biological cleanliness must be exercised during the fertilising opera- tions. This is readily attained by sterilising fingers and forceps with a little strong spirit before each operation, thereby ensuring the death of any foreign pollen grains which may be present. The above method applies also to sweet peas, with these slight modifications. As the anthers ripen relatively sooner in this species, emasculation must be performed at a rather earlier stage. It is generally safe to choose a bud about three parts grown. The interval between emas- culation and fertilisation must be rather longer. Two to three days is generally sufficient. Further, the sweet pea is visited by the leaf-cutter bee, Megachile, which, unlike the honey bee, is able to depress the keel and gather pollen. If the presence of this insect is suspected, it is desirable to guard against the risk of admixture of APPENDIX 189 foreign pollen by selecting for pollinating purposes a flower which has not quite opened. If the standard is not erected, it is unlikely to have been visited by Mega- chile. Lastly, it not infrequently happens that the .little beetle Meligethes is found inside the keel. Such flowers should be rejected for crossing purposes. INDEX Abraxas grossulariata, gg "Acquired" characters, 14 Adaptation, 143 Agouti mice, 50 Albino mice, 50 Albinos, nature of, 53 Amauris, 144 Analysis of types, 156 Ancestral Heredity, Law of, 13 Andalusian fowls, 70 Axil colour in sweet peas, g3 Bateson, W., 14, 2g, 55, 116, 132, 141 Biffen, R. H., 157 Blue Andalusian fowls, 71 Brachydactyly, 171 Bryony, 120 Bush sweet peas, 63 Castle, 132 Cattle, horns in, 86, 166 Colour, nature of, in flowers, 48 Colour-blindness, 117 Combs of fowls, 33, 43 Correns, C., 2g, 120 Coupling of characters in gametes, 93 Cuenot, 50, ng "Cupid" sweet peas, 62 Currant moth, gg Darwin, C., 10, 65, 147, 163 De Vries, H., 15, 2g, 141 Discontinuity in variation, 14 Dominant characters, 18 Doncaster, L., gg Drinkwater, H., 172 Dutch rabbits, 60 Eggs, 2 Environment, influence of, 137 Euralia, 144 Evolution, 10, 85, i3g Eye, in primulas, 55 Eye-colour, in man, 176 Factor, definition of, 31 Factors, interaction of, 42 Fertilisation, 3 Fertilisation, self- and cross-, 163 Fixation of varieties, 153 Fluctuations, 138 Fowls, coloured from whites, 4g, 73 Gal ton, 13, i7g Gametes, nature of, 6 Gregory, R. P., 55, g3 Haemophilia, 176 Hardy, G. H., 147 Heterozygote, definition of, 28 Heterozygote, of intermediate form, 68 Hieracium, 27, 132 Himalayan rabbits, 60 Homostyle primulas, 56 Homozygote, definition of, 28 Hooded sweet peas, 8g Horses, bay and chestnut in, 167 Hurst, C. C., 62, 176, 180 Immunity in wheat, 158 Individuality, 135 Inhibition, factors for, 74, 108 Intermediates, 125 191 MENDELISM Johannsen, W., 160 Lop-eared rabbits, 132 Mendel, 8, 17, 26, 132 Mental characters, 180 Mice, inheritance of coat colour in, 50 Mimicry, 143 Mirabilis, 151 Morgan, T. H., 116 Mulattos, i2g Mutation, 83, 138 Nageli, C., 26 Natural selection, n, 140, 142, 149 Nettleship, E., 175 Night-blindness, 175 Pararge egeria, 132 Parkinson, J., 122 Pea comb, 33 Peas, coloured flowers in, 24 Peas, tall and dwarf, 18 Pigeons, 86 Pin-eye in primulas, 55 Pisum, 17 Primulas, 31, 55, 68, 93 Pollen, 3 Pollen of sweet peas, 92 Pomace fly, 115 Population, inheritance of characters in a, 147 Presence and Absence theory, 35 Pure lines, 162 Purity of gametes, 24 Purity of type, 155 Rabbits, 53, 60 Ratios, Mendelian 3:1, 20 9:3:3:1, 25, 34 9:3:4, Si 9 : 7, 49 Ray, John, 143 Recessive characters, 19 Repulsion between factors, oo Reversion, 59, 165 in rabbits, 59 in sweet peas, 62 in fowls, 65 in pigeons, 65 Rose comb, 33 Saunders, E. R., 54, 122 Seeds, nature of, 4 Segregation, 22 * Selection, 162 Sheep, horns in, 76 Silky fowls, 30, 105 Single comb, 32 Species, nature of, 150 Species, origin of, n Speckled wood butterfly, 132 Spermatozoa, 3 Sports, 147 Staples-Browne, R., 66 Sterility, 151 Sterility in sweet peas, 93 Stocks, double, 122 Stocks, hoariness in, 54 Sweet pea, colour in, 44, 79 history of, 82 inheritance of hood in, 89 inheritance of size in, 62 Telegony, 167 Thrum-eye in primulas, 55 Toe, extra toe in poultry, 76 Tscherrhak, E., 29 Unit-character, definition of, 31 Variation, 14, 137, 139 Walnut comb, 33 Weismann, A., 13 Wheat, beard in, 74 experiments with, 157 White, dominant in poultry, 72 Wilson, J., 1 68 Yellow mice, 119 Zygotes, nature of, 5 THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW AN INITIAL FINE OF 25 CENTS WILL. BE ASSESSED FOR FAILURE TO RETURN THIS BOOK ON THE DATE DUE. THE PENALTY WILL INCREASE TO SO CENTS ON THE FOURTH DAY AND TO $1.OO ON THE SEVENTH DAY OVERDUE. BIOLOGY LIBRARY OCT 21 1932 SEP 18 1933 / 1933 OCT 11 1933 1933 / SEP 6 193 4 APR 19 1935 181935 1936 HOV 1 1* J937 OCT 27 1938 APR 10 1939 APR 24 1942 NOV 13 1950 LD 21-Jm-G, . YwrBTKi BIOLOGY LIBf- THE UNIVERSITY OF CALIFORNIA LIBRARY