INHERITAN«^MBIT| College This book luas presented • ^^ I • L 1 • 656 THCA«UI1NA-ME UNWU'jirMlBRAR'ES S00764707 V 2 _> QH431 C38 •->, '2€8 Castle Studies of Inhe-pl- Ot^* 7U LIBrai 502 i8 This BOOK may be kept out TWO WE:^KS ONLY, and is subject to a fine of F VE CENTS a day thereafter. It is due on the da3'^ indicated below: APR M^ gj ^t^ ^^ igg^ A ^-^B'l STUDIES OF INHERITANCE IN RABBITS W. E. CASTLE IN COLLABORATION WITH H. E. WALTER, R. C. MULLENIX, and S. COBB WASHINGTON, D. C. Published by The Carnegie Institution of Washington 1909 Carnegie Institution of Washington, Publication No. 114. Papers of the Station for Experimental Evolution, No. 13. Contributions from the Zoological Laboratory of the Museum of Comparative Zoology at Harvard College. E. L. Mark, Director. No. 199. The Plimpton Press Norwood Mass. U.S.A. CONTENTS. Kagt Preface 5 Part I. — Ear-size 9-35 Introduction 9 Characteristics of lop-eared rabbits; sterility and its inheritance 9 Growth-rate of lop-eared and of short-eared rabbits in size and in ear-length . la Matings of short-eared rabbits iuler se 14 Matings of lop-eared rabbits inter se 16 Cross I. — Lop-eared female X short-eared male 18 Cross 2. — Short-eared female X lop-eared male 18 Cross 3. — Belgian hare female X lop-eared male 20 Cross 4. — Lop-eared female X half-blood lop male 21 Cross 5. — Half-blood lop female X lop male 179 22 Cross 6. — Half-blood lops mated ititer se 23 Other matings of half-blood lops and additional Cross 6 matings 26 Matings of three-quarter-blood lops 3I Cross 7. — Quarter-blood lop female X short-eared male 3i Cross 8. — Quarter-blood lop X three-quarter-blood lop 34 Limitations of the data studied 34 Conclusions 35 Part II. — Weight 37-4° Part III. — Skeletal Dimensions 4i-44 Part IV. — Color 45-68 Color variation in relation to color factors 45 Development of the factor hypothesis 46 The general color factor, C 46 The specific pigment factors, B, Br, and Y 46 The intensity factor, I or D 46 The factor for a pigment pattern of the individual hair, A 47 The factor for uniformity of pigmentation, U, or spotting with white, S . . 47 The factor for extended distribution of black or brown, E, alternative with R (restricted distribution) 47 Interrelations of factors E and U 48 Interrelations of factors B, Br, and V 49 Gametic structure and variation 49 Gametic and zygotic formulae 5° Color varieties of the rabbit 5' Gray type 5 2 Black type 5^ Yellow type 53 White type 53 Zygotic variation within each color variety 54 Gray 54 Blue-gray 60 Black 61 Blue 6a Yellow 63 Sooty 64 White 65 The material basis of heredity factors 68 Bibliography 69 Description of Plates 7° 3 PREFACE. In this paper arc recorded observations upon inheritance in rabbits which were made in the Harvard Zoological Laboratory with the aid of a grant from the Carnegie Institution of Washington. The authors desire to express their appreciation of that aid, without which these observations could not have been made. The experiments described were planned by the senior author, and this report also was written by him. Dr. H. E. Walter has made the majority of the extremely laborious observations and computations con- cerning the inheritance of ear-length and of body-weight. Dr. R. C. MuUenix prepared and measured the rabbit skeletons as a foundation for Part III of this paper; while both Dr. Walter and Mr. Cobb rendered valuable assistance in connection with the study which has been made of color inheritance. The senior author alone is responsible for the analytical treatment of the observations. STUDIES OF INHERITANCE IN RABBITS BY W. E. CASTLE IN COLLABORATION WITH H. E. WALTER, R. C. MULLENIX, AND S. COBB. PART I. — EAR-SIZE. INTRODUCTION. The inheritance of ear-size in rabbits has been characterized as blend- ing, in certain preliminary publications, (Castle, :o5, :05a).' The experi- mental evidence for such a characterization is described in the following pages. It consists of results obtained by experimental cross-breeding of lop-eared rabbits with ordinary short-eared ones. A detailed account of this evidence is of little interest to the general reader, who therefore may advantageously omit pages 14-34. For consultation on the part of the critical student of heredity, it has been thought essential to present this evidence in some detail, even though it is intrinsically uninteresting. CHARACTERISTICS OF LOP-EARED RABBITS; STERILITY AND ITS INHERITANCE, Lop-eared rabbits are distinguished from ordinary ones chiefly by the enormous size of their ears, which are so large as to hang down, touching the ground on either side of the head. (See plate i, fig. 2, and plate 2, fig. 8.) This breed of rabbit is characterized also by a long tail and unusual size, being one of the largest breeds known. The characters of large size and long tail, however, have probably not been sought for their own sake, but have been incidentally obtained in the production of the breed as a result of selection for ears of large size; for among lop-eared rabbits, as a rule, those of the largest size have longest ears. In the winter of 1904 a pair of lop-cared rabbits was obtained from a fancier and used in various breeding experiments. Matings of the two together were for the most part fruitless, only one litter of 2 young being successfully reared. These were similar to the parent rabbits in size and ear-character. Of the two, one was a male, which was used extensively in breeding experiments, including one successful mating with his mother, from which came a good-sized litter. But only two out of this litter at- tained the age of 20 weeks and they ultimately succumbed to disease under conditions not unfavorable to other rabbits. The second of the two young reared by the original lop-eared pair was a female. Only twice did this rabbit bear young by any sort of mating. In one case she failed to rear any of the young. In the other case she reared, when mated to her own • For complete titles see^Bibliography, p.. 69. ■'■ '■'-.-TO T"^ 10 INHERITANCE IN RABBITS brother, 2 young out of a litter of 3. Both were males. The larger one, although apparently healthy, failed to breed; the smaller one was not tested. Accordingly, out of 5 pure-bred lop-eared rabbits with which we have experimented, 2 (a male and a female) were infertile — one of the two largely so, and the other completely. Infertility has also been encountered among a few of the female descendants of this lop-eared stock produced by cross-breeding, but in no other stock of rabbits with which we have experimented. Sterile individuals have not been observed among half-blood lops of generation Fi, but a few have occurred in later genera- tions. In the majority of cases, however, the sterile individuals have been three-quarter-blood lops. From these facts we conclude that a tendency to sterility is inherent in the lop-eared stock used, and is transmitted, not to the immediate off- spring (FJ if they are cross-breds, but to the next generation, when it is produced by a back-cross between Fj and the pure lop-eared stock; less frequently sterility reappears in F2, produced by breeding the half- blood lops inter se. We should expect the infertility to occur only half as frequently in this latter sort of mating as in the former, where it has been oftenest observed. On the whole, it seems probable that a ten- dency to sterility is inherited in rabbits, as in Drosophila (see Castle et al., :o6), after the manner of a Mendelian recessive character, i. e., skipping a generation in crosses. Why lop-eared rabbits more than other breeds should show a tendency to sterility is not known; but as they are extensively inbred, it seems highly probable that inbreeding is largely responsible for this sterihty. The lop-eared character is one which, from the manner of its inheritance, we may be sure, has been built up slowly as the result of selection. In this process inbreeding must have been continuously practised, for since every out-cross would result in loss of half the ground gained by selection, it would be practised only when absolutely necessary. At birth rabbits have ears quite undeveloped, and the ears do not attain their full growth until an age of 5 to 8 months have been reached. Ear- growth is well advanced, however, at 20 weeks, after which time it becomes very slow. Accordingly 20 weeks has been found a convenient age at which to institute comparisons as to ear-character between different lots of rabbits. Frequently, however, it is impossible to rear an entire Utter of rabbits to the age of 20 weeks, in which case an earlier determination of ear-character becomes desirable. For this reason, after some experi- mentation, we adopted the plan of making weekly measurements of the ear dimensions at ages from 2 to 20 weeks inclusive. This process, while laborious, fully eliminates errors due to observation, as well as those due to temporary growth conditions. The weekly observations upon each rabbit included taking its weight, the maximum length and maximum width of its right ear, and finally the EAR-SIZE 11 spread of the ears, i. e., the distance from ear-tip to ear-tip when the ears are extended in a horizontal position and stretched slightly. Since the measurements in nearly all cases were made by the same observer (Walter), the personal equation is a fairly constant factor and may be disregarded Ear length in mm. / a 3 4 5 6 7 8 9 10 II 12 l^ 14- fS I6 I7 id 19 ' ' I I I ' I I I I » I I I I I I I 120 110- 100 SO- SO- 70 60- 50- 40 30 JL ,0 o_-a' 0--0841 84-4 y, •' 1 1 1 1 r 1 1 1 1 1 "I 1 1 I I I I I I Agem , 2 3 4 S 6 7 B S 10 II IZ 13 14 IS I6 I7 JQ 19 weeks. Fig. I. Chart showing growth in ear-length, and body-weight of a litter of six short-eared rabbits between the ages of two and eighteen weeks. See table i. Weight in grams. ■1500 ■1400 1300 1200 1100 1000 Ysoo 800 700 600 \-500 400 300 200 100 12 INHERITANCE IN RABBITS in comparing one set of observations with another. Records of this sort, more or less complete, were made for 70 dilYerent litters of rabbits, containing 341 individuals. An inspection of figs, i to 3 shows that the growth-curve for ear-length ^ from 2 weeks after birth is of the same general form in the case of both long-eared and short-eared rabbits. It is a curve convex above, indicating a steadily diminishing daily increment in ear-lengtli. GROWTH-RATE OF LOP-EARED AND OF SHORT-EARED RABBITS IN SIZE AND IN EAR-LENGTH. The theoretical growth-curve of an organism in weight (Houssay, loy; Robertson, :o8) is at first concave upward, but later becomes convex. When the curve is concave upward the daily growth increment is increas- ing. But when the growth-curve becomes convex upward, it is evident that the growth increment is decreasing. Therefore the period of greatest daily growth occurs when the growth-curve is changing from a concave to a convex one. In rabbits this occurs at an age of from 6 to 8 weeks after birth (see figs, i to 3). According to Robertson (:o8) the period of maximum growth corresponds with the middle point of a growth-cycle which in character resembles an autocatalytic monomolecular chemical reaction. In the rabbit this growth-cycle probably has its beginning at some time prior to birth and ends before puberty is attained. It is possible that this same form of curve would be observed in respect to ear-length also, if the measurements began at a period sufficiently early. Growth of the ears is completed before increase in body-weight ceases, and it is possible that the growth-curve for ear-length has already changed from a concave to a convex form at the age of 2 weeks, when our measure- ments begin. But it is, on the other hand, possible that the growth-curve for ear-length would not show a convex form upward even if completed for the period prior to 2 weeks of age; for ear-length is a linear dimen- sion, whereas body- weight depends on volume, i. e., size in three dimen- sions, and a doubHng of any linear dimension should be attended by an eight-fold increase of volume. A comparison of fig. i with fig. 2 shows a considerable difference between ordinary short-eared (fig. i) and lop-eared (fig. 2) rabbits as regards size, at corresponding ages; the difference is even more striking in regard to ear-length. Crosses between the two varieties produce rabbits inter- mediate in character as regards both weight and ear-length. But before considering further the character of the cross-breds, it will be well to inquire how each variety breeds by itself. ' The measurements for ear- width and "spread" are closely correlated with those for ear-length. For the sake of simplicity we shall deal with the statistics for ear-length only. EAR-SIZE 13 Ear length in mm. 220- 210 200- 190 130 no- lea- 150- 140 130 120 110 100- 90 80 70 60 SO- 40 30 ti J ^ 4 cf i ¥ i 1 i i ' * i > 4t • JO i 1 cr .■ 7 ?' f Aqe in ' ' — ' — ■ — ' — i — ' — ' — r— i 1 1 — r 1 — i — \ 1 1 1 — r- v/eeks. ' 2 3 4 5 6 7 6 9 10 II 12 13 14 I5 16 17 18 13 20 Fig. 2. Growth-curves for a litter of five lop-eared rabbits. See table 2 and compare fig. i. 14 INHERITANCE IN RABBITS MATINGS OF SHORT-EARED RABBITS ENTER SE. Several matings of short-eared rabbits inter se are recorded in table i. They show great uniformity of result. The young differ little in ear- length from their parents, which in no case differed from each other by more than 5 mm. Table i. Mating i. Mating 2. Ear- length. Weight. Age. Ear- length. Weight. Age. Parents : 9 255 d'497 Mid-parental Offspring: Litter i — 9768 9 769 9770 c?77i 9772 c?773 Litter 2 — c?88i ?882 c?883 d>884 mm. "5 no 112.5 107 in "5 "3 106 III no no 114 "5 gms. 2,650 2,04s 2,347 1,445 1,480 1,780 1,740 1,550 1,650 1,380 1,380 1,500 1,560 Adult.» 7 mos. 20 weeks. Do. Do. Do. Do. Do. 14 weeks. 16 weeks. 14 weeks. Do. Parents : 9498 (^497 Mid-parental Offspring : Litter i — 9782 9783 9784 ,cf785-- Litter 2 (see fig. I) - 9840 9841 9842 c?843 9844 9845 Tntn, no no no no 106 107 no 112 1x6 106 in "5 no gms. 1,980 ( 1.91S \ 2.045 1.947 1.365 1.575 1.375 1.695 1,450 1,460 1,460 1,510 1,340 1,420 27weeks. 7 mos. 27 weeks. 20 weeks. Do. Do. Do. 18 weeks. Do. Do. Do. Do. Do. Mating 3. Mating 4. Parents: $268 (5^497 Mid-parental Offspring : 9859 c?86i d>862 c?863 105 no 107-5 no 112 no no 2,280 2,045 2,162 1,445 1,340 1,420 1,295 Adult. 7 mos. 15 weeks. Do. Do. Do. Parents: 9268 c? 56 Mid-parental Offspring: 0^774 105 102 103-5 108 2,280 2,500 2,390 1,715 Adult. Do. Do. 20 weeks. 1 By adult is meant i year or more old. pTn mating i , the extreme deviations from the mid-parental * ear-length are —6.5 mm. and +2.5 mm., the average deviation being only 2.5 mm. The total range of variation is 9 mm. In mating 2, between brother and sister, the extreme deviations are —4 mm. and -f- 6 mm., giving a total range of variation of 10 mm. The average deviation from the parental ear-length (no mm.) is, as in mating i, 2.5 mm, 1 By mid-parental, as we shall use the term in this paper, is meant a magnitude exactly halfway between the magnitudes of the respective parents. It is the mean of the parental magnitudes. EAR-SIZE 15 The growth-curves for litter 2, which were produced by this mating, are shown in figure i. In mating 3, the deviations are all plus in character, but are small in amount, namely, 2.5, 4.5, 2.5, and 2.5 mm. Ear length IT) mm. 160- no 160- 150- 140 130 120 no 100 90- BO 10 60 SO 40- 30- Age m weeks. ^ ■(/ ^789 p. ci f788 -0 V / r t j>-o 787 —r—r—i 1 1 J I I I I J 1 1 1 I ' I I I I I I / 2 3 4 5 6 7 8 9 10 n 12 13 14 15 16 17 18 19 20 2/ Fig. 3. Growth-curves for a litter of five second-generation (F,^) half-lop rabbits. See table 9 and compare figs. I and 2. From mating 4, by the same female that was concerned in mating 3, a single young one was reared, which showed a plus deviation of 4.5 mm. 16 INHERITANCE IN RABBITS Another mating which falls in this category was made between the Belgian hare (9 431) and the short-eared c? 56 (see table ia). It shows a complete blending in the offspring of the parental ear-lengths, with a very small range of variation, viz, 6 mm. Table ia. Ear- length. Weight. Age. Parents : 9431 r?> 1:6 mm. n8 102 no I 102 I 105 III 108 J 108 1 no 108 gms. 3.400 2,500 2.95° 2,700 2,945 Adult. Do. Do. 21 weeks. Adult. 21 weeks. Do. Do. Adult. 21 weeks. Mid-parental .... Offspring : 9232 Q 2X1 6^234 c?23S 9236 The mid-parental ear-length was exceeded by i of the young at 21 weeks of age; 3 others came within 2 mm. of the mid-parental ear-length at 21 weeks of age, and i of these equaled it when adult. If the other 2 did as well they too must have attained the expected ear-length. Only I individual (9232), then, fails to attain the mid-parental ear-length. This result is almost identical in general character with that shown by table I. We may conclude that short-eared rabbits breed true within a range of fluctuating variability not exceeding 10 mm. MATINGS OF LOP-EARED RABBITS ENTER SE. Our original stock of lop-eared rabbits consisted of a single pair. Both of them gave vigorous young in matings with short-eared rabbits, but not with each other. Consanguinity may have been the reason for this lat- ter fact. They were obtained from the same source, and doubtless were nearly related, as well as inbred. Nevertheless we did obtain from them two good-sized and healthy young, c? 179 and 9 180. The former appears in many of the crosses to be described, but the latter proved a very poor mother, producing only occasional htters of young, none of which attained maturity. Table 2 shows the only results obtained from mating lop-eared individuals inter se. Mating i produced 2 young, one (^ 179) very similar to the father, the other (9 180) very similar to the mother, but not quite so large and with ears 5 mm. shorter. The deviations from the mid-parental ear-length are —7.5 and —2.5 mm., respectively. EAR-SIZE 17 Mating 2 (between brother and sister) produced 2 young, which reached the age of 20 weeks. Though they were not large, their ears attained a good length, the deviations from the mid-parental ear- length being — 5 mm. and —2 mm. Table 2. Mating i. Mating 3. Ear- length. Weight. Age. Ear- length. Weight. Age. Parents : Old 9 lop (pi. I, fig. 2). . Old c? lop . . Mid-parental Ofifspring: c?i79(pl. 2,fig. 8) 9 180 mm. 225 210 217-5 210 220 gms. 4,600 3.450' 4,025 3.410 3.765 Adult. Do. Do. Adult. Do. Parents: Old 9 lop (pi. I, fig- 2) . 0^179 (Pl- 2, fig- 8) Mid-parental Ofifspring (see fig. 2): cf667 9668 9669 c?670 (^671 mm. 225 210 215-7 1 320 1 223 300 215 330 190 gms. 4,600 3.410 4,005 2,010 1. 590* «,37o 2,030 1,900 1.205 Adult. Do. Do. 14 weeks. 20 weeks. 14 weeks. Do. Do. Do. Mating 2. Parents: 9 180 ro.\imate the mid-parental conditions both of ear-length and of weight, these two being (^319 and 9322. The same is measurably true of a third individual, s 506. But 9 504 and 9 508 fall considerably short of the mid-parental ear-length and the mid-parental weight; while c? 505 and 9 509 con.siderably exceed the mid- parental ear-length, and approximate the mid-parental size. The greatest deviation from the mid-parental ear-length is a minus one of 15 mm. (recorded at the age of 18 weeks), but a \Aus deviation of nearly the same amount (14.5 mm.) is also observed, though not until the age of a year had been attained. The average deviation from mid-parental ear-length is 9 mm. The lowest measurement, 180 mm., stands almost exactly midway between the ear-length of the father (166 mm.) and the mid- parental ear-length 195.5 mm.; while the largest measurement (210 mm.) stands midway between the condition of the mother (225 mm.) and the mid-parental ear-length. The result observed in this cross may be de.scribed as blending inheri- tance, with fluctuation about the mid-parental ear-length, in about the same degree as in the case of lop-eared rabbits i)urely bred. 22 INHERITANCE IN RABBITS Two of the offspring produced by cross 4 were mated with each other, viz, 9 504 and c? 506. They produced a htter of 3 young, the character of which is shown in table 7. Table 7. Ear- length. Weight. Age. Parents: 9 <;o4 mm. 186 192 189 191 201 185 gms. 3.430 3.150 3.290 2,255 2,255 1,900 I year. 30 weeks. Do. 20 weeks. Do. 19 weeks. f^irod Mid-parental .... Offspring: rt^TAO r?'?^.! $742 The deviations from the mid-parental ear-length are +2, —12, and — 4 mm. respectively, which lie within the limits of variation observed among lop-eared rabbits purely bred. The range of variation is chiefly upward (plus), doubtless because of the small number in the htter, which would make the food supply of the individual better than usual. Cross 5. — Half-blood Lop Female x Lop Male 179. Female 167 was produced by a cross similar to cross 2, in which the mother was a short-eared Himalayan rabbit (9 23), and the father the male lop used in cross 3. She was mated with lop c? 179 and produced 3 htters of young, as indicated in table 8. The range of variation in ear-length in these 3 htters of rabbits is wide, extending from 165 mm. in a rabbit 7 months old (having full-grown ears) to 194 mm. in one only 18 weeks old, or to 200 mm. in one 30 weeks old, a total range of 35 mm. The largest minus deviation from the mid- parental ear-length is 5 mm. ; the largest plus deviation, 30 mm. ; the least deviation from the short-eared parent is 35 mm., but from the long-eared parent only 10 mm. Hence in this cross the long-eared parent is approx- imated more closely than the short-eared one. This, however, is not of necessity evidence of Mendehan recessiveness of the character long-ear in 9 167. Another and, we believe, better way of viewing the matter is this: 9 167 transmitted a greater ear-length than she had herself attained. It is known that conditions which influence general growth, during the first 20 weeks, influence also ear-length. But at 20 weeks of age ear-growth is practically complete, although growth in other respects continues some time longer. It is possible, therefore, for an animal to be stunted in ear- EAR -SIZE 23 size and yet lo attain a normal or nearly normal general size. It is not to be expected, however, that such an animal will transmit to voung reared under normal conditions the diminished ear-size which it shows, but rather the ear-size which it would have attained harl it been reared under normal conditions. Table 8. — Cross 5. Parents: ?i67 cJ>i79(lop) . Mid-parental Offspring: Litter i — c^437 9438 Litter 2 — ^^566 crs68 (^'sfig 9570 Litter 3 — 9644 (^645 9646 9647 9648 cr649 Ear- length. mm. 130 210 170 1 184 / 200 I 177 / 185 Weight. gms. 2,55° 3.410 2,980 2.550 3.140 3,510 3. no 194 1.945 181 1.915 170 1,865 183 1 164 1.655 1 i6s 3,430 170 1,460 175 1,430 171 2,030 180 1.930 178 2,080 Age. Adult. Do. Do. 20 weeks. 30 weeks. 20 weeks. 30 weeks. 18 weeks. Do. Do. Do. 20 weeks. 7 months. 20 weeks. Do. Do. Do. Do. Cross 6. — Half-blood Lops Mated inter se. The same female half-blood lop already mentioned (9167, cross 8), was mated with a male produced by cross i (^"248). Their young con- stitute an F2 generation of half-blood lops. In this litter the deviations from the mid-parental ear-length are all, with one exception, positive (upward). This result accords with that observed among the young of this same mother (9 167), in connection with cross 5. She evidently transmitted a greater car-length than she manifested. The range of variation, 35 mm., while high, docs not exceed that found among lop-eared rabbits mated inter se, as is clear from a comparison of fig. 2 with fig. 3, the former showing growth-curves for lop-eared rabbits, the latter for the litter of F, half-blood rabbits under consideration. The range of variation in this cro.ss also agrees exactly with that observed in cross 5, in which the same mother was matccl with a full-blood lop. We get, therefore, from this case no evidence of Mendelian sj)litting as regards the character ear-length. 24 INHERITANCE IN RABBITS Table 9. Ear- length. Weight. Age. Parents: Q167 mitt. 130 153 141-5 140 153 170 175 140 gms. 2,550 3.930 3.240 1.730 1.915 2,060 2,170 2,155 Adult. Do. Do. 20 weeks. Do. Do. Do. Do. c?248 Mid-parental. . . . Offspring : c?786 d^787 c?788 Q 780 (^792 Two of the young produced by cross 5 were mated with each other, viz, c?437 with 9438. Their young (table 10) vary closely about the mid-parental ear-length. Table 10. Ear- length. Weight. Age. Parents: $4.78 mm. 184 177 180.5 180 185 180 176 gms. 2,550 2,510 2,530 1,970 2,030 2,010 1,825 20 weeks. Do. 20 weeks. Do. Do. Do. (^Aiy Mid-parental .... Offspring : r^yiQ $720 (^721 r?*725 Another cross 6 mating was obtained between 9 247 and d 248 (pro- duced in cross i). The character of the young is shown in table 11. Table ii. Ear- length. Weight. Age. Parents : Q247 mm. 152 153 152-5 122 130 135 135 130 gms. 3,290 3,930 3,610 1,375 1,770 2,460 1,860 1,755 Adult. Do. Do. 19 weeks. Do. 8 months. 19 weeks. Do. c?248 Mid-parental .... Offspring : Q -loy (f-iaS Q 400 Q401 (^402 Contrary to the result shown in table 9, the young obtained from this mating all fall short of the mid-parental ear-length by from 17 to 29.5 mm., indicating probably conditions of nutrition below the normal, dur- ing the period of principal growth of the ears, or of the transmission by the parents of a condition of ear-length inferior to that which they mani- c. EAR-SIZE 25 fested. The young vary in normal fashion about a mean ear-length of 130.4 mm. The total range i.s only 13 mm., indicating no Mendclian heterogeneity among the gametes [produced by the jjarents, though both were Fj half-lops. One of the young produced in this Utter (9400) was mated with the lop-eared cfiyg, table 2. The result is shown in table 12. Six offspring were obtained from this mating; they vary rather closely about the mid- parental ear-length, though chiefly below it, as we might e.xpect from the fact that the mid-parental value given is based upon measurements of adults, and that of the young upon measurements at the age of 20 weeks. The total range of variation is 15 mm. Table 12. Ear- length. Weight. Age. Parents: Q AOO mm. 135 310 172.5 162 166 160 171 160 175 gms. 2,460 3.410 2.935 1,680 1,670 1,520 1,880 1.715 1,720 8 months. Adult. 20 weeks. Do. Do. Do. Do. Do. r^iTO Mid-parental. . . . Offspring : c^'JOX C^noA 2 70c r?7o6 $ 707 Q 708 Female 400 was hkewise mated with her father (d"248), producing a litter of 4 young, all of which fell below the mid-parental ear-lengths. (See table 13.) Table 13. Ear- length. Weight. Age. Parents : Q AOO mm. 135 153 144 135 137 138 126 gms. 2,960 2,930 3.445 1,500 1.710 1.780 1,560 8 months. Adult. 20 weeks. Do. Do. Do. f?248 Mid-parental .... Offspring : 9818 C^SlQ 9820 9821 The deviations are —9, —7, —16, and —18, average —12.5 mm. But the total range of variation is only 11 mm., or scarcely greater than that observed among short-eared rabbits. Certainly tliis result affords no evidence of heterogeneity as regards ear-character among the gametes formed by the parents, though one was an Fj and the other an Fj cross- bred between the lop-eared and the short-eared races. 26 INHERITANCE IN RABBITS OTHER MATINGS OF HALF-BLOOD LOPS AND ADDITIONAL CROSS 6 MATINGS. Other matings in which the rabbits 9 247 and c? 248 were concerned are recorded in tables 14 and 15. In mating i (with d 179) 9247 gave a fully normal blending result. Of the 5 young produced, 2 siu-passed the mid-parental ear-length, i equaled it, and 2 fell below it. All were intermediate, and the range of variation was 14 mm., or about one-fourth of the difference between the parents. In mating 2 (with c?3i9) 9247 gave a result similar to that which she had given with c? 248. All the young were intermediate in ear-length, but all fell short of the mid-parental ear-length, by from 3 to 16 mm. This was not due to consanguinity, for 9247 and (^319 were not closely related. It may, however, have been due to inferior condi- tions of nutrition, perhaps resulting from the large size of the litter. The whole litter seems to have been affected ahke, the total range of variation among the seven young being only 11 mm. Table 14. Mating i. Mating 2. Ear- length. Weight. Age. lenSh. Weight. Age. Parents: 9247 d'179 Mid-parental Offspring: 9635 9636 9637 0^638 ^^639 mm. 152 210 181 170 183 180 170 184 gms. 3.290 3.410 3.350 1,740 1,620 1,990 1,630 1,505 Adult. Do. Do. 20 weeks. 19 weeks. 20 weeks. 20 weeks. 20 weeks. Parents : 9247 c?3i9 Mid-parental Offspring: 9731 9732 9733 9734 (5^736 c?737 9738 mm. 152 200 176 170 170 160 173 165 160 171 gms. 3.290 3.955 3,622 1.970 2,100 2,020 1,710 r.785 1.700 2,020 Adult. Do. Do. 20 weeks. Do. Do. Do. Do. Do. Do. Male 248 was mated with three different short-eared females, none of which was nearly related to him. The results are shown in table 15. In mating i, c? 248 gives a result like that which he had given when mated with his sister (9 247). All but i of the 6 young fell below the mid- parental ear-length by from 8 to 19 mm. The shortest-eared one had exactly the same ear-length (115 mm.) as the short-eared parent, a result unparalleled elsewhere in these experiments except in one case, presently to be noticed. The shortness in this case can not be attributed to the poor condition or small size of the individual, for it was the largest rabbit but one in the litter, a position which it maintained throughout the growth period. Apparently this individual represents an extreme variate of a fluctuating group. The extreme range of variation in this litter was 23 mm.; the difference between the parents, 38 mm. EAR-SIZE 27 Mating 2 shows a result more nearly normal. Two individuals exceed the mid-parental ear-length by 3 mm., 2 fall short of it 7 mm., and i by 9 mm. The total range of variation is 12 mm. In mating 3, which jjroduced 2 litters of young, the variations are again chiefly below the mid-parental ear-length, but to no greater extent than we might expect, in view of the difference in age between parents and children, when measured. In litter i the deviations are —2, +3, —4, and —2 mm., a nearly normal result; but in litter 2, four individuals show a deviation of —7 mm., and one a deviation of +3 mm. The range in htter i is 7 mm.; in Utter 2, 10 mm. There is no evidence of hetero- geneity among the gametes. Table 15. Mating i. Mating 3. Ear- length. Weight. Age. Ear- length. Weight. Age. Parents : 9255 C?248 Mid-parental Offspring: 9619 9620 C?62I 9622 d'624 (^625 mm. "5 153 134 125 120 138 "5 126 125 gms. 2,650 3.930 3.290 1,820 1.750 1,700 1,885 1,720 1,910 Adult. Do. Do. 19 weeks. 20 weeks. Do. Do. 19 weeks. 20 weeks. 1 Parents : 9389 (^248 Mid-parental Offspring: Litter i — 1 9653 9654 ?655 9656 Litter 2 — c^793 9794 9795 c?796 d'797 mm. los 153 129 127 132 125 127 120 120 120 130 120 gms. 3,090 > 3.930 1.465 1.845 1,840 1.745 1,700 1.740 1,860 1,690 1,770 Adult. Do. Do. 20 weeks. Do. Do. Do. 20 weeks. Do. Do. Do. Do. Mating 2. Parents : 9 269 6^248 Mid-parental Offspring: 9726 9727 c?728 6^729 ?730 92 153 122 "5 "3 "5 125 125 1.935 3.930 2,932 1.730 1,685 1.695 1,500 1.930 Adult. Do. Do. 20 weeks. Do. Do. Do. Do. • At 20 weeks. Some other matings which fall in the category of cross 6, and consti- tute a second generation (Fj) from cross 3, are included in table 16. The statistics contained in table 16 are not very satisfactory because the observations are made at such dilTerent ages, and because, in one case at least, that of (?38i, a remarkable increase in ear-length is recorded subsequent to the age of 18 weeks. For observations made at the same age, however, the variability in ear-length is considerable. Tlie range of variation in mating i, litter i, is 17 mm.; in litter 2 it is 15 mm.; in mating 2, it is 18 mm. In generation F,, cross 3, the range of variation was only 28 INHERITANCE IN RABBITS slightly less, viz, 14 mm. So far, then, as table 16 is concerned, we get no clear evidence of heterogeneity among the gametes formed by the cross-breds produced by cross 3. Table 16. Mating i. Mating 2. Ear- length. Weight. Age. Ear- length. Weight. Age. Parents: 9i7S(pI-3, fig. 10) . . . c?i76 Mid-parental Offspring: Litter i — c?37S d^376 c?38i (pl. 3, fig. 12) .. . Litter 2 — c?759 c?76o 9761 mm-. 170 166 168 1 60' 172I i '"'. . ] 168 180 170 185 gms. 4,305 4,130 4,218 2,960 2,975 2,800 3,125 3,800 2,220 2,200 2,410 Adult. Do. Do. 6 months. Do. 22 weeks. 6 months. I year. 20 weeks. Do. Do. Parents: 9178 c?i76 Mid-parental Offspring: 9471 9472 d'474 9475 9476 (^480 mm. 170 166 168 147 168 163 150 150 150 gms. 4,070 4,130 4,100 1,470 1,865 1,733 1,515 1,750 1,610 Adult. Do. Do. 17 weeks. Do. Do. Do. Do. 15 weeks. 1 18 weeks. The female half-blood lop 175 (plate 3, fig. 10) produced by cross 3 was mated with the three-quarter-blood lop c?437 (cross 5), and produced a litter of 5 young, the character of which is shown in table 17. Table i 7- Ear- length. Weight. Age. Parents: 9i7S c?'437 Mid-parental .... Offspring: 9847 mm. 170 I184 / 200 185 176 188 180 174 190 gms. 4,305 2,550 3,140 3,722 1,920 1,800 1,970 1,750 1,790 Adult. 20 weeks. 30 weeks. Adult. 18 weeks. 17 weeks. Do. 16 weeks. 18 weeks. 9 848 Q 840 c?'8<;i Ji8<:2 These young early (16 to 18 weeks) attained a large size, indicating conditions favorable for growth. In ear-length they fluctuated about the mid-parental condition, which was exceeded by 2 individuals, while 3 fell short of it. All had ear-lengths intermediate between those of their respective parents. The range of variation among them at 18 weeks was 14 mm., exactly the same as in cross 3, from which the mother was derived. The greatest deviations from the mid-parental were —11 (at 16 weeks) and -I-5 (at 18 weeks). No e^^dence is afforded of unusual EAR-SIZB 29 heterogeneity among the gametes of either parent, although both were cross-bred individuals. Females 175 and 178 (cross 3) were also used in back-crosses with a lop-eared male (179), resulting in the production of three-quarter-blood lops. The character of these young is shown in table 18. Table 18. Mating i. Mating 2. Ear- length. Weight. Age. Ear- length. Weight. Age. Adult. Do. Do. 20 weeks. 26 weeks. 19 weeks. 26 weeks. Do. Do. Parents : 9175 ^"179 Mid-parental Offspring: 9487 (5*491 (5'492 c?493 mm. 170 210 190 182 182 I 190 / 200 170 gms. 4.305 3.410 3.857 2.740 1.795 2.035 3.330 1,900 Adult. Do. Do. 6 months. 16 weeks. Do. Adult. 16 weeks. [ Parents: ! 9178 6^179 Mid-parental Offspring: 9546 ?547 (5*548 SS49 9550 9552 mm. 170 210 190 190 '85 »9S 193 200 185 gms. 4,070 3.410 3.740 2,242 3.«6o 2,320 2,900 2,460 2,360 The data for mating i are incomplete; but those recorded for mating 2 are entirely satisfactory. They show a total range of variation in ear- length at 26 weeks of 15 mm. which is very similar to that found in cross 3, by which the mother w^as produced. The greatest minus deviation from the mid-parental ear-length is 5 mm.; the greatest plus deviation, 10 mm.; the average deviation, 4.6 mm. The nearest approximation to the short-eared parent is 15 mm., to the long-eared parent 10 mm. The inheritance may fairly be described as blending, with no evidence of seg- regation in Fj. The half-blood lop (^176 was also employed in a back-cross with his mother, the Belgian hare, 9431 (plate 3, fig. 9). Table 19 shows the result obtained. Table 19. Ear- length. Weight. Age. Parents: 94^1 mm. 118 166 142 145 142 135 135 128 gms. 3.400 4.130 3.765 2,660 2.87s 2,520 2,690 2,180 Adult. Do. Do. 27 weeks. Do. Do. Do. Do. (?i76 Mid-parental .... Offspring : (5*52o 9 C2I (5^522 9 ea-i 9 C24 Average 137 2,581 30 INHERITANCE IN RABBITS The offspring fluctuate in ear-length about the mid-parental condi- tion; 2 of them exceed it, 3 fall short of it. The minus deviations, how- ever, are greater than the plus ones, precisely as in lop-eared rabbits bred inter se (p. 17). The range of variation is 17 mm., which is greater than that occurring among short-eared rabbits, but less than that occur- ring among lop-eared rabbits. Another son of 9431, own brother to J' 176, was Ukewise mated with his mother. This male (177, cross 3) had ears 10 mm. shorter than those of his brother (d'176). The mid-parental ear-length, accordingly, was only 137 mm. Two young only were reared to an age of 20 weeks, and each of them had an ear-length of 125 mm. Further tests of the half-blood lop females 175 and 178 are afforded by the crosses recorded in table 20, with a related three-quarter-blood lop male (319) produced by cross 4. Table 20. Mating i. Mating 2. Ear- length. Weight. Age. Ear- length. Weight. Age. Parents: 9i7S c?3i9 Mid-parental Offspring : 9674 c?67S d'677 c?678 mm. 170 200 185 185 175 192 180 gms. 4,305 3,955 4,130 2,400 2,350 2,410 1,420 Adult. Do. Do. 20 weeks. Do. Do. Do. Parents: 9178 (5^319 Mid-parental Offspring: Litter i — ?66o 6^661 Litter 2 — 0^754 c?75S mm. 170 200 185 195 191 150 162 gms. 4,070 3,955 4,012 2,100 2,750 1,460 1,860 Adult. Do. Do. 20 weeks. Do. 19 weeks. 18 weeks. The young produced by mating i are all intermediate in ear-length between their parents. One ( 9 674) exactly attains at 20 weeks of age the mid-parental ear-length, a second ( s 678) would doubtless have done so had he not fallen into bad condition at about 13 weeks of age. Previous to that he had been one of the largest and longest-eared rabbits in the htter. Of the remaining 2, both of which developed normally and were of large size at 20 weeks of age, one exceeded the mid-parental ear-length by 7 mm. and the other fell 10 mm. short of it, approaching to within 5 mm. of the ear-length of the short-eared parent. The range of variation (17 mm.) is not excessive, and the result may be described as a fully normal blend, with no indication of heterogeneity among the gametes of the cross-bred parents. Mating 2 yielded 2 htters very different in character and illustrating rather strikingly the influence of external conditions on growth. Litter I consisted of 2 young only. They were born in summer and developed under optimum conditions as regards food supply. At 20 weeks of age they had attained large size and had ear-lengths exceeding by 6 and 10 EAR-SIZE 31 mm. respectively the mid-parental ear-k-rif^th. Litter 2, on the other hand, was born in the winter. It consisted originally of 8 individuaLi. The 2 weakest ones in the litter died, one previous to, the other subsequent to weaning. The 4 largest ones were stolen, leaving 2 survivors, cT 754 and c? 755, both of which when last measured gave evidence of having been ])ermanently stunted in size and ear-length by the hard conrlitions under which they had developed. They are too abnormal to throw any light on the inheritance of ear-length in this cross. MATINGS OF THREE-QUARTER-BLOOD LOPS. The male 319, employed in the matings last described, was also used in crosses with short-eared rabbits and w:th a three-quarter-blood lop, his sister. The results of the crosses with short-eared females are shown in table 21. Table 21. The 7 young produced b\' mating i iluctuate about the mid-parental condition of ear- length. The greatest minus variation is 11 mm., the greatest plus variation 8 mm., giving a total range of variation of 19 mm. This is not large, considering that the difference between the parents is 95 mm. The greatest deviation from the mid-parental, 11 mm., is 36 mm. removed from the nearest parental ear-length, that is, it is less than one-third as great as the least deviation from either parent. The inheri- tance is unmistakably blending. Even more clearly is this the case in mating 2. The parents dilTer in ear-length by 100 mm. The young are all almost exactly intermediate. The entire range of variation in the 6 young is only 5 mm., while the nearest approximation to the car-length of either parent is nine times this amount. A better example of fully blending inheritance can scarcely be imagined. In neither mating do we get evidence of heterogeneity among the gametes formed by the three- quarter-blood father (,J3i9). 32 INHERITANCE IN RABBITS It is of interest to note that both the mothers employed in matings i and 2 were employed also in cross 2, with the lop $ 179. In that case, also, they gave a distinctly and fairly uniform blending result. Male 319 was mated also with his sister (9322), producing a litter of only 2 young. These closely resembled their parents in ear-character. (See table 22.) Table 22. Ear- length. Weight. Age. Parents: Q 122 mm. 195 200 197-5 208 190 gms. 4.450 3.955 4,202 2,680 1.950 Adult. Do. Do. 20 weeks. 16 weeks. (^319 Mid-parental .... Offspring : 9 770 9780 The latest measurement recorded for one of them ( 9 780) was made at 16 weeks of age, but aheady she had attained an ear-length of 190 mm. At maturity she would doubtless have equaled or exceeded the mid- parental ear-length. The other one (9 779) did exceed the mid-parental ear-length at 20 weeks of age by more than 10 mm., and she exceeded by 8 mm. the ear-length of the long-eared parent. Her size also at 20 weeks of age was very large, viz, 2,680 grams. This unusually great plus vari- ation was doubtless due in part to extremely favorable conditions during the growth period, especially during the period of lactation. During that period the mother's milk was divided among 3 young only, but i of these died soon after the young were weaned. At the last measurement recorded, its ear-length was a httle less than that of 9 780, while in size it was inferior to both 9 779 and 9 780. Table 23. Ear- length. Weight. Age. Parents : Q 722 mm. 195 210 gms. 4.450 3.410 3.930 2,460 2,78s 2,220 2.510 1,970 2,86s 2,080 2.235 Adult. Do. Do. 20 weeks. 25 weeks. 20 weeks. 25 weeks. 20 weeks. 33 weeks. 20 weeks. 25 weeks. r?l70 Mid-parental .... Offspring: Q c8o 202.5 205 210 200 205 195 200 190 195 Q coo rT'coi 9592 The same three-quarter-blood female (322) which was mated with c?3i9 (table 22) was mated also with the lop s 179, producing a litter of seven-eighth-blood lops. (See table 23.) Two of the 4 young reared had EAR-SIZE 33 at 25 weeks of age ear-lengths identical with those of the respective parents, viz, of 195 and 210 mm. The other two had intermediate ear-lengths of 200 and 205 mm. resjjectively. This is a fully normal blending result. The total range of variation is 15 mm. In both car-length and size the young are similar to those produced by the mating with (^319 (table 22). Cross 7. — Quarter-blood Lop Fem/Vle X Short-eared Male. Three different quarter-blood lop females, 521, 522, and 524 (table 18), produced by a mating of the Belgian hare with her son (d" 176), were mated with a son of the same Belgian hare by an unrelated short-eared male. (See table 14.) The outcome of these matings is shown in table 24. Table 24. Mating i. Mating 3. Ear- length. Weight. Age. Ear- length. Weight. Age. Parents: 9521 (^235 Mid-parental Offspring: dSog c?8io 98ii 9813 c?8i4 9815 mm. 142 no 126 "5 1 30 "5 125 126 gms. 2,875 2,945 2,910 2,080 1,890 1,820 2,03s 1,960 2,150 27 weeks. Adult. 20 weeks. Do. Do. Do. Do. Do. Parents: 9524 c?235 Mid-parental Offspring: cJ;834 <^835 9836 9837 9838 mm. 128 no ,19 "5 "S 121 116 134 gms. 2,160 2,945 2.552 1,600 1,700 1,820 1,450 2,000 27 weeks. Adult. 20 weeks. Do. Do. Do. Do. Mating a. Parenu: 9522 (5*235 Mid-parental Offspring: 9823 d'824 13s no 122.5 125 125 2,520 2,945 2,732 2,300 2,250 27 weeks. Adult. 20 weeks. Do. The offspring show, as regards ear-length, a rather wide range of vari- ation, 20 mm., which is nearly two-thirds of the difference in ear-length between the parents. The average ear-length of the offspring corresponds, in each Utter, closely with the mid-parental ear-length, the j^lus and minus deviations being, except in mating 3, about equal in number and amount. In mating i, 3 of the 6 young have api)roximately the mid-parental ear- length, but 2 show minus deviations of 6 and 11 mm. respectively, and i shows a plus deviation of 9 mm. The 2 young produced by mating 2 were of large size at 20 weeks of age, indicating conditions of nutrition above the average. The ear-length of each exceeds by 2,5 mm. the mid-parental ear-length. The 5 young produced by mating 3 show 2 minus deviations of 3 and 4 mm. res])cctively, and 3 j)lus deviations of 2, 6, and 15 mm. respectively. 34 INHERITANCE IN RABBITS Cross 8. — Quarter-blood Lop x Three-quarter-blood Lop. A single mating of this sort produced a litter of 3 young, all very sim- ilar and close to the mid-parental ear-length. (See table 25.) The observations were discontinued when the young were 14 or 16 weeks old, but the mid-parental ear-length of the parents, when they were 20 weeks old, had already been closely approximated. The deviations were —2, — 5, and —5 mm. If growth progressed normally from the age of 14 or 16 weeks on, they would surely have attained the adult mid-parental ear-length, viz, 167.5 mm. Table 25- Ear- length. Weight. Age. Parents : 9 C2-* mm. \ 130 1 13s 1 19s 1 200 162.5 1 167.5 160 157 157 gms. 1,930 2,690 2,540 3,330 2,235 3,010 1,450 1,300 1,450 20 weeks. 27 weeks. 20 weeks. 43 weeks. 20 weeks. Nearly grown. 14 weeks. 14 weeks. 16 weeks. r^AQ 2 Mid-parental. . . . Offspring: 884 885 886 LIMITATIONS OF THE DATA STUDIED. In attempting to draw conclusions from the statistics presented in the foregoing pages, one must bear in mind certain of their Hmitations and imperfections. (i) Ear-length is modified to some extent by external conditions. If the young rabbit is well nourished up to the age of 20 weeks, its ear-length will be greater than if it is poorly nourished, other conditions being equal. While we have attempted to give our rabbits the best of care at all sea- sons, it is inevitable that the quality of food supplied at different seasons of the year should vary, and with variation in the quaUty of the food goes variation in the growth rate. This renders it difficult to compare with each other, as regards ear-character, rabbits reared at different seasons of the year. But it has been impossible for us to rear enough rabbits at any one season to afford adequate material for comparisons. Hence we are forced to utihze material produced at different seasons of the year. (2) Size of litter is of some consequence in determining the growth rate of a rabbit. If there are several young in a litter each gets a smaller amount of food during the period of lactation than it would have received had the litter been smaller. Our material, however, is not extensive enough to allow us to institute comparisons merely between litters of substantially the same size. EAR-SIZE 35 (3) It is the belief of fanciers that a warm, moist atmosjihere, during the period of active growth of the ears, favors the attainment of large ear-size. This view we have not been able to juit to an experimental test, but we are inclined to think that the temperature and humidity arc much less important factors than abundant food supply. (4) Rabbits of the small, short-cared races have a shorter growth period than the larger races. Their ears are more likely to be full-grown at 20 weeks of age than are those of loj)-eared rabbits. Therefore, in comi)aring rabbits of ditTerent ancestry at the same age, say 20 weeks, one is in dan- ger of underestimating the ear-length of the larger-sized rabbit. (5) A cross between rabbits of entirely different races is likely to result in young of unusual vigor, which causes them to attain a greater weight and ear-length than the hereditary constitution of either race by itself would result in. This is illustrated notably in cross 3, page 20. Supe- rior size or ear-length, induced by crossing, we should not e.xpcct to be per- manent in later generations. (6) Disease frequently interrupts the orderly progress of a growth-curve and necessitates the omission altogether of certain series of observations. CONCLUSIONS. Notwithstanding these limitations, which manifestly restrict the scope of our conclusions, certain generaUzations are clearly justified. (i) A cross between rabbits differing in ear-length produces offspring with ears of intermediate length, varying about the mean of the parental ear-lengths. (2) It is immaterial whether the larger parent was father or mother; the result is the same in either case. As regards ear-length, then, we may say, reciprocal crosses give the same result. This shows that ear-size is a character inherited with equal intensity through father or mother. (3) A study of the offspring of the primary cross-breds shows the blend of the parental characters to be permanent. No reappearance of the grand- parental ear-lengths occurs in generation F2, nor are the individuals of that second generation as a rule more variable than those of the first gener- ation of cross-breds. Fig. 3 shows the most extreme case of "scatter" in F2 that we have observed. Yet the variation in this case is no greater than among the young of lop-eared rabbits bred inler se. (4) The extreme range of variation in ear-length among short-eared rabbits is about 10 mm.; in lop-eared rabbits it is two or three times as great, or from 20 to 30 mm. Among rabbits produced as crosses of vari- ous sorts between short-eared and lop-eared rabbits the range of varia- tion in ear-length is mostly intermediate in amount. (5) The form of the growth-curve for ear-length from the age of 2 weeks on is convex upward, indicating a steady diminution in the daily growth increment. PART II. — WEIGHT. Our statistics for size inheritance arc not very satisfactory, because we were unable to keep any considerable number of rabbits until they were full grown, owing to the smallness of our breeding room, so that a large number of weighings of adults is not available for jjurposes of compari- son. But the size of a growing rabbit varies greatly with the character of its food, and this in turn is dependent upon a variety of conditions which it was not possible for us fully to control. A com[)arison of the weights of growing rabbits at correspomh'ng ages is, therefore, not alto- gether satisfactory, yet it is the best material we have. In tables i to 25 the latest available weighing, or the heaviest weight, is recorded for each rabbit. But since the weighings there recorded were made at very different ages, it is necessary to select some particular age at which to make comparisons. The age of 18 weeks has been selected, because the weighings for that age are most numerous. In table 26 are shown the average weights, at 18 weeks of age, of different lots of rabbits, each lot containing those of like ancestry. The number of individuals in each lot is also shown in the table, as well as the greatest range of variation in weight found in any litter of each lot. The statistics in table 26 are fullest for those crosses (left section) in which ordinary short-eared rabbits were concerned. The average weight of such rabbits, in a lot of 17 individuals, is seen to be 1,412 grams. For lop- eared rabbits it is something over 1,743 grams, the weight given in the table from observations on 2 rabbits. This weight, however, has been exceeded at 14 weeks of age by a majority of the lop-eared rabbits which we have reared, so that it is certainly too low. The lots of rabbits, partly of short-eared, partly of lop-cared ancestry, have intermediate weights, the weight tending to increase with increase in the proportion of lop blood. The variability (range) in weight, which was found to be twice as great in lop-eared as in short-eared rabbits, is intermediate in the cross-bred lots, increasing with increase in the propor- tion of lop blood. Both the position of the average for each lot, and the amount of variation within it, indicate that weight-inheritance, like the inheritance of ear- size, is blending in character. Neither dominance nor segregation in the Mendelian sense are recognizable. The Belgian hare crosses and mixed crosses, recorded in the last sec- tions of tabic 26, show, in general, results similar to those given by the crosses with short-eared rabbits, but man\- of the averages are less reliable 37 38 INHERITANCE IN RABBITS because based on too few individuals (in 4 cases, a single litter each time). The Belgian hare was heavier than the short-eared stock used, and it will be seen that, in all cases, her descendants exceed in size animals of the short-eared series having a like amount of lop blood. Further, a mixture of short and Belgian blood tends to produce a rabbit intermedi- ate in weight between those of the short and of the Belgian series, respec- tively. (See table 26, right section.) All these observations confirm the idea that body-weight is a character blending in its inheritance. Table 26. — Size at 18 weeks of age of rabbits of different proportions of lop ^^ blood," from crosses with short-eared, with the Belgian hare, or with both. Lop blood. Short-eared. Belgian hare. Mixed. Average weight. Max. varia- tion in weight. Individ- uals ob- served. Average weight. Max. varia- tion in weight. Individ- uals ob- served. Average weight. Max. varia- tion in weight. Individ- uals ob- serv e. None gms. 1,412 1.592 1.463 1,700 1.585 1,652 1.743* gms. 315 345 315 420 420 495 8oo2 17 20 17 18 35 17 7 gms. 1,788 2,076 1.965 1,940 1.954 1.936 gms. .... 627 260 820 890 890 590 5 4 .... 12 10 gms. 1.791 1,580 1,888 1.754 gms. 490 205 575 420 13 4 14 II One-eighth One-fourth Three-eighths. . . . One-half: Gen. I Gen. 2 Both Three-fourths : Gen. I Gen. 2 Both Seven-eighths . . . All 4 1 This is certainly too low, for in litter 70, table 2, mating 3, it was surpassed by three of the five rabbits of the litter, already at 14 weeks of age. The average given (1,743) is for the two ani- mals, 6^179 and 9 180 (table 2, mating i). 2 At 14 weeks. When the parents differ in size, the young are clearly of intermediate size, but our observations are too incomplete to show in most cases whether the size is midway between that of the respective parents or not. Prof. W. C. Sabine has kindly pointed out that if linear dimensions give a mid- parental condition (the mean of the respective parental conditions), then we should expect the weights to be less than the mean of the parental weights, provided the proportions of parts are the same in all cases. But the proportions of the parts are different in the two parents, when rabbits of different size are mated with each other, and the proportions in the off- spring are unmistakably intermediate between those of the respective parents. This, perhaps, accoimts for some of the peculiarities observed in com- paring weights of the rabbit c? 248 with those of his parents, and with the mid-parental weights. All three rabbits were fully grown (2 or more years old) when the observations were made, and these are fairly com- plete. The maximum body-weight recorded for S 248 was somewhat WEIGHT 39 in excess of the mid-parental (table 27), but since he shows a less per- centage of bone to total body-weight than either i)arent it is j)robable that the excess is due in part at least to some tem[)orary condition (fat- ness). If he showed a mid-j)arental ])ercentage' of bone to body-weight, and this would possibly be the case if all 3 rabbits had been in like con- dition, as regards fatness, then his weight should be something less than the mid-parental weight, or about 3,326 grams, instead of the mid-parental weight (3,800) or the observed maximum weight (3,930). But in the absence of more extensive observations, we can not be certain that the percentage ratio of bone to body-weight is a mid-percentage. The ques- tion must remain an o])en one until further data can be accumulated. Table 27. Relation between hone-weights and total body-weight in the rabbit <3 248 and in his parents. Bone- weight. Bodv- weight. Per cent bone- weight. Old 9lop J>4<; gms. 77-15 39-35 58-a5 49-7 1 4,600 1.67 Mid-parental Son, (^2^9> 3,800 3.930 .... 1.36 As regards bone-size, however, we can reach more satisfactory conclu- sions, for this character is unaffected by temporary conditions of the tlesh. In table 29 are recorded bone measurements of this family of rabbits, which show the measurements of the son ( The percentages given are based upon the combined weights of particular bones, not of all the bones of the body. ' A comparison of the weights and volumes of corresponding bones in t.iblc 28 indicates that the specific gravity of the bones of the son (d^248) was slightly less than that of either parent, \\z, about 1. 19 for the son, i.ao for the mother, and i.a6 for the father. 40 INHERITANCE IN RABBITS observed falls below the mid-parental by about three times that amount. It is in fact removed from the mid-parental only a little less than from the weight of the smaller parent. It is difficult to explain this extensive deviation, but it undoubtedly exists and is apparently fairly uniform, though possibly the method of computing the "expected" magnitudes is faulty. The computation is made in the following way. The cube root of the weight for each parent was found. These two roots were then added together, and their half-sum found, which was then cubed. B I E Fig. 4. Relation of the bone-weights of rabbit 248 to those of his parents and to the mid- parental bone-weights. If distances are laid off from a point at the left proportional to the bone-weights, they bear to each other the spacial relations of the following points : A, bone-weights of the mother; B, of the father; D, of the son, c? 248; C, the mid-parental; E, the expected bone-weights of the son. Observed and expected deviate in the same sense from the mid-parental, that is, are less, but the uniform difference in amount between observed and expected is something requiring fuller analysis. Table 28. — Weights and volumes of skeletal parts of c? 248 in relation to those of his parents. [Mother, old female lop; father, (^45; son, (5*248. Plate i, figs. 2, 3, i.] Part. Weights of bones (grams). Mother. Father. Mid- parental. Son. Ob- served. Ex- pected. Devia- tion of observed from ex- pected. Devia- tion of expected from mid- parental. Devia- tion of observed from mid- parental. Humerus Femur Tibia-fibula Innominate (of one side) 1 2 ribs (of one side) Vertebras: I to 6 7 to I 2 13 to 20 Total, I to 20 . Total, counting weight of verte- bras once only . 5-9 10.8 9.1 8.4 6-95 3-2 5-9 5-45 4-1 3-2 4-55 8.35 7.27 6.25 S-07 3-7 7-5 6.1S S-4 3-9 4.4 7.8 7-13 5-9 4-7 -0-7 -0-3 -0.98 -0.5 -0.7 -0.1 -0-5 -0.14 -0-35 -0.3 -0.8s -0.85 -1. 12 -0.85 -1. 17 7-4 9-9 18.7 3-3 4-35 9-8s 5-35 7.12 14.27 4.6 5-95 12. 5 5-04 6.6 13-7 -0.44 -0.65 -1.2 -0.31 -0.52 -0.5 -0.75 -1. 17 -1.77 36.0 17-5 26.75 23-05 25-5 -2.4 -1.25 -3-7 77-15 39-35 58.24 49-7 55-43 5-58 2.64 8-54 Volumes of bones (cubic centimeters). Humerus Femur Tibia-fibula 5-2 9.1 7-1 2-5 5-0 4.0 3-85 7-05 5-55 3-1 6-5 5-0 3-65 6.94 5-40 -o-SS -0.44 -0.40 —0.20 -O.II -0.15 -0-75 -0.55 -0.55 PART III. — SKELETAL DIMENSIONS. Skeletons were prepared of certain of the rabbits concerned in this series of experiments, and uj)on these several series of measurements were made. The most complete scries arc recorded in tables 29 and 30. In one case (cross i, table 3) the skeletons of both parents were pre- served, as well as that of one of the fully-grown young, viz, d" 248. The measurements of this animal (recorded in table 29) are approximately intermediate between those of his respective parents. They include 7 different skull measurements and 7 of other parts, chiefly bones of the appendages. The skull of the lop-eared rabbit is relatively much longer and more slender than that of short-eared rabbits. (See plate 4.) The proportions of half-blood lops (like their absolute dimensions) are intermediate, cor- responding closely with the mid-parental or mean of the parents in this respect. (Sec table 29, ratios.) The limb bones are shorter in proportion to the length of the innominate bone in lop-eared than in short-eared rabbits. In this j)articular also part-blood lops are intermediate. (See tables 29 and 30, ratios.) In the case of the rabbit igment i)at- tern of the individual hair has been jjresented elsewhere (Castle, :07a). It was first recognized in the case of the guinea-pig (Castle, :o6) as an essential factor of the "agouti" coat, indeed as the only feature which differentiates the agouti variety from black. Hence the symbol A (agouti) was adopted to designate it. Cuenot (104) employed the symbol G to designate in mice the agouti or gray coat, and designated black by a differ- ent symbol, but he failed to recognize that gray is simply black plus a second factor. Hence his G equals B (black) plus A. Hurst has inde- pendently discovered the existence of the A factor in rabbits (Proceedings Seventh International Zoological Congress, unpublished). In the guinea- pig, a new color variety, cinnamon-agouti, has been dehberately produced through the agency of the independent factor A. (See Caistle, : 08.) The Factor for Uniformity of Pigmentation, U, or Spotting with White, S. The factor U (uniformity of pigmentation) is alternative with spotting with white, S. Its existence was first established by Cudnot (104). Like I, the intensity factor, it may be regarded as a modifier of C, though not identical with it; for U and S are transmissible through albinos, which themselves have no pigmentation and which by h}pothesis lack the fac- tor C. U is also demonstrably independent of any particular color, for spotting with white is transferable in crosses from one color variety to another, as, for example, from black to yellow. The Factor for Extended Distribution of Black or Brown, E, Alternative with R (Restricted Distribution). The assumed factor E is a modifier of black and brown, but not of yellow pigmentation. It is alternative to R, a restricted distribution of black and of brown jMgments, in which distribution they are conlined to the eyes and to the skin of the extremities. The distribution of yellow pigment (Y) is wholly unaffected by this factor. When black and brown are restricted, yellow remains as the principal or even as the exclusive pig- mentation of the hair (yellow varieties). That E really exists as an independent factor, and not as a condition merely of black or of brown, is shown by the following ex|X'riment. If one crosses a brown ("chocolate") guinea-pig with an ordinary yellow 48 INHERITANCE IN RABBITS one (black-eyed), the young are black pigmented, but in F^ 4 varieties are obtained, viz, black, brown, black-eyed yellow, and brown-eyed yellow. The case, at first thought puzzUng, is entirely plain if we con- sider the distribution independent of the kind of pigment. In the original cross extended brown was combined with restricted black. Extension dominated restriction, and black dominated brown, but in Fj black and brown each occurred both in the extended and in the restricted condi- tion. Plainly the case is one of MendeUan dihybridism, in which two independent pairs of alternative characters are concerned. The extension factor (E) may be replaced, not merely by the extreme condition (R) in which black and brown pigment are absent from the fur, but also by conditions of restriction less extreme, in which spots of black (or brown) occur on a background of yellow. Such intermediate con- ditions (E', E", etc.) are heritable, and are alternative with E and R, respectively. In some of these intermediate conditions the spots are of large size and sharply limited, in others the spots are numerous and small. Each condition has a tendency to breed true, i. e., is alternative to other conditions of E. INTERRELATIONS OF FACTORS E AND U. Spotting with black or brown on a yellow background is independent of spotting with white, though the two may coexist. The one is due to a modification of E, the other to a modification of U. When E and U are both unmodified the animal is of course black (or brown) pigmented all over. When U alone is modified (and occurs in condition S) , the animal is black (or brown) but spotted with white. When E is modified (to E' or E") but U is unmodified, the animal is spotted with black (or brown) on a yellow background, but is devoid of white. When both E and U are modified (to E' or E" and to S, respectively) the animal bears two differ- ent sorts of colored spots on a white background. The spots are either black and vellow or brown and vellow, and constitute with the white background on which they lie the so-called "tricolor" condition, well known in the case of guinea-pigs, dogs, cats, and mice. It is a singular fact that spots of black and of brown do not occur on the same animal, so a 4-colored condition is never attained. The reason for this is apparent, if the hypothesis stated in this paper is correct. The distribution of black and of brown is controlled by the same factors, E and S, so that when black and brown are present together, their distribution is the same, and black because of its greater opacity covers up the brown. The " black-and-tan " dog is, we believe, an apparent, not a real, excep- tion to this generahzation; for the "tan" is not a chocolate-brown pigment such as is found in the brown water-spaniel, but merely a yellow pigment. The black-and-tan dog is not a spotted dog, but is a black dog plus a color- pattern, similar to the agouti-pattern of guinea-pigs and rabbits. In COLOR 49 this pattern black is largely excluded from the lower surfaces and from a spot over each eye, where yellow then shows. The correctness of this hypothesis is shown by the existence of this same i)attern anmng brown- pigmenled dogs. The brown-and-tan has chocolate-brown i)igment above and tan (yellow) below, as well as a spot over each eye. It bears the same relation to self-brown that black-and-tan docs to self-black. On this interpretation brown-and-tan is brown plus |)attern, and black-and-tan is black plus pattern. If, then, brown-and-tan is crossed with self-black, black-and-tan offspring should result in Fj, and in F, there should be obtained black-and-tan, brown-and-tan, self-black, and self-brown, in the proportions 9:3:3: i. The experiment is commended to dog breeders. INTERRELATIONS OF FACTORS B, Br, AND Y. Returning, after this digression, to a consideration of the interrelations of the three pigments, black, brown, and yellow, the fact seems clearly established that black and brown are closely related but alternative con- ditions dependent for their distribution upon two factors, which we may designate E and S, whereas yellow is dependent for its distribution solely upon one of these two factors, S. It would seem probable, therefore, that in the genesis of the hair jjigments, yellow is a first product of the interaction of C and Y, which may or may not be further modified to pro- duce brown or black, depending upon whether certain other factors (B and Br) are or are not present. The amount and distribution of the yellow pigment produced is conditioned by a factor which may assume phases U, S, S', etc. The amount of the yellow pigment which is converted into black or brown and its distribution is conditioned by another factor which may assume phases E, E', etc., to R. Gametic Structure and Variation. A diagram Hke those employed by the organic chemist may help to show the relationships to each other of these 8 assumed pigment factors. C, the general color factor, is indisj)ensable y B to the manifestation of any of the others. All the others may be represented as linked directly or indirectly with it. E, however, is a modifier of B and Br alone, and is there- fore joined with them alone in the diagram; and since B and Br are assumed to act only after Y has acted, they are represented as joined with it. Homozygous gray rabbits, wild ones for exam|)le, possess and transmit all these 8 factors in each of their gametes. The diagram, therefore, expresses their gametic composition. A homozygous black rabbit lacks, of all these 8 factors, A alone. A yellow rabbit has R (restricted) in place of E (extendcfl black or brown), but otherwisi- is like- the gray, or else the .\ C Y E 50 INHERITANCE IN RABBITS black rabbit. Those with A and those without A are, however, visibly different. Theoretically, if each factor is capable of independent variation, 256 different gametic combinations should be possible. In reahty we are acquainted with 18 visibly different color varieties, and we have evidence that 48 different gametic combinations are capable of realization. This leaves still a wide discrepancy between theoretical and known, and leads to the conclusion either that many as yet unknown mutations are possible in the rabbit, or that couplings may exist among these factors which pre- vent their independent action. We have evidence of independent variation on the part of the factors A, C, I, U, and E, each of which has in one case or another either been lost or been replaced by the alternative condition already described; but B, Br, and Y are unvariable; at least we have not ourselves seen evidence among rabbits of independent variation on the part of these factors. There can be no question, however, that both in the guinea-pig and in the mouse such variation has occurred, resulting in the complete loss of B from the gamete, and it is possible, as elsewhere stated, that such a change has already occurred among European rabbits. Supposing, how- ever, that B, Br, and Y are all constant constituents of the rabbit gamete and that each of the five others may be either present or absent, the num- ber of different gametic combinations theoretically possible becomes 32. We have reason to believe that this entire assortment is produced and that 16 other ones also occur owing to a second and different sort of vari- ation in factor C. Gametic and Zygotic Formul-e. The diagram given on page 49 was intended to express the known aggre- gate of independent factors which a pure gray rabbit transmits in each of its reproductive cells (gametes). In producing a new individual each re- productive cell must unite with another reproductive cell, the two together forming a zygote. An individual resulting from the union of two gam- etes of hke constitution will be double as regards each hereditary factor. It is known as a homozygote (Bateson). This double condition we might express by a subscript 2 following the symbol for each factor indicated. We should then have a zygotic formula for the individual. But it sometimes happens that a gamete unites with another gamete having a composition slightly different from its own — one which, for example, lacks one or more factors found in itself. The zygote produced is then a heterozygote and will be double as regards certain factors, but single as regards others. But in sexual reproduction, as is well known, there is a return from the double to the single condition. So that when a heterozygous individual attains sexual maturity, it forms gametes each of which contains the factor double in the zygote, but as regards those which were single in the zygote, half the time they will be present, half COLOR 51 the time absent from the f^amete (or if not absent, then represented by an ahernativc condition). This is simply another way of stating the funda- mental ^Slendelian i)rinciple that heterozygotes do not breed true, but form at least two difTerent kinds of rej)roductive cells. The breeder has to deal ahvavs with individuals, and onlv inrlirecth with gametes. Therefore zygotic formulae are to him quite as im[K)rtant as gametic formula?. Accordingly in what follows we shall endeavor to give the zygotic formula of each variety described. Its breeding capac- ity may quickly be inferred from an insi)ection of its zygotic formula. Kach factor which is double in the zygote will be rejiresented in every gamete formed, each factor which is single in the zygote will be present in only half the gametes formed, or will be represented b\' the alternative (reces- sive) condition expressed in the zygotic formula by a symbol in paren- thesis. The zygotic formula of a gray ra])bit which breeds true (an ordinary wild one, for example) is BoBroEjAX^LU^Yz, and the interrelations of these factors, as at present understood, may be expressed in a diagram. A, C, Y, E2 Other gray rabbits arc single (or heterozygous) as regards one or more of the factors enumerated in this formula, though none of them lacks altogether any one of these 8 factors. When a factor drops out altogether a new color variety is produced. New color varieties have undoubtedly originated in this way in the past, and are still doing so at the present time. A maturation division in which the two components of a double factor should fail to separate (as they do normally) might be the starting- point of a new color variety, since it would result in the ])roduction of a gamete which lacked a particular factor. Abnormal maturation diu- sions, therefore, may be the immediate cause of color variations. COLOR VARIETIES OF THE RABBIT. It is impossible to make a scientific classification of the color varieties of the rabbit without discarding or modifying some of the names now in use; for many of these names are either without significance or are misleading. From a perusal of the literature of the rabbit-fancy, we arc unable to decide what certain named varieties are, and it is more than likely that we are not acquainted at first-hand with many varieties known to the fancy in Europe. All such cases must necessarily be omitted, for the present, from our classification. For convenience we may recognize 4 general color types, viz, (i) gray, (2) black, (3) yellow, and (4) white. Each of the pigmented varieties 52 INHERITANCE IN RABBITS (gray, black, and yellow) may have either intense or dilute pigmentation (disregarding intermediate shades, which, however, exist and are heri- table) . Further, each may either have uniform pigmentation or be spotted with white (disregarding differences in the fineness of the spotting, which, however, exist and are heritable). Further, the yellow may have either pigmented or white under surfaces. Even with categories so inclusive as these, the number of visibly different pigmented varieties rises to i6, and since albinos may either have or not have pigmented extremities, the total number of visibly different varieties mounts to i8. There is every reason to suppose that each of these i8 varieties may be obtained in a homozygous condition. Most of them, indeed, have been so obtained in our experiments. But for each homozygous condition there are possible several heterozygous conditions. An enumeration of all these is unnecessary, as the number is truly stupendous. With 5 inde- pendently variable characters (the number known to be independently variable in the rabbit) the number of different zygotic combinations theoret- ically possible is 243. We shall content ourselves with enumerating the 18 different known gametic combinations, and in giving examples of a few of the different zygotic combinations. Gray Type. (i) Gray, found in wild rabbits; gametic composition — A C Y E I \ / I Br (2) Blue-gray, same as the foregoing, with the substitution of D (dilute) for I (intense). (3) Spotted gray, same as i, with the substitution of S (spotted) for U (uniform pigmentation). (4) Spotted blue-gray, same as 2, with the substitution of S for U. Black Type. (5) Black, same as i without A, namely, f /\ C Y E t \/ (6) Blue (i. e., dilute black), same as 5, with the substitution of D for I. (7) Spotted black, same as 5, with the substitution of S for U. (8) Spotted blue, same as 6, with the substitution of S for U. COLOR 53 Yellow Type. (9) Yellow (with white belly and tail), same as i, with R (restricted) substituted for K (extended black or brown pigmentation), namely, y /\ A C Y R 1 \/ (10) Cream (/. e., dilute yellow), same as 9, with D substituted for I. (11) Spotted yellow, same as 9, with S substituted for U. (12) Spotted cream, same as 10, with S substituted for U. (13) Sooty (yellow with pigmented belly and tail), same as 9 without A, or as 5 with R substituted for E, namely, I Br (14) Pale sooty, same as 13, with D substituted for I. (15) Spotted sooty, same as 13, with S substituted for U. (16) Spotted pale sooty, same as 14, with S substituted for U. WmTE Type. (17) White (wholly unpigmented), in any of the foregoing 16 varieties with C omitted. (18) Himalayan white, a pink-eyed albino variety differing from 17 in appearance, in having black pigmented extremities (nose, ears, feet, and tail) and in having fur of a creamy white, not of a snowy white as in 17. Those with which we have exjierimented seemed to be of the formula ' — y - Y E 1 \./ That is, they were black pigmented rabbits (see 5) in all points except C. It would seem that we must assume the presence of C in some form in an animal which like these does bear a certain amount of pigment. Nevertheless this C is not the same as the C found in dark-eyed pigmented varieties, for a cross of Himalayan with other albinos produces no dark-eyed ofTs])ring, and gives no increase of pigmentation over that found in the Himalayan parent, but rather a diminution of it (see Castle, :o5). If, then, we assume C to be j^resent in the Himalayan, it must be in a greatly modified form, as compared with its condi- tion in dark-eyed animals. This is why we use C rather than C in the formula. The factors E, I, and U, were all found to be present in our Himalayan rabbits, but not A, for crosses of Himalayan with homozygous gray gave only gray in •April, 1909. Himalayan ral>l)its have now iK-cn produri-d whidi contain al.so factor .A. They have extremities lc.i this variety were produced by the Belgian hare (9 431) in a mating with an albino rabbit (c? 56). These gray ral»bits (2 males and t, females, 232 to 236), when mated inter se produced 17 gray, 5 black, 6 yellow, 2 sooty yellow and 6 white young, the expected Mcndelian proportions being 27:9:9:3: 16. When mated with albinos these same gray rabbits produced 9 gray, 6 black, 2 yellow, and 16 white young. The zygotic composition of the aIl»ino mates in this case is not fully known, so that the theoretical jjroportions of the pigmented young can not be stated. The albinos are as expected approximately half the total young, and all expected color varieties are represented except sooty yellow. Table 36. — Matings and young of 9 178, gray. Mating. With 6^176, gray (variety 5) . Expected Withc?i77, gray (variety 6) . With 0^505, gray (variety 6)' Total for last two matings Expected With o'i79, yellow With c?3 19, yellow Total for last two matings Expected Gray. Black. YeUow. Sooty. 8 2 3 I I a S 3 I 6 5 I (?) 9 3 3 I I 3 3 8 3 4 6 9 6 7 6 I I I I 1 Plus 3 yellow or sooty, died young. For simplicity the young of all 5 gray rabbits are grouped together in the foregoing account. In reality, however, one of the males was heterozygous in factor U, and at least i male and i female were heterozygous in factor I, as is shown by the facts (i) that several of the young produced in matings with white individuals bore spots of white, the whitest of all being belted with white ("Dutch marked"); and (2) that one of the gray offspring of the gray parents was of a pale blue-gray color. (9) The gray rabbits just described, which produced a blue-gray young one, in reality belong to a ninth variety of gray rabbit, indistinguishable in appear- ance from the other eight, but producing a different assemblage of young, viz, dilute-colored as well as intensely colored young in each of the pigmented vari- eties, and also albinos. The whole assemblage of visibly different varieties is gray, black, yellow, sooty yellow-, blue-gray, blue, pale yellow, pale soot)' yellow, and white. Not all of these varieties were obtained directly from the pair of rabbits in question, doubtless because too small a number of young was produced, but in later generations all were obtained. Thus from the single blue-gray individual, when mated within the same family, were obtained blues, pale sooties, and pale yellows, as well as individuals of normal intcnsitv. The zygotic formula of this ninth variety of gray ral)bit is B,BrJ-"(R).\ri(n)V,Uj. Since it indicates a heterozygous condition in 4 character-units, we should expect a pair of individuals of this formula to produce 16 different gametic combinations. Eight of these are represented in the enumerated 8 classes of pigmented young. 8 others would occur among the albinos which would differ from the pigmented classes in the absence of C, but all of which would look alike, though breeding differently in crosses with pigmented animals. 58 INHERITANCE IN RABBITS (10) The gray rabbit which produced spotted offspring in crosses with albinos was undoubtedly heterozygous in regard to the factor U, for experience has shown (in agreement with Hurst, 105) that uniform pigmentation is in the main domi- nant over spotting. We might then recognize as a tenth variety one of the formula B2Br2E(R)ACl2U(S)Y2. (11) Had the rabbit in question produced pale-pigmented as well as spotted young (and such we have since derived from this stock of rabbits), we should need to modify the formula as given by writing 1(D) instead of Ij, i. e., intense (dilute recessive). The formula of such a rabbit would be B2Br2E(R)ACI(D)U(S)Y2. This indicates a heterozygous condition as regards 5 character-units, and rabbits of this formula should be capable of producing 32 different gametic combinations, 16 of which would be visibly expressed in different pigmented varieties, while an equal number lacking the factor C would produce albinos visibly alike but gametically different. If, then, we were to carry to its logical conclusion the enumeration of the conceivable different varieties of gray rabbit, all alike in appearance but all different in breeding capacity, i. e., of different zygotic formula, w^e should need to mention 32 varieties: 8 of these would correspond with the first 8 which have already been enumerated and the existence of which has (except for variety 7) been demonstrated, namely: (i) Gray producing gray only. (2) Gray producing gray and black. (3) Gray producing gray and white. (4) Gray producing gray, black, and white. (5) Gray producing gray and yellow. (6) Gray producing gray, black, yellow, and sooty. (7) Gray producing gray, yellow, and white. (8) Gray producing gray, black, yellow, sooty, and white. Eight other varieties would produce the same sorts of 3'oung as these 8, but would produce in addition dilute pigmented ones of the same color types, i. g., blue-grays as well as grays, blues as well as blacks, pale yellows (cream) as well as yellows, and pale sooties as well as sooty yellows. The 16 remaining varieties would produce the same sorts of young as the 16 varieties already described, but would produce spotted as well as uniformly pigmented (self) individuals. Table 37. Color. Observed. Expected. Gray Black Yellow Sooty Blue-gray Blue Cream Pale sooty 24 8 16 2 8 2 3 2 27 9 9 3 9 3 3 I COLOR 59 Not every one of these 32 varieties of gray rabbit has actually been demonstrated to exist in the course of our experiments; 10, however, which we have shown to exist, have already been mentioned, and several others are known. For example, by crosses of black with pale yellow (cream) or of blue with yellow, we have obtained grays which produced the same sorts of young as variety 6, and in addition blue-grays, blues, creams, and pale sooties. Such was the character of our gray females 1413, 1457, '525> 1526, and 2009. By a male of hke character they have produced young as shown in table 37. Other gray rabbits producetl by the same cro.sses, black X cream, or blue X yellow, produce the same assortment of young, and in addition albinos. That is, they are hke variety 8, but heterozygous in intensity of pigmentation. These gray rabbits, females 1423, 1443, and 1505, and males 1351 and 1458, mated inter se, have produced young as indicated in table 38. Table 38. Color. Observed. Expected. Gray Black Yellow Sooty Blue-gray Blue Cream Pale sooty White 20 8 13 I 7 4 (?) I 8 a? 9 9 3 9 3 3 I 31 The category yellow is probably too large because of a failure on our part to discriminate between yellow and cream, a difference which at t'lrst we failed to record. It is possible also that albion \oung were not enu- merated in all the records which we have combined, and so albinos are apparently deficient in number. It is needless to go farther in the enumeration of zygotic varieties of gray rabbits. There is Uttle doubt that the entire 32 varieties theoretically possible coukl readily be protluccd; or we have found that a spotted coat may be transferred from one color variety to another by means of crosses, and the same is true of a dilute condition of the jjigmentation in contrast to intense pigmentation. It is known also from a variety of sources, including besides our own observations the valuable experiments of Hurst (."05), that albinism may occur as a recessive character in any and all color varieties of rabbits. Additional evidence seems to be desir- able chiefly as concerns the assumed factor E; therefore, we may proceed to the consideration of color varieties other than gray, in the course of which this evidence will be produced. 60 INHERITANCE IN RABBITS BLUE-GRAY. A blue-gray rabbit differs from a gray one only in the intensity of its pigmentation, which is always dilute. As regards the intensity factor, therefore, it is invariably homozygous, D2, since D is recessive to I, whereas a gray rabbit may be either homozygous, 1^, or heterozygous, I (D). Con- sequently only half as many zygotic combinations are possible among blue- gray as among gray rabbits, 16 instead of 32 being the maximum. The 16 conceivable varieties of blue-gray rabbits, all of which should be similar in appearance but different in breeding capacity, are: (i) Blue-gray producing only blue-gray; formula, B2Br2E2A2C2D2U2Y2. (2) Blue-gray producing blue-gray, and blue; formula, B2Br2E2AC2D2U2Y2. (3) Blue-gray producing blue-gray, and white; formula, B2Br2E2A2CD2U2Y2. (4) Blue-gray producing blue-gray, blue, and white; formula, B2Br2E2ACD2U2Y2. (5) Blue-gray producing blue-gray, and cream; formula, B2Br2E(R)A2C2D2U2Y2. (6) Blue-gray producing blue-gray, blue, cream, and pale sooty; formula, B2Br2E(R)AC2D2U2Y2. (7) Blue-gray producing blue-gray, cream, and white; formula, B2Br2E(R)A2CD2U2Y2. (8) Blue-gray producing blue-gray, blue, cream, pale sooty, and white; formula, B2Br2E(R)ACD2U2Y2. The 8 remaining varieties would be identical with these, except for the factor U, in which they would be heterozygous, U (S), producing spotted as well as self-pigmented young. Three blue-gray rabbits, all females, have been tested, and each of these is of a different zygotic formula. Female 389, the original blue-gray individual, proved to be of variety 4. When mated with 2d(D)U2Y2. (7) Black producing black, blue, sooty, and pale sooty; formula, B2Br2E(R)C2l(D)U2Y2. (8) Black producing black, blue, sooty, pale sootv, and while; ft)rmula, B2Br2E(R)CI(D)U2Y2. The 8 remaining varieties would be identical with these 8, except that they would be heterozygous as regards U, viz, U(S) instead of U. Con- sequently they would produce spotted as well as self-pigmented young. All the black rabbits (with one exception) which we have used for breed- ing purposes were produced in the course of our experiments from animals of other color varieties. All were in one or more respects heterozygous, except possibly the recently purchased black rabbit, not yet full\ tested, but apparently homozygous in all particulars and so of variety i. Hurst (:o5) obtained rabbits of variety 2, but we do not hajjpen to have hail any of this variety, nor of variety 3. Variety 4 is re])resented in a modified form in our rabbits 104, 105, 167, 247, and 255. The modification consists in this: the albino otTs|)ring arc, at least in part, of the HimalaNan type, having pigmented extremities. 62 INHERITANCE IN RABBITS This we might express by adding the Himalayan factor (C) to the for- mula as given for variety 4. Variety 5 is represented in our black X, which when mated with blue c? 1434 (variety 3) produced 4 black and 4 blue young. Variety 7 is represented in our rabbits 1230 and 1231, 201 1, and 2038; and variety 8, in a modified form, in our d 248, which has sired black, blue, sooty, pale sooty, white, and Himalayan w^hite offspring by black, sooty- yellow, or blue-gray mates. He therefore differs from variety 8 as previ- ously described in that he is heterozygous in the Himalayan factor C His formula accordingly is B2Br2E(R)C'(C)I(D)U2Y2. Some evidence for this classification of our black animals will be found in the table 41. Other evidence is derived from matings with yellow or gray animals. Table 41. — Matings of black rabbits with black or sooty individuals. Mating. Black. Sooty. Hima- layan. White. Blue. Pale sooty. $ 105 black with c?io4 black 9 167 black with c?248 black 9 247 black with (^248 black 9 25s black with 0^248 black Total 9 2 3 10 I 3 2 I I I 2 2 24 9 7 3 4 3 2 I Expected proportions 9 1230 black with c?i340 sooty Expected 3 I 4 3 4 I 4 3 E ;::; I I ■ V 9 1230 black with (^1414 sooty Expected BLUE. "Blue" pigmentation in rabbits and other rodents is merely a dilute condition of black. The zygotic formula of a blue rabbit is the same as that of a black one, if we substitute D2 for the I2 or 1(D) of the black vari- eties. Blue rabbits may occur theoretically of 8 dift'erent sorts, viz: (i) Blue producing blue only; formula, B2Br2E2C2D2U2Y2. (2) Blue producing blue, and white; formula, B2Br2E2CD2U2Y2. (3) Blue producing blue, and pale sooty; formula, B2Br2E(R)C2D2U2Y2. (4) Blue producing blue, pale sooty, and white; formula, B2Br2E(R)CD2U2Y2. Table 42. — Matings and young of S 1434, blue. Mating. With 9647, sooty, variety 2 Expected With $1471, sooty, variety 3 or 4. Expected With 9 black, variety 5 or 6 Expected Black. Blue. Sooty. Pale sooty. CO LOU 63 The 4 remaining varieties would be identical witii these 4 except as regards the factor U, in which they would be heterozygous, U(S), instead of homozygous, Uj. We have determined the zygotic formuke of 2 blue rabbits only, botii of which were produced in the course of our experiments. One (s 1434, table 42) was of variety 3, the other (S 1228, table 43) was of variety 4. We shall pass by the spotted black and spotted blue varieties of rabbit, of both which sorts a certain number of individuals have been jiroduced in our experiments, but wliich have not been thoroughly tested. Table 43. — Malings and young of cf 1228, blue. Mating. With 9647, sooty, variety 2 Expected With 9 1280, sooty, variety 3 or 4 Expected With 9656, blue-gray, variety a . . Expected , Blue- gray. Black. IS 3 2 I Blue. Sooty. Pale sooty. White. 17 3 2 I o I YELLOW. Yellow rabbits differ from gray ones only in the factor E (extended black or brown pigmentation), in place of which they bear the alternative condition R (restricted black or brown pigmentation). Since R is reces- sive in relation to E, yellow rabbits are invariably homozygous, R,, as regards this factor. Theoretically sixteen different varieties are possible, as follows: (i) Yellow producing yellow only; formula, BjBrjRoAjCoIjUjYj. (2) Yellow producing yellow, and wliite; formula, BoBtjRj.VjCIjUjYj. (3) Yellow producing yellow, and sooty; formula, BiBrjRoACoIjUjYj. (4) Yellow producing yellow, sooty, and white; formula, BiBtjRo-^CIjUjYj. Four other varieties should differ from these 4 in factor I only, being 1(D) instead of I2, and producing dilute as well as intensely jiigmcnted individuals. Eight others should differ from these eight in producing spotted as well as uniformly pigmented individuals. We shall content ourselves with giving examjiles of the first four varieties enumerated. Variety i produces only yellows when mated with other yellows or with sooties, and only yellows and grays when mated with blacks or blues of any sort whatever. It is represented in our yellow (^381, a son of 2 gray rabbits of variety 5, viz, 9 175, and cJiyC. He was mated with 6 different yellow females, 3 of which had produced sooty offspring by other mates, and there resulted 61 young, all yellow. In matings with 2 dilTer- ent sooty females he produced 12 young, all yellow; and in a mating with a gray female of variety 6 (9 178) he produced 3 yellow and 6 gray 64 INHERITANCE IN RABBITS young; expected i: i. We have had several other yellow rabbits which were probably of this same variety, but they were less extensively and inconclusively tested. Variety 2 is represented in a yellow rabbit obtained by purchase (c?i256). He was mated with 9 547 yellow, variety 3, and produced 9 young, all yellow (as expected) ; by yellow 9 745^, variety 4, he produced 4 yellow and 3 white young (expected 3:1); and by black 9 1230 and 9 1 23 1, variety 7, he produced 8 yellow, 3 gray, and i blue-gray young; expected 4: 3: i. Variety 3 is represented in yellow females 547, 714, and 11 15, which in matings with yellow males of variety 4 produced 24 yellow and 2 sooty young, but no white ones. Had these females been of variety 4 they should have produced 25 per cent of white young in the mating mentioned. It is possible that the recorded number of sooties is too small, owing to a failure in our earUer records to discriminate sooty from yellow. No such possibility exists in the case of the records for albinos. One of the females already mentioned, of variety 3 (9 1115), when mated with sooty J 1340 produced 2 yellow and 6 sooty yoimg. Another j^ellow rabbit of variety 3 was the lop cf 179 (plate 2, fig. 8). When mated with the sooty "old female lop" (plate i, fig. 2) he produced 4 yellow and 4 sooty young (expected 1:1); and when mated with black females of variety 4 (9 9 105, 167, and 247) he produced 7 gray, 5 black, 6 yellow, and 7 sooty young (expected 1:1:1:1). Notice in the matings with black females the total absence of albinos, though all these females had produced albinos by other mates. Still another male of variety 3, c?3i9, son of the sooty old female lop by gray J 176, variety 5, when mated with the black 9 247 (variety 4) produced 2 gray, 2 black, 2 yellow, and i sooty young (expected i: i: i: i). Variety 4 is represented in our "Cutler's yellow" and in 9 745^ pro- duced by black 9105 (variety 4) mated with yellow c?3i9 (variety 3). When "Cutler's yellow" was mated with the above female, 745-2-, he pro- duced 4 yellow, 3 sooty, and i white young (expected 9:3:4), WTien mated with sooty females 632 and 647 (variety 2), he produced 7 yellow, 7 sooty, and 2 white young (expected 3:3:2). SOOTY. Sooty rabbits differ from yellow ones only in the factor A, which they lack. Theoretically 8 varieties are possible, viz: (i) Sooties producing sooties only, when mated inter se; formula, B2Br2R2C2l2U2Y2. (2) Sooties producing sooty, and white; formula, B2Br2R2Cl2U2Y2. (3) Sooties producing sooty, and pale sooty; formula, B2Br2R2C2l(I^)U2Y2. (4) Sooties producing sooty, pale sooty, and white; formula, B2Br2R2CI(D)U2Y2. The 4 remaining varieties would be like these, except as regards the factor U, in which they would be heterozygous, U(S), instead of homozy- COLOR 65 gous, U3. They would produce spotted as well as uniformly j^igmented young. An exam|)lc' of variety i is the "old female lop" (plate i, fig. 2). When mated with an albino of black ancestry, 6 45 ([)late i, fig. 3), she producccl a litter of 8 black young (plate i, fig. i). This experiment shows her to have been homozygous in C, /. e., to have been Cj in character, anfl to have lacked factor A. When mated with yellow .i 179 (plate 2, fig. 8), variety 3, she jjroduced 4 yellow and 4 sooty young, exactly the exi>ect«l equaUty of yellow and sooty. Another individual probably of this same variety was 9 1472, which when mated with a sooty male, 1414, produceroduced by 9632 and cf 402 was a Himalayan albino. This shows one or both of the parents to have been slightly different from typical variety 2, and to have carried C. Variety 3 is represented probably by our 9 147 1 which, when mated with blue d* 1434, produced 4 black, 2 blue, 5 sooty, and 2 pale sooty young (expected 1:1:1:1). The possibility is not excluded that this female was of variety 4 (capable of producing also albino young), but she can not have been of either variety i or variety 2. Another j^robable example of variety 3 is 9 1280. (See matings of d* 1228, blue, p. 63). Variety 4 we have not identified with certainty. Neither have we made a detailed study of pale yellows, pale sooties, or spotted rabbit.s of any color variety. We have observed, liowever, that dilute pigmentation, as well as spotting, occurs in all the fundamental color varieties and are entirely satisfied of the independent inheritance of both. WHITE. Albino rabbits differ from i)igmented ones onl\- in regard to the factor C, which they either lack, or possess only in a greatly modified form, C. If C is absent, there are jiossible i6 dilTerent combinations of the 4 remain- ing variable factors, which combinations corresj)ond with gametes of the 16 visibly different i)igmented varieties of rabbit, minus C. But if C is present in the modified form, C, found in Himalayan albinos. 16 other gametic combinations should be ])Ossible, only slightly difierent from the foregoing, making in all 32 difierent gametic possibilities, or 232 zygotic possibilities. 66 INHERITANCE IN RABBITS Plainly it is unprofitable to attempt to find illustrations of all these con- ceivable variations. We shall content ourselves with noticing some of the more important varieties of albinos and presenting evidence that each of the 4 variable factors, A, E, I, and U, is transmitted through albinos. The foUovv^ing albino varieties may be expected to occur: (i) White producing gray only (in crosses with any pigmented variety); for- mula, BjBrjEjAjIjUjYj. (2) White producing black only (in crosses with black or any pigmented variety recessive to black); formula, BjErjEjIjUaYj. (3) White producing yellow only (in crosses with yellow or sooty individuals) ; formula, BjBrjRjAjIjUjYg. (4) White producing sooty only (in crosses with sooty) ; formula, BoBrjRjIzUjYj. (5) White producing gray, and black (in crosses with black or any pigmented variety recessive to black); formula, BjBrjEjATjUjYj. (6) White producing gray, and yellow (in crosses with yellow or sooty) ; formula, B2Br3E(R)A2l2U2Y2. (7) White producing gray, black, yellow, and sooty (in crosses with sooty) ; formula, B2Br2E(R)Al2U,Y2. (8) White producing black, and sooty (in crosses with sooty); formula, B2Br2E(R)l2U2Y2. (9) White producing yellow, and sooty (in crosses with sooty); formula, B2Br2R2Al2U2Y2. Another set of 9 varieties, quite similar to these, w^ould produce only pale-pigmented offspring. As regards the intensity factor they would be D2 instead of I2. Another set of 9 varieties would produce both dilute and intensely pigmented offspring, being heterozygous, 1(D), as regards the intensitv factor. Nine other varieties, in which S2 replaces U2, would produce only spotted young; and another set of 9 would produce both spotted and self-colored offspring; in these U(S) would replace U2. Another set of 9 varieties would produce only pale-pigmented spotted individuals, another would produce pale-pigmented individuals, both self and spotted; and lastly a set of 9 varieties would produce both dilute and strongly pigmented indi- viduals, both spotted and self-colored. It is probable that the foregoing list of 72 varieties could be duplicated in varieties having the Himalayan modification, and duplicated a second time in varieties heterozygous in the two sorts of albinism. A few examples will now be mentioned of some of the 9 varieties of albinos first enumerated, or of animals differing from those 9 varieties in one or two characters only. Variety i is represented in our s 1425, which when mated with black 9 1 541 produced 11 young, all gray, and when mated with yellow 9 547 (variety 3) produced 4 young, all gray. Variety 2, but heterozygous in the Himalayan modification, C, and in spotting with white, U(S), is rep- COLOR 67 resented in our (^45 (plate i, fig. t,), which when mated with the sooty lop (plate I, fig. 2) produced 8 young, all black. One of these is shown in plate i, fig. 1. When mated with black 9 105, he proflucetl 8 black jiig- mented and 3 Himalayan albino young, but .several of the {iigmtntcnl young were spotted with white, this character being recessive in 9 105, which had a Dutch-marked father. Variety 5 is represented in 9 269, which when mated with sooty d" 402 produced i gray and 3 black young (expected 1:1). When materoduce gray otT- spring in crosses with black j)igmented animals, while others (lacking A) never produce gray offspring, though mated to the same black animals. That albinos transmit the factor E is shown clearly by extensive ex|x>ri- ments with guinea-pigs carried out by one of us. One family of albino guinea-pigs has been found invariably to produce black offsi>ring in mat- ings with any pigmented variety devoid of factor A, whether that varirty has the extended or the restricted distribution of black or brown pigment; a second family of guinea-pigs, with equal uniformity, produces colortd offspring having a restricted distribution of black pigment, if cro.ssed with colored individuals having the restricted distribution. This second vari- ety produces black-eyed yellows, if crossed either with black-eyed yellow or with brown-eyed yellow individuals. Of the 2 albino varieties men- tioned, the first evidently carries B with E, the second B with R. These same two famihes of albino guinea-pigs likewise ditTer in factor I, which is present in the first family, but rej^laced by D in the second. If each is crossed with pale yellow (cream) individuals, the former produces 68 INHERITANCE IN RABBITS in Fj black offspring, and in Fj black, blue, red, and cream, as well as albinos; whereas the latter produces in Fj cream, and in F2 cream and albino offspring only. As regards the factor U, Hurst ( : 05) has shown clearly that some albino rabbits transmit a uniformly colored coat, others a spotted coat, in crosses with colored rabbits. The former we may regard as carrying U, the latter S. In rabbits we have not made an extensive study of this matter. We have found, however, in agreement with Hurst, and Woods (:o3) that spotted rabbits in general produce only spotted young, when mated with each other, i. e., that spotting with white is recessive to uniform pigmenta- tion, and the case has been mentioned of an albino (c? 45) which produced spotted young when mated with a black rabbit that had a spotted father. In guinea-pigs also, spotting is in the main recessive, and spotting is clearly transmitted by albinos. Thus the e Note.) Arch, de Zool. exper. et g^n. (4), tome 2, Notes et revue, pp. 45-56. Donaldson, H. H. : 06. A comparison of the white rat with man in respect to the growth of the entire body. Boas Memorial Volume, New York. pp. 5-26, 2 pi. Farabee, W. C. : 05. Inheritance of digital malformations in man. Papers of Pealxxly Museum, Harvard Univ., vol. 3, No. 3, pp. 69-77, p'- 23-27. HOUSSAY, F. 107. Variations e.xperimentales. Etudes sur six generations de poules carnivores. Arch, de 2k)ol. exp^r. et g^n. (4), tome 6, pp. 137-332, 47 fig. Lock, R. H. :o6. Studies in plant breeding in the tropics. III. Experiments with maize. Annals Roy. Bot. Gardens, Peradeniya, vol. 3, pt. 2, pp. 95-184. Robertson, T. B. : 08. On the normal rate of growth of an individual, and its biochemical significance. Arch. f. Entwicklungsmechanik der Organismen, 25, pp. 581-614. TSCHERMAK, E. :o3. Die Theorie der Kryptomerie und des Kryptohybridismus. Bcihefte zum Botan. Centralblatt, Bd. 16, pp. 1-25. Woods, F. A. : 03. Mendel's laws and some records in rabbit breeding. Biometrika, v. 2, pp. 299-306. 69 DESCRIPTION OF PLATES. Plate i . — Photographs from life of rabbits described in the text. Fig. I. (5^248, son of the rabbits shown in figs. 2 and 3; ears of intermediate length, hair short, color black. 2. " Old female lop," sooty yellow in color. 3- C?45) ^ short-eared, Himalayan albino, angora rabbit. 4. 9 400, daughter of c? 248, fig. i, and of his sister 9 247, a rabbit of like character. Plate 2. — Photographs from life of rabbits described in the text. Fig. 5. 9 269, a short-eared albino rabbit. 6. A litter of four rabbits (640 to 643) borne by 9 269, fig. 5, when mated with c?i79, fig. 8. Three are gray, one is black; all have ears of intermediate length, as compared with the parents. 7. Gray quarter-blood lops borne by 9 43i, plate 3, fig. 9, in a mating with her son C? 176, a half-blood lop similar in appearance to his sister 9 I75) plate 3, fig. 10. 8. c?i79, a full-blood yellow lop, son of the "old female lop," plate i, fig. 2, and of the "old male lop," a yellow rabbit. Plate 3. — Photographs from life {except fig. 9) of rabbits described in the text. Fig. 9. Mounted skin of 9 431. the "Belgian hare," a gray rabbit with short ears. ID. 9 175) ^ gray half-blood lop, daughter of 9 43i> %• 9, and the old (J* lop, a yellow rabbit similar in appearance to his son (^179, plate 2, fig. 8. 11. 9322, a gray three-quarter blood lop, daughter of old female lop, plate i, fig. 2, and the half-blood lop (^176; compare fig. 10, which gives a good idea of the appearance of (J* 176. 12. c?38i, son of 9 175, fig- 10, and her brother, c?i76; an F2 half-blood lop, with the same general ear-character as his parents, but yellow in color, like his grand- father. Plate 4. — Dorsal and ventral views of the skulls of 3 rabbits. In the middle the skull of c?248 (compare plate i, fig. i); at the right the skull of his mother "old female lop" (plate i, fig. 2); and at the left the skull of his father 6^45 (plate i, fig. 3). 70 CASTLE PLATE i CASTLE PLATE 2 ■-^-iw*-^ CASTLE PLATE 3 •