EXCHANGE BIOLOGY LIBRARY G HABIT FORMATION IN A STRAIN OF ALBINO RATS OF LESS THAN NORMAL BRAIN WEIGHT A DISSERTATION SUBMITTED TO THE BOARD OF UNIVERSITY STUDIES OF THE JOHNS HOPKINS UNIVERSITY, IN CONFORMITY WITH THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY By GARDNER CHENEY BASSET 1913 (Reprinted from Behavior Monographs, Volume 2, Number 4, 1914) HABIT FORMATION IN A STRAIN OF ALBINO RATS OF LESS THAN NORMAL BRAIN WEIGHT A DISSERTATION SUBMITTED TO THE BOARD OF UNIVERSITY STUDIES OF THE JOHNS HOPKINS UNIVERSITY. IN CONFORMITY WITH THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY By GARDNER CHENEY BASSET 1918 (Reprinted from Behavior Monographs, Volume 2, Number 4, 1914) we . LIBRARY G CONTENTS PAGE I. Introduction 1 II. Methods 2 III. Experiment 1 : the Maze 5 IV. Experiment 2: the Preliminary Inclined Plane 25 V. Experiment 3: the Inclined Plane 30 VI. Summary and Conclusions 43 m 291041 ACKNOWLEDGMENTS . Before entering upon the body of this presentation I desire to express my obligations to those without whose co-operation my experiments would have been of less value. Above all am I indebted to Professor John B. Watson, Direc- tor of the Johns Hopkins Psychological Laboratory, who has kept himself informed of the progress of my experimentation and who has been ready at all times with helpful suggestions and encouragement. To Dr. Henry H. Donaldson I owe much: for suggesting the experiment; for placing the facilities and materials of the Wistar Institute at my disposal; for much helpful advice as to the evaluation of my results. To Dr. Shinkishi Hatai for preparing the anatomical data referring to the rats used in my experiments. To Dr. Helen D. King for keeping her sequences of in- breeding moving so perfectly that it was possible at any time to procure inbred rats of the desired age. GARDNER CHENEY BASSET. IV I. INTRODUCTION A few years ago experimental inbreeding of the albino rat, Mus norvegicus albinus, was started at the Wistar Institute of Anatomy and Biology in order to determine the anatomical consequences of such procedure upon successive generations of progeny. Among other results obtained was a distinct and progressive decrease in actual and relative brain weight (relative, that is, in reference to body length) for four generations of close inbreeding. At the end of the fourth generation the rats seemed lacking in vitality and, for this reason, were subjected to a change in food. From this period until the end of the tenth generation (the extent of inbreeding at the time this paper was prepared) the relative brain weight remained, on the average, constant at six and one-half per cent less than that of the normal control rats. When, early in October, 1911, Dr. Donaldson suggested to Professor Watson that the decrease in brain weight might be accompanied by a similar decrease in ability to form habits a new line of investigation in comparative psychology was opened up. The problem was offered to the writer and gladly accepted. It is no part of the purpose of this paper to raise the question as to whether inbreeding, per se, results deleteriously upon the progeny. In this, as in all disputed questions, it is unsafe to be arbitrary, and authoritative testimony must await the results of further investigations. We know, upon the authority of historians, that the Incas of Peru for many generations married their sisters and were physically and mentally superior to their subjects. Breeders of domestic animals frequently resort to inbreeding in order to perfect desirable qualities in the strain. It may be, as many claim, that inbreeding results deleteriously only in cases where an hereditary taint, occurring in the common ancestor, is strengthened in the progeny of a consanguineous union. Of the rats used in the experiments hereinafter described, it is not postulated that the lesser ability to form habits is neces- sarily due either to inbreeding or to the environmental factor of 2 GARDNER CHENEY BASSET insufficient nourishment during the first four generations; but, the rats used for purposes of inbreeding produced a strain hav- ing a lesser relative brain weight on the average. This strain of rats I shall hereafter, for convenience, refer to as the Inbred Strain. The object, then, of the following experiments is to compare the habit-forming ability of ' the inbred strain with lesser brain weights, with the ability of a normal control series. Owing to the fact that experimental work on the brain weight problem has not before been attempted there is no history and little literature to be presented. Donaldson 1 reproduces tables from Manouvrier 2 showing the brain weights of eminent men to be, on the average, greater than those of average Parisians. It is not necessarily true that the specific individual with greater brain weight is more intelligent or will contribute more to the world's arts and sciences than the specific individual of lesser brain weight; but, if the conclusions of Manouvrier are to be believed, individuals of brain weight above the average are more liable to be of superior intelligence and to do the world greater service. The results of the experiments described in this paper agree closely with Manouvrier 's conclusions. Tables of distribution of brain weights of the inbred strain and normal control series overlap; but the normal series, having a greater brain weight average, show greater ability t in habit formation. All the experiments here described were carried out at the Psychological Laboratory of the Johns Hopkins University. II. METHODS All the inbred rats used in this investigation were bred at the Wistar Institute of Anatomy and Biology by Dr. Helen D. King. Two strains were used, referred to in this paper as strains A and B. The original parents of each strain were taken at random, a male and female from each of two unrelated lit- ters. The A male was mated to his sister, A female, and the B male to his sister, B female. Their respective litters consti- tuted generation 1A and IB. From this point inbreeding was carried on by selecting from the litter the healthiest appearing 1 Donaldson: The Growth of the Brain. London and New York, 1909, pp. 128 ff. 2 Manouvrier, Sur Interpretation de la quantite dans 1'enc^phale, etc., Paris, 1885. HABIT FORMATION IN ALBINO RATS 3 rats and mating brother to sister within the same litter, this constituting the closest possible inbreeding. At about thirty days of age the young rats were taken from their mother, and those to be used by the writer were shipped to the Johns Hop- kins University. There were no fatalities in transit and all arrived apparently in good condition. The system of number- ing individual rats for identification was as follows: the first number referred to the generation of inbreeding, the letter ( A or B) to the strain, and the last number was that applied to the individual. For example, 7A90? may be analyzed as follows: 7th generation inbred, A strain, individual 90, female. Each rat had one or both ears punched or clipped to agree with the individual number, a system in use by Professors Castle and Yerkes at the Harvard laboratories. It seemed advisable to secure normal control mating strains from different laboratories in order to avoid any possibility of inbreeding. In addition to our own Hopkins stock there were obtained rats from the Wistar Institute, Columbia University, animal dealers in Baltimore, and from Dr. Herbert M. Evans of the Johns Hopkins Medical School. Care was taken in mat- ing the control series to avoid any approach to inbreeding. As in the case of inbred rats the young were taken from the mother at the age of thirty days. The system of numbering individual control rats for identification was as follows: the first letter, S, signified that it was a standard or normal control rat, letters within parentheses gave the pedigree, and the figures gave the individual number. For example, S(C/EB)70c^ may be analyzed as follows: standard, or normal control series, Columbia father, Hopkins Medical maternal grandfather, maternal grandmother purchased from a Baltimore dealer, individual 70, male. The same system of ear marking was used as in the case of the inbred rats. When taken from the mother males and females were kept in separate cages. According to Watson 3 the bearing of young has some effect upon the central nervous system of the white rat; for this reason, and in order to keep conditions constant, neither males nor females used in these experiments were al- lowed to mate. As solitude may be a factor affecting behavior, 3 Watson: The Effect of the Bearing of Young Upon the Body- Weight and the Weight of the Central Nervous System of the Female White Rat. Journ. of Comp. Neur. and Psych., Vol. XV., No. 6, 1905. 4 GARDNER CHENEY BASSET from three to five rats were kept in each cage, the cages being made sufficiently large (24" x 15" x 15") to permit it. Cages were frequently disinfected with a preparation the principal ingredient of which is carbolic acid. The rats were occasionally immersed in a solution of this preparation in order to destroy skin parasites. A layer of clean chips and shavings was kept on the floor of all cages. The food, from the time of weaning, consisted of bread soaked in milk every day, grain and sun- flower seeds twice a week, and banana or carrot once a fort- night. Temperature was kept as uniformly as possible at 70 F. In order to facilitate this a small gas heater was installed and it proved very efficient even during the coldest days of winter. As the animal laboratory is located in the basement the temperature, during the summer, rarely rose above our norm. At the age of sixty days the rats intended for experimental purposes were placed on a short allowance .of feeding time (thirty minutes) in order to prepare them for experimentation. The experiments were begun with both inbreds and normal control at the age of seventy days. Care was taken in each experiment to use the same number of males and females in the control series as in the inbred; this was necessary because, as in man, the relative brain weight of the female is greater than that of the male. Experiments upon individual rats were con- ducted as nearly as possible at the same time of day, both to form feeding rhythms and in order not to interfere with other rhythms. There are three methods of estimating perfection in experi- ments relating to the habit-forming abilities of animals: the number of errors, the distance traversed, the time consumed. It is hard, in any case, to form a judgment as to what consti- tutes an error in the behavior of an animal; especially is this true in a comparative study of this kind where it is possible for the personal prejudices of the experimenter to become a factor. At the time this investigation was begun there was no adequate means of measuring the distance traversed. This left at the disposal of the experimenter but one criterion: the time consumed. However, time consumed in learning is the criterion most frequently used by experimenters in the animal field. HABIT FORMATION IN ALBINO RATS 5 Hicks, 4 in summing up the experimental results of several in- vestigators, concludes that " time is the best single criterion, inasmuch as it represents all phases of the process of learning, and since it will yield the most comparable results at the hands of different investigators." In timing the rats a very accurate Swiss split-second stopwatch was used. Under ideal conditions, perhaps, the animal should be presented to the problem by one person, timed by another, while the experimenter himself should merely record the results. But timing very soon becomes auto- matic; when the rat is crossing the line it is almost impossible to inhibit the impulse to press the stem of the watch. At the conclusion of the experiments all the rats were shipped to the Wistar Institute where Dr. Shinkishi Hatai ascertained the anatomical data necessary for the formulation of the com- parison between relative brain weight and the ability to form habits. III. EXPERIMENT 1: THE MAZE The apparatus used in this experiment was the Watson Maze (see Plate I) . This maze is circular in form, five feet in diameter, with entrances from outer runways to the next inner at alternate ends of a quadrant arc. The runways are each four inches wide, and the centre, F, eight inches in diameter. The partitions are of aluminum and rise to a height of five inches above the floor of the maze. A heavy wire screen resting on the top serves the purpose of preventing the rats from climbing over the par- titions, and also allows the experimenter to observe all move- ments within. The perfect course of the animal running is, from the entrance, E, through runway entrances 1, 2, 3, 4, 5, 6, and 7 to F (food). Each side of runway entrances 2 to 6 inclusive lead into cul-de-sacs. The object of the experiment was to have each rat learn to reach the centre, F, in the least possible time, the starting time being taken when the animal crossed runway entrance 1, and the finishing time when he crossed entrance 7. In preparation, each animal, beginning at the age of sixty- five days, was fed alone in the centre, F, ten minutes daily for 4 Hicks, The Relative Values of the Different Curves of Learning. Jour. Animal Behavior., Vol. I, pp. 138 ff. GARDNER CHENEY BASSET The Uratson ftlaje. Plate I. HABIT FORMATION IN ALBINO RATS 7 five consecutive clays. During this period the centre was barred from the rest of the maze at entrance 6. At the age of seventy days the experiment began. Eleven males and ten females from the inbred strain were used and, as control, an equal num- ber of males and females from the normal control series. Of the inbred rats, fourteen were from the sixth generation and seven from the seventh. The stimulus used was the food to which they had become accustomed, bread soaked in milk. From the beginning of the experiment each rat was required to run from E to F five times daily. At the end of the fifth trial it was allowed to feed in the centre, F, for five minutes, but permitted no more food until the completion of the next day's experiment. Each rat was used daily until it had learned the course perfectly, the criterion of perfection being five perfect trials for each of three successive days. A perfect trial consisted in running the course within six seconds, a period of time so short that it was practically impossible for the rat to make a detectable error and reach the centre within that time. Those rats failing to learn within one hundred days (five hundred trials) were no longer used for experimentation. Such rats as learned the maze were, at the conclusion of the experimentation, fed for sixty days in a runway twenty-five feet long with a feed- ing-box at the right of the far end. At the end of this period they were tested for retention and relearning. The shortest period of time required by an inbred rat to learn the maze perfectly was twelve days; for a control rat, ten days. Two inbred rats and one control failed to learn the maze habit within the one hundred days allowed. During the process of learning certain of the normal control series showed peculiarities of behavior similar to those exhibited by the inbred strains. These peculiarities, for the greater part, consisted in disorientation and persistent errors. All the normal control series, with the exception of five rats containing germ-plasm of the B strain, had perfected the maze habit by the twenty-fourth day. The control rat mentioned above as having failed to learn the maze within one hundred days was from this group. So erratic in behavior and so slow in learning were the B strain rats that the investigator suspected them to be of less than normal brain weight; and, when the returns were received from the Wistar Institute, this was indeed found to be the case. 8 GARDNER CHENEY BASSET For greater convenience in making a comparative study, I have placed together in Table I a summary consisting of the daily averages of the entire inbred group and, directly beneath, the corresponding daily averages of the entire normal control group. From this table, too, are constructed the comparative curves of learning. Table I compares: (a) the progress of learning by days; (b) the ' 'absolute retention" (this being a term used here to represent the time required to complete the first trial of the relearning series after the sixty days' rest; the greater the re- tention, the less is the time) ; (c) the progress of relearning by days; (d) the anatomical data. Table la shows that two of the inbred and one of the control rats (the latter from the" B strain) failed to learn the maze habit. The inbreds required, on the average, 36.62+ days to learn; the control but 24.67+ days. The absolute retention of the inbred rats (Table Ib) was, on the average, 81.558 seconds; of the control series, 59.640 seconds. The two inbreds and one control failing to learn the maze were not, of course, tested for retention and relearning. Of the inbreds so tested (Table Ic), two failed to relearn within fifty days, in consequence of which it was thought useless to carry them further. All the control series had relearned at the end of twenty-two days. The inbreds required, on the average, 12.68+ days to relearn; the normals but 5.75 days. In all these criteria of ability: learning, absolute retention, and relearning, the rats of the normal control series are shown, on the average, to be superior to those of the inbred series. There are two methods in use for estimating the relative brain weight: in reference to body length and in reference to body weight. In a healthy normal rat the difference between body weight in grams and body length in millimeters is slight; but, under conditions of overfeeding, underfeeding, or of sick- ness the body weight varies greatly while the body length remains constant. For this reason Dr. Donaldson of the Wistar Institute has accepted body length as the better method. I have laid greater stress on the body length criterion, although both are presented in the tables of anatomical data. Both body length and body weight of the inbred rats used in the maze (Table Id) are, on the average, slightly greater than is HABIT FORMATION IN ALBINO RATS 9 the case with the normal controls. The relative brain weight (in reference to body length) of the inbreds is 4.43% less than that of the normals. The relative brain weight (in reference to body weight) of the inbreds is 7.99% less than that of the normal control. The percentage of water in brain and cord normally decreases with age ; but in the inbreds used in the maze experiment, although killed, on the average, fourteen days later than the control rats, the percentage of water was greater. The figures presented in Table I support the hypothesis that a less than normal average brain weight in a strain of rats is accompanied by an average lesser ability to form habits. TABLE la THE MAZE DAILY LEARNING AVERAGES OF INBRED AND NORMAL CONTROL RATS (Time in seconds) Inbred Average 531 .665 Control Average 505 . 128 Inbred Average. , Control Average Inbred Average . Control Average . Inbred Average . Control Average , Inbred Average . . Control Average. Inbred Average . , Control Average . Inbred Average . . Control Average . Inbred Average. Control Average . Inbred Average . Control Average , Day 1 31.665 05.128 Day 2 91.404 110.739 Day 3 68.160 53.851 Day 4 39.459 28.404 Day 5 25.568 25.613 Day 6 20.015 17.366 Day 7 13.899 13.381 Day 8 11.061 11.994 Day 9 9.709 12.440 Day 10 9.522 9.619 Day 11 10.217 7.295 Day 12 7.937 7.748 Day 13 8.708 7.354 Day 14 7.600 6.904 Day 15 7.000 7.277 Day 16 6.439 6.687 Day 17 6.585 6.428 Day 18 6.305 5.900 Day 19 6.492 6.209 Day 20 6.458 5.851 Day 21 6.362 5.630 Day 22 5.749 5. $16 Day 23 5.978 5.710 Day 24 5.753 5.675 Day 25 6.248 5.420 Day 26 5.734 5.389 Day 27 7.130 5.442 Day 28 5.669 5.479 Day 29 6.387 5.496 Day 30 5.697 5.437 Day 31 5.384 5.700 Day 32 5.708 5.502 Day 33 5.702 5.378 Day 34 ' 6.084 5.499 Day 35 5.731 5.308 Day 36 5.590 5.272 Day 37 5.476 5.249 Day 38 5.494 5.327 Day 39 5.540 5.363 Day 40 5.848 5.316 Day 41 5.640 5.343 Day 42 5.978 5.573 Day 43 5.631 5.375 Day 44 5.526 5.250 Day 45 13.456 5.444 10 GARDNER CHENEY BASSET Day 46 Day 47 Day 48 Day 49 Day 50 Inbred Average 6.734 7.400 6.602 5.713 5.840 Control Average 5.282 5.162 5.147 5.198 5.223 Day 51 Day 52 Day 53 Day 54 Day 55 Inbred Average 5.844 5.522 5.353 5.416 5.707 Control Average 5.322 5.360 5.192 5.615 5.261 Day 56 Day 57 Day 58 Day 59 Day 60 Inbred Average 5.924 5.640 5.458 5.621 6.740 Control Average 5.286 5.181 5.358 5.286 5.257 Day 61 Day 62 Day 63 Day 64 Day 65 Inbred Average 8.347 6.442 6.177 5.425 5.686 Control Average 5.398 5.952 5.743 5.299 5.288 Day 66 Day 67 Day 68 Day 69 Day 70 Inbred Average 5.88U 0.630 5.442 5.821 5.396 Control Average 5.345 5.339 5.173 5.360 5.223 Day 71 Day 72 Day 73 Day 74 Day 75 Inbred Average 5.419 5.737 5.457 5.587 5.928 Control Average 5.244 5.170 5.130 5.206 5.110 Day 76 Day 77 Day 78 Day 79 Day 80 Inbred Average 5.686 5.798 5.817 5.627 5.432 Control Average 5.183 5.288 5.170 5.143 5.421 Day 81 Day 82 Day 83 Day 84 Day 85 Inbred Average 6.095 7.335 5.379 5.535 6.345 Control Average 5.263 5.236 5.211 5.208 5.160 Day 86 Day 87 Day 88 Day 89 Day 90 Inbred Average 5.495 5.316 5.560 5.421 7.290 Control Average 5.288 5.156 .5.181 5.152 5.152 Day 91 Day 92 Day 93 Day 94 Day 95 Inbred Average 5.829 7.015 6.204 6.011 5.556 Control Average 5.257 5.095 5.141 5.210 5.166 Day 96 Day 97 Day 98 Day 99 Day 100 Inbred Average 5.893 5.665 6.017 5.973 5.958 Control Average 5.217 5.149 5.118 5.187 5.215 , Failed Days required to learn to learn Inbr d Average , . 2 36.62+ Control Average 1 24.67+ TABLE Ib THE MAZE AVERAGE ABSOLUTE RETENTION OF INBRED AND NORMAL CONTROL RATS Absolute Retention after 60 days' rest Inbred Average 81 . 558 seconds Control Average 59 . 640 seconds HABIT FORMATION IN ALBINO RATS 11 TABLE Ic THE MAZE DAILY RELEARNING AVERAGES OF INBRED AND NORMAL CONTROL RATS (Time in seconds) Day 1 Day 2 Day 3 Day 4 Day 5 Inbred Average 35.415 12.208 10.069 9.560 8.069 Control Average 28.574 18.752 9.530 7.996 6.548 Day 6 Day 7 Day 8 Day 9 Day 10 Inbred Average 7.672 7.659 6.642 6.232 6.604 Control Average 7.076 6.064 5.922 5.670 5.630 Day 11 Day 12 Day 13 Day 14 Day 15 Inbred Average 6.200 5.966 6.067 5.660 5.587 Control Average 5.508 5.434 5.430 5.414 5.468 Day 16 Day 17 Day 18 Day 19 Day 20 Inbred Average 5.634 5.669 5.680 6.029 5.718 Control Average 5.970 5.424 5.490 5.354 5.440 Day 21 Day 22 Day 23 Day 24 Day 25 Inbred Average 6.046 5.834 5.844 6.061 5.771 Control Average 5.614 5.300 5.300 5.300 5.300 Day 26 Day 27 Day 28 Day 29 Day 30 Inbred Average 6.166 5.697 5.914 5.842 5.817 Control Average 5.300 5.300 5.300 5.300 5.300 Day 31 Day 32 Day 33 Day 34 Day 35 Inbred Average 5.901 5.905 5.640 5.846 5.939 Control Average 5.300 5 300 5.300 5.300 5.300 Day 36 Day 37 Day 38 Day 39 Day 40 Inbred Average 5.920 5.956 5.903 5.766 5.726 Control Average 5.300 5.300 5.300 5.300 5.300 Day 41 Day 42 Day 43 Day 44 Day 45 Inbred Average 5.657 5.848 5.779 5.745 5.861 Control Average 5.300 5.300 5.300 5.300 5.300 Day 46 Day 47 Day 48 Day 49 Day 50 Inbred Average 6.032 5.815 5.920 5.762 5.697 Control Average 5.300 5.300 5.300 5.300 5.300 Failed Days required to relearn to relearn Inbred Average 2 12 .68 + Control Average 5 .75 TABLE Id THE MAZE ANATOMICAL DATA OF INBRED AND NORMAL CONTROL RATS Inbred Average Control Average . . Body Body Brain Length Weight Weight in mm. in grms. in grms. . 195.38 180.55 1.71122 191.00 171.41 1.74930 Cord Water Weight in Brain in grms. per cent .52852 78.497 .52740 78.319 Inbred Average Water Per cent in Cord Brain Weight per cent in Relation to Body Length 71 723 87685 Per cent Age Brain Weight killed in Relation to Days Body Weight 97052 200 Control Average. . . 71.666 .91745 1.05479 186 12 GARDNER CHENEY BASSET In Plate II is shown the curve of learning (below) and of relearning (above) of the inbred rats compared with those of the normal control. These curves are constructed from figures given in Table I. The curve of the inbred rats is indicated by the solid line, that of the normal control by the broken line. The ordinates give the average daily time in seconds for each group, and the abscissae the number of the day in which such time was made. The time required by both inbred and control rats 'for the first four days was so long that it is represented here by figures and does not appear in the curve. For the first few days the descent in time for both the inbreds and the con- trol is very rapid. From the twentieth day the curve of the control rats lies entirely below the six-second mark. The curve of the inbred rats never reaches even an approximately flattened appearance, but exhibits great irregularities, particularly on the forty-fifth, sixty-first, eighty-second, ninetieth and ninety- second days. The inbreds' curve of relearning is more similar to that of the controls, but it must be borne in mind that the two inbreds and one control rat that failed to learn the maze are not represented in the relearning curve, and for this reason this curve applies to selected groups. From the twenty-second day the control curve of relearning is perfectly flat at 5.3 seconds, all the control rats having relearned. Two of the inbred rats having failed to relearn, their curve of relearning remains slightly irregular and above that of the control in time. In Plate III may be seen the curves representing the distri- bution of learning and relearning of both inbreds and control for the maze experiment. The time is given in days in groups of five for learning, in groups of two for relearning. As may readily be seen, the advantage from the standpoint of time (days required to learn and relearn) lies wholly in favor of the normal control group. The question arises as to whether the later generations of inbred rats differ from the earlier in the ability to form habits; that is, is decrease in this ability progressive even if, as earlier stated, decrease in relative brain weight after the 4th genera- tion is not. Of the inbred rats used in the maze experiment, fourteen were from the 6th generation and seven from the 7th generation. In Table II is presented a comparative summary consisting of the daily averages of the 6th and 7th generation HABIT FORMATION IN ALBINO RATS 13 14 GARDNER CHENEY BASSET 7 1 -a d d " -& -f/r - //r - ^f f-/f if-Si />?-( ST rt-'f .5 e - A-Z C s? - /:r *; r ^ r --w o L -rir-fT -; rr - n ry r- ^/ ** V -si hi- ci 'i - 1 1 1 - f> -L -f - f ^& fh-ll, c 5- SL-IL ?:S n f- ? HABIT FORMATION IN ALBINO RATS 15 inbred rats used in this experiment. Two of the 7th generation failed to learn the maze; all the 6th generation had learned it after eighty-three days. The 6th generation required, on the average, 32.93 days in which to learn; the 7th generation, 44.00 + days. The absolute retention of the 6th generation was, on the average, 65.443 seconds; of the 7th generation, 126.680 seconds. Two of the 6th generation failed to relearn; all the 7th genera- tion had relearned at the end of fourteen days. The 6th genera- tion required, on the average, 14.14+ days to relearn; the 7th generation but 8.60 days. In these criteria of ability, the 6th generation excelled in learning and absolute retention; the 7th in relearning. It must be remembered, however, that the 7th generation rats used in the relearning test formed a selected group, the two rats having failed to learn having been from this generation. On the whole, the ability of the 7th generation inbreds in the maze experiment appears to be somewhat inferior to that of the 6th generation. The body length and body weight of the 6th generation aver- age greater than those of the 7th. The average actual brain weight of the 6th generation is greater than that of the 7th. The relative brain weight (in reference to body length) of the 6th generation is .91% less than that of the 7th generation. The relative brain weight (in reference to body weight) of the 6th generation is 1.50% less than that of the 7th generation. The relative brain weight of the inbred rats used in the maze has not decreased from one generation to the next; but all the 7th generation rats were females, and the females normally have relatively greater brain weights than the males. The percentage of water in brain and cord is within .03% of the same figure in the two generations. TABLE Ha THE MAZE DAILY LEARNING AVERAGES OF SIXTH AND SEVENTH GENERATION INBRED RATS (Time in seconds) Day 1 Day 2 Day 3 Day 4 Day 5 Sixth Average 541.423 73.343 79.997 37.449 30.014 Seventh Average 512.149 127.526 44.486 43.480 16.674 Day 6 Day 7 Day 8 Day 9 Day 10 Sixth Average 24.457 16.371 10.646 10.486 9.789 Seventh Average 11.131 8.954 11.891 8.154 8.989 16 GARDNER CHENEY BASSET TABLE Ha (Continued) Day 11 Day 12 Day 13 Day 14 Day 15 Sixth Average 11.157 8.294 9.825 7.958 7.130 Seventh Average 8.337 7.223 6.760 6.926 6.741 Day 16 Day 17 Day 18 Day 19 Day 20 Sixth Average 6.767 6.690 6.350 6.376 6.421 Seventh Average 5.781 6.376 6.216 6.724 6.530 Day 21 Day 22 Day 23 Day 24 Day 25 Sixth Average 6.276 5.741 5.899 5.576 6.004 Seventh Average 6.536 5.764 6.136 6.107 6.736 Day 26 Day 27 Day 28 Day 29 Day 30 Sixth Average 5.604 5.716 5.455 5.441 5.466 Seventh Average 5.993 9.959 6.096 8.279 6.159 Day 31 Day 32 Day 33 Day 34 Day 35 Sixth Average 5.320 5.674 5.594 5.658 5.464 Seventh Average 5.513 5.776 5.919 6.936 6.267 Day 36 Day 37 Day 38 Day 39 Day 40 Sixth Average 5.406 5.444 5.284 5.316 5.236 Seventh Average 5.959 5.541 5.913 5.987 7.073 Day 41 Day 42 Day 43 Day 44 Day 45 Sixth Average 5.399 5.853 5.396 5.261 5.461 Seventh Average 6.124 6.227 6.101 6.056 29.444 Day 46 Day 47 Day 48 Day 49 Day 50 Sixth Average 5.364 5.259 5.361 5.396 5.387 Seventh Average 9.473 11.684 9.084 6.347 6.747 Day 51 Day 52 Day 53 Day 54 Day 55 Sixth Average 5.713 5.447 5.230 5.244 5.647 Seventh Average 6.107 5.673 5.599 5.759 5.827 Day 56 Day 57 Day 58 Day 59 Day 60 Sixth Average 5.336 5.359 5.301 5.210 5.241 Seventh Average 7.101 6.204 5.770 6.444 9.736 Day 61 Day 62 Day 63 Day 64 Day 65 Sixth Average 5.333 5.366 5.312 5.198 5.209 Seventh Average 14.376 8.593 7.907 5.879 6.639 Day 66 Day 67 Day 68 Day 69 Day 70 Sixth Average 5.198 5.264 5.186 5.286 5.166 Seventh Average 7.244 6.364 5.953 6.890 5.856 Day 71 Day 72 Day 73 Day 74 Day 75 Sixth Average 5.158 5.184 5.192 5.158 5.212 Seventh Average 5.941 6.844 5.987 6.444 7.359 Day 76 Day 77 Day 78 Day 79 Day 80 Si th Average 5.249 5.189 5.209 5.269 5.169 Seventh Average 6.559 7.016 7.033 6.341 5.959 Day 81 Day 82 Day 83 Day 84 Day 85 Sixth Average 5.155 5.149 5.158 5.153 5.158 Seventh Average 7.976 11.707 5.821 6.290 8.730 HABIT FORMATION IN ALBINO RATS 17 TV Sixth Average ....... LBLE Ila (Continue Day 86 Day 87 5.158 5.158 6.170 5.633 Day 91 Day 92 5.158 5.158 7.170 10.730 Day 96 Day 97 5.158 5.158 7.364 6.679 Failed to learn d) Day 88 Day 89 5.158 5.158 6.364 5.947 Day 93 Day 94 5.158 5.158 8.296 7.719 Day 98 Day 99 5.158 5.158 7.736 7.604 Days required to learn 32.93 44.00+ Seventh Average Sixth Average Seventh Average Sixth Average .... Seventh Average Sixth Average Seventh Average. 2 Day 90 5.158 11.553 Day 95 5.158 6.353 Day 100 5.158 7.559 TABLE lib THE MAZE AVERAGE ABSOLUTE RETENTION OF SIXTH AND SEVENTH GENERATION INBRED RATS Absolute Retention after 60 Days' Rest Sixth Average 65.443 seconds Seventh Average 126 . 680 seconds TABLE lie THE MAZE DAILY RELEARNING AVERAGES OF SIXTH AND SEVENTH GENERATION INBRED RATS (Time in seconds) Sixth Average Seventh Average. Sixth Average. . . Seventh Average. Sixth Average. . . Seventh Average. Sixth Average. .. Seventh Average. Sixth Average . . . Seventh Average . Sixth Average Seventh Average. , Day 1 31.866 45.352 D*y 6 8.174 6.264 Day 11 6.163 6.304 Day 16 5.628 5.656 Day 21 6.186 5.656 Day 26 6.349 5.656 Day 2 10.357 17.392 Day 7 7.426 8.312 Day 12 6.054 5.720 Day 17 5.674 5.656 Day 22 5.897 5.656 Day 27 5.711 5.656 Day 3 11.043 7.344 Day 8 6.351 7.456 Day 13 5.886 6.576 Day 18 5.689 5.656 Day 23 5.911 5.656 Day 28 6.006 5.656 Day 4 9.009 11.104 Day 9 6.111 6.584 Day 14 5.660 5.656 Day 19 6.163 5.656 Day 24 6.206 5.656 Day 29 5.909 5.656 Day 5 8.071 8.064 Day 10 6.129 7.936 Day 15 5.563 5.656 Day 20 5.740 5.656 Day 25 5.811 5.656 Day 30 5.874 5.656 18 GARDNER CHENEY BASSET TV Sixth Average Seventh Average LBLE He (Continue Day 31 Day 32 5.989 5.994 5.656 5.656 Day 36 Day 37 6.014 6.063 5.656 5.656 Day 41 Day 42 5.657 5.917 5.656 5.656 Day 46 Day 47 6.166 5.871 5.656 5.656 Failed to Relearn 2 d) Day 33 Day 34 5.634 5.914 5.656 5.656 Day 38 Day 39 5.991 5.806 5.656 5.656 Day 43 Day 44 5.823 5.777 5.656 5.656 Day 48 ' ,Day 49 6.014 5.800 5.656 5.656 Days Required to Relearn 14.14 + 8.60 Day 35 6.040 5.656 Day 40 5.751 5.656 Day 45 5.934 5.656 Day 50 5.711 5.656 Sixth Average Seventh Average Sixth Average. ., Seventh Average Sixth Average Seventh Average Sixth Average . Seventh Average . TABLE lid THE MAZE ANATOMICAL DATA OF SIXTH AND SEVENTH GENERATION INBRED RATS Sixth Average Body Length in mm. 200 71 Body Brain Cord Weight Weight Weight in grms. in grms. in grms. 195 04 1 75234 53881 Water in Brain per cent 78 49 Seventh Average. . . Sixth Average .... 184.71 Water in Cord per cent ,. 71.73 152.47 1.63286 .50794 Per cent Per cent Brain Weight Brain Weight in Relation to in Relation to Body Length Body Weight .87418 .91653 78.51 Age killed Days 196 Seventh Average 71.70 .88219 1.07851 207 . In Plate IV is shown the curve of learning (below) and of relearning (above) of the 6th generation of inbred rats compared with those of the 7th. These curves are constructed from figures given in Table II. The curve of the 6th generation rats is indi- cated by the broken line, that of the 7th generation by the solid line. The ordinates show the average daily time in seconds for each group, and the abscissae the number of the day in which such time was made. From the twenty-second day the curve of learning of the 6th generation lies below the six-second mark, and from the eighty-third day is flat at 5.158 seconds, signi- fying that on that day the last of this group had perfected the habit. The 7th generation learning curve is very irregular HABIT FORMATION IN ALBINO RATS 19 throughout its length and never approaches the appearance of perfect learning. The 7th generation relearning curve, however, is slightly better than that of the 6th, being flat from the four- teenth day at 5.656 seconds. But it is again necessary to call attention to the fact that this was a selected group, the two Plate IF, The 7 Sn. ln>d au- fctt5.n--nH5t, >> Siti.tiii sii.mi R.Icatrmnj). rats failing to learn having been thrown out and not tested for relearning. The similarity in behavior of the rats of the control series containing blood of the B strain (of which the original parents were purchased from a Baltimore dealer) to the behavior of the inbreds has already been mentioned. The length of time re- 20 GARDNER CHENEY BASSET quired by them to learn the maze had led the investigator to suspect a less than normal brain weight; and, when the brains were weighed, this was found to be the case. Table III presents a comparative summary consisting of the daily averages of the nine rats containing B blood and of the twelve control rats lacking it. Eight of the rats containing B are one-half C and one-half B; the remaining rat is one-half C, one-fourth E, and one-fourth B. That the C' blood is not a factor in their erratic behavior is proven by the fact that most of the rats of the co- trol series not containing B blood do also contain C. In order to compare the behavior of control rats having B and those lacking it with that of the inbred rats, cross reference may be made from Table III to the inbred averages of Table I. The control rats having B blood shall be referred to in Table III as Control +B; those lacking B blood as Control B. The tables (I and III) show that two of the inbreds and one of the +B failed to learn the maze; the B controls had all learned at the end of the twenty-fifth day. The inbred rats required, on the average, 36.62-f- days to learn; the -f-B 35.67 + days, and the B but 16.42 days. The absolute retention of the inbreds was, on the average, 81.558 seconds; of the +B, 72.475 seconds; and of the B, but 51.083 seconds. Two of the inbreds failed to relearn; all the + B had relearned at the end of the twenty-second day; while all the B had relearned at the end of the eighth day. The inbreds required, on the average, 12.68+ days to relearn; the +B, 8.24 days; the B, but 4.08 days. In these criteria of ability to learn the maze, the inbred rats did the least well; the +B rats were, in each instance, above, but not far from, the record of the inbreds; the B were much superior to either. Both body length and body weight were greatest m the in- breds, next in the +B, and least in the B. Actual brain weight was least in the inbreds, much greater in the +B, and slightly greater in the B than in the +B. The relative brain weight (in reference to body length) of the inbreds was 5.46% less than that of the B; that of the +B was 2.53% less than that of the B. The relative brain weight (in reference to body weight) of the inbred rats was 10.02% less than that of the B; that of the +B was 5.15% less than that of the B. As might HABIT FORMATION IN ALBINO RATS 21 be expected, from our hypothesis and the behavior, the average relative brain weight of the +B> rats lies between that of the inbreds and of the B. The results obtained from the supposedly normal B rats reinforce the former conclusion that a lesser relative brain weight is accompanied in a similar degree by a lesser ability 'to form habits. TABLE Ilia THE MAZE DAILY LEARNING AVERAGES OF +B AND B NORMAL CONTROL RATS (Time in seconds) Day 2 Day 3 Day 4 Day 5 187.729 88.511 36.160 29.689 52.997 27.857 . 15.085 22.557 Day 1 Control +B 849.458 Control B.. 248.380 Control +B. Control B. Control +B Control B, Control +B. Control B, Control +B. Control B Control +B. Control B. Control +B Control B Control -t-B. Control B. Control +B. Control B. Control +B. Control B. Control +B. Control B. Control +B. Control B. Day 6 20.093 15.320 Day 11 8.142 6.652 Day 16 6.702 6.675 Day 21 6.529 4.956 Day 26 6.031 4.908 Day 31 6.756 4.908 Day 36 5.758 4.908 Day 41 5.923 4.908 Day 46 5.791 4.908 Day 51 5.874 4.908 Day 56 5.679 4.908 Day 7 15.822 11.550 Day 12 9.544 6.393 Day 17 7.822 5.382 Day 22 6.973 4.948 Day 27 6.156 4.908 Day 32 6.294 4.908 Day 37 5.704 4.908 Day 42 6.461 4.908 Day 47 5.501 4.908 Day 52 5.963 4.908 Day 57 5.546 4.908 Day 8 13.066 11.223 Day 13 8.907 6.190 Day 18 7.067 5.025 Day 23 6-804 4.888 Day 28 6.240 4.908 Day 33 6.006 4.908 Day 38 5.887 4.908 Day 43 5.999 4.908 Day 48 5.466 4.908 Day 53 5.572 4.908 Day 58 5.959 4.908 Day 9 10.276 14.063 Day 14 7.476 6.475 Day 19 7.467 5.265 Day 24 6.719 4.892 Day 29 6.280 4.908 Day 34 6.287 4.908 Day 39 5.971 4.908 Day 44 5.706 4.908 Day 49 5.586 4.908 Day 54 6.559 4.908 Day 59 5.690 4.908 Day 10 12.964 7.110 Day 15 9.289 5.768 Day 20 7.040 4.959 Day 25 6.102 4.908 Day 30 6.142 4.908 Day 35 5.842 4.908 Day 40 5.861 4.908 Day 45 6.159 4.908 Day 50 5.643 4.908 Day 55 5.732 4.908 Day 60 5.723 4.908 22 GARDNER CHENEY BASSET Control +B. Control B. Day 66 Day 67 Control +B 5.928 5.914 Control -^B , 4.908 4.908 Day 71 Day 72 Control +B.. 5.692 5.519 Control B 4.908 4.908 Day 76 Day 77 Control +B 5.550 5.794 Control B 4.908 4.908 Day 81 Day 82 Control +B 5.746 5.673 Control B 4.908 4.908 Day 86 Day 87 Control +B 5.794 5.488 Control B 4.908 4.908 Day 91 Day 92 Control +B 5.723 5.346 Control B 4.908 4.908 v Day 96 Day 97 Control +B 5.630 5.470 Control B 4.908 4.908 Failed to Learn Control +B 1 Control B TABLE Ilia (Continued Day 61 Day 62 Day 63 Day 64 Day 65 6.052 7.346 6.857 5.821 5.794 4.908 4.908 4.908 4.908 4.908 Day 68 Day 69 Day 70 5.528 5.963 5.643 4.908 4.908 4.908 Day 73 Day 74 Day 75 5.426 5.603 5.381 4.908 4.908 4.908 Day 78 Day 79- Day 80 5.519 5.457 6.106 4.908 4.908 4.908 Day 83 Day 84 Day 85 5.617 5.608 5.497 4.908 4.908 4.908 Day 88 Day 89 Day 90 5.546 5.479 5.479 4.908 4.908 4.908 Day 93 Day 94 Day 95 5.452 5.612 5.510 4.908 4.908 4.908 Day 98 Day 99 Day 100 5.399 5.559 5.626 4.908 4.908 4.908 Days Required to Learn 35.67+ 16.42 TABLE Illb THE MAZE AVERAGE ABSOLUTE RETENTION OF +B AND B NORMAL CONTROL RATS Absolute Retention After 60 Days' Rest Control +B 72.475 seconds Control B . . 51 . 083 seconds TABLE IIIc THE MAZE DAILY RELEARNING AVERAGES OF +B AND B NORMAL CONTROL RATS Control +B. Control B. Control +B, Control B. Day 1 36.215 23.480 Day 6 8.075 6.410 Day 2 29.230 11.767 Day 7 7.200 5.307 Day 3 10.930 8.597 Day 8 6.970 5.223 Day 4 11.180 5.873 Day 9 6.340 5.223 Day 5 7.990 5.587 Day 10 6.240 5.223 HABIT FORMATION IX ALBINO RATS 23 TABLE IIIc (Continued) Day 11 Day 12 Day 13 Day 14 Control +B 5.935 5.750 5.740 5.700 Control B 5.223 5.223 5.223 5.223 Day 15 5.835 5.223 Day 20 5.765 5.223 Control +B Day 16 7.090 5.223 Day 21 6.200 5.223 Day 17 5.725 5.223 Day 22 5.415 5.223 Failed to Relearn Day 18 Day 19 5.890 5.550 5.223 5.223 to Day 50 Days Required to Relearn 8.24 4.08 Control B Control -fB Control B Control +B Control B. . Body Brain Cord Weight Weight Weight in grms. in grms. in grms. 175.73 1.75378 .5419'. 168.18 1.75428 .51643 Per cent Per cent Brain Weight Brain Weight in Relation to in Relation to Body Length Body Weight .90406 1.02303 .92748 1.07861 Water in Brain, per cent 78.25 78.37 Age killed, Days 200 175 TABLE Hid THE MAZE ANATOMICAL DATA OF +B AND B NORMAL CONTROL RATS Body Length in mm. Control +B 193.00 Control B 189.50 Water in Cord, Per cent Control +B 71.48 Control B 71.81 In Plate V is shown the curve of learning (below) and of relearning (above) of the inbred and +B rats compared with those of the B. The inbred curve is represented by the solid line, the +B by the heavy broken line, and the B by the lighter broken line. The ordinates show the average daily time in seconds for each group, and the abscissae the number of the day in which such time was made. From the twentieth day the B curve is flat at 4.9 seconds. Neither the inbred nor the +B curves flatten entirely, although the -f B curve of learning is more regular than that of the inbreds. The curve of relearning (lacking the two inbreds and one -f-B that failed to learn) shows little difference between the three groups. But even here, although the inbred and +B are selected groups, the B re- mains superior to both, and the +B is slightly superior to the inbred rats. 24 GARDNER CHENEY BASSET -p ^ - i s - - 1 5 P- l P* M pQ >i/ I 1 - HABIT FORMATION IN ALBINO RATS 25 IV. EXPERIMENT 2: THE PRELIMINARY INCLINED PLANE The apparatus used in this experiment (see Plate VI) was designed especially to make a problem exceptionally difficult to learn, and in this purpose it exceeded expectations. The basic principle is the same as that of the apparatus designed and used Plate 1ZI. The Inclined "Plant. by Watson in his experiments at the University "of Chicago, called by him the " Inclined Plane," and which is described and illustrated in his monograph "Animal Education," page 37. 5 But my apparatus differs from his in several respects. Plate VI shows in detail the construction and method of operation. The food box, A, is framed of wood, eleven by 5 Watson: Animal Education, Chicago, 1903. 26 GARDNER CHENEY BASSET twelv.e inches base, eleven inches in height, and is covered on top and sides with three-eighths inch heavy wire mesh. It is fitted with a hard rubber door, d, three-sixteenths inch thick, five inches high, and four and one-fourth inches wide. To the inner side of the door is fastened a cord which passes over a pulley, p, and is weighted at the other end with a piece of lead, 1, of sufficient weight to insure the opening of the door upon releasing the latch. B shows the device for latching and re- leasing the door. A short distance above the door is fastened a three-inch electrical magnet, m; directly below that is a steel wire, s.w., surmounted by a steel disk, s.d., of the same diameter as the core of the magnet. The steel wire holds the door by dropping through holes in two brass plates, g, which serve as guides, to a point, behind another brass plate which is set at the top of and behind the door, one and one-half millimeters below the top. The setscrew, s.s., placed on the steel wire above the lower guide prevents any further drop. When the steel wire holds the door the disk is two mm. below the magnet ; when the disk is drtiwn up to the magnet one-half mm. clearance is allowed for the door to swing back. Back of the feeding box, A, is placed the inclined plane, I. P. The inclined plane has a hard rubber base three-eighths inch thick, six inches long, and .two and three-eighths inches wide. Upon pivot standards rising from the middle of the base rests the plane itself. The plane is of wood fibre and of the same dimensions as the base. It is weighted at the end nearest the feeding box in order to insure its return to position after use. At the end opposite the weight and farthest from the feeding box, platinum electrical contacts, e.c., are placed in both base and plane. The power is provided through wires connecting the regular electric lighting system, 115 volts, direct current, with the wired apparatus. A 32 candle power lamp is placed in the series in order to avoid any danger of short-circuiting. To make the contact and allow the current to pass through the magnet, thus raising the steel wire and releasing the door, it is necessary for the rat to step on the point of operation, o, which lies well out toward the end of the plane. On account of a certain amount of latency in the operation of the magnet, the rat must not only make the contacts touch, but must also inhibit further action, remaining on the point of operation until HABIT FORMATION IN ALBINO RATS 27 click of the disk meeting the magnet is heard. Over the food box and plane is placed a cage, C, constructed of one-half inch heavy wire mesh, the base measurements of which are twenty- four by twenty-four inches and the height fourteen inches.. This allows the rat ample room to explore all sides of and above the food box. When the rat is placed within, the entrance, e, to the cage is closed. The preliminary inclined plane experiment was not intended so much as a decisive experiment as to test the efficiency of the apparatus. The results, however, are significant and, there- fore, included here. The object of this experiment was to have each rat learn to reach the , interior of the food box from the cage entrance in the least possible period of time. The procedure of a perfectly trained rat was to run from the entrance, e, to the point of operation, o, remaining there until the click of the disk against the magnet insured the door being open, then running through the door of the box to the food which was placed within at point f. The starting time was taken when the animal entered at e, another when the magnet clicked, and the final time when the food box was entered. The object of recording the two periods of time was that it had been anticipated that differences in association between the inbred series and the control rats might appear. But, as in both series the association was practically perfect by the third day, a comparison of such differences was thought useless. In preparation for the experiment each animal, beginning at -the age of sixty-five days, was fed alone in the food box, the door remaining open, ten minutes daily for five consecutive days. This gave the rat an opportunity to become acquainted with .all parts of the interior of both box and cage, and also accustomed him to a reduced feeding time. At the age of seventy days the experiment began. Six males and five females from the inbred strain were used and, as control, an equal number of males and females from the normal series. Ail the inbred rats were from the 6th generation. The stimulus used was their regular food, bread soaked in milk. As one of the first rats used consumed fourteen hours before his first accidental success, it was decided to use "cumulative time" for the first few trials. By this method each rat was 28 GARDNER CHENEY BASSET allowed to work thirty minutes and then, if unsuccessful, he was taken out, the food box door was opened, and he was then re- turned to feed for five minutes and used no more that day. When they began to succeed within the half hour, each rat was required to open and enter the food box five times daily. At the end of the fifth trial it was allowed to feed for five minutes, but was permitted no more food until the completion of the next day's experiment. Each rat was used daily for- twenty days, making one hundred trials. As a time limit had been placed, no criterion of perfect learning was established for this experiment. At the conclusion of the learning experiment the rats were fed in the runway, which has already been described, for sixty days. At the end of this period they were tested for absolute retention .and relearning, and were worked for five days, twenty-five trials, in order to ascertain the effects of the previous training. In Table IV is a comparative summary consisting of the daily averages of the entire inbred group and, directly beneath, the corresponding daily averages of the entire normal control group. From the eleventh day the daily time averages of the control rats are less than those of the inbreds. The absolute retention of the control rats is superior to that of the inbreds. In the five days allotted to testing the effects of previous training, the average time of the control rats is less each day than that of the inbreds. In these criteria of ability the rats of the normal control series are shown to be, on the average, superior to those of the inbred series. Body length of the inbred rats used in the preliminary inclined plane is, on the average, slightly greater than is the case with the control; body weight, however, is a trifle less. The average actual brain weight of the inbreds is less than that of the control. The relative brain weight (in reference to body length) of the inbreds is 11.61% less than that of the control. The relative brain weight (in reference to body weight) of the inbreds is 1 1.65% less than that of the control. Although killed at a later age, the percentage of water in brain and cord of the inbreds is greater than is the case with the control. The preliminary inclined plane figures presented in this table (IV) support the hypothesis that a less than normal average brain weight in a strain of rats is accompanied by a lesser ability to form habits. HABIT FORMATION IN ALBINO RATS 29 TABLE IVa THE PRELIMINARY INCLINED PLANE DAILY LEARNING AVERAGES OF INBRED AND NORMAL CONTROL RATS (Time in seconds) Inbred Average Control Average Inbred Average Control Average Inbred Average . . Control Average . Day 1 1965.858 2470.393 Day 2 1631 . 178 971.102 Day 3 60.302 79.775 Day 4 Day 5 45.916 28.116 27.564 21.840 Inbred Average . . Control Average . Day 6 11.120 20.305 Day 7 14.833 10.375 Day 11 6.262 5.342 Day 12 9.465 5.015 Day 16 7.360 5.062 Day 17 6.247 3.727 Day 8 9.095 9.869 Day 9 16.855 8.971 Day 10 7.109 8.058 Day 13 7.658 5.055 Day 14 10.916 4.425 Day 15 10.291 4.513 Day 18 5.531 4.800 Day 19 8.811 5.815 Day 20 10.251 5.622 TABLE IVb THE PRELIMINARY INCLINED PLANE AVERAGE ABSOLUTE RETENTION OF INBRED AND NORMAL CONTROL RATS. Absolute Retention After 60 Days' Rest Inbred Average 59.309 seconds Control Average 49 . 164 seconds TABLE IVc THE PRELIMINARY INCLINED PLANE DAILY RELEARNING AVERAGES OF INBRED AND NORMAL CONTROL RATS (Time in seconds) Day 1 Day 2 Day 3 Day 4 Day 5 Inbred Average 24.302 9.905 11.516 8.149 7.869 Control Average 17.102 5.498 5.869 4.262 6.651 TABLE IVd THE PRELIMINARY INCLINED PLANE ANATOMICAL DATA OF INBRED AND NORMAL CONTROL RATS Body Length in mm. Inbred Average 190.82 Control Average 189 . 45 Water in Cord, Per cent Inbred Average 71 .436 Control Average 71 . 128 Body Weight in grms. 166.50 168.08 Brain Weight in grms. 1.62031 1.81946 Cord Weight in grms. .47535 .52819 Water in .tsrain, per cent 78.565 77.982 Per cent Brain Weight in Relation to Body Length .84929 .96084 Per cent Brain Weight in Relation to Body Weight .98140 1.09571 Age killed, Days 194 170 30 GARDNER CHENEY BASSET In Plate VII is shown the curve of learning (left) and of re- learning (right) of the inbred rats compared with those of the normal control. These curves are constructed from figures given in Table IV. The curve of the inbred rats is indicated by the solid line, that of the normal control by the broken line. The ordinates give the average daily time in seconds for each group, and the abscissae the number of the day in which such time was made. The time required by both inbred and control rats for the first four days was so long that it is represented here by figures and does not appear in the curve. Both learning curves are irregular, but on the eleventh day that of the control passes permanently below that of the inbred. The curves of relearning show that the inbreds had failed to benefit by practice to so great an extent as the normal control. V. EXPERIMENT 3: THE INCLINED PLANE The apparatus used in this experiment was the same as that used in Experiment 2 : the Preliminary Inclined Plane. The animals were prepared in the same way as for the previous experiment, and began work at the age of seventy days. Sixteen males and fourteen females from the inbred strain were used and, as control, an equal number of males and females from the normal series. Of the inbred rats, fifteen were from the seventh genera- tion, fourteen from the eighth, and one from the ninth. As the behavior of the single ninth generation rat did not vary greatly from -the average of the eighth generation, her results have been included in the tables and curves of the eighth generation. The stimulus used in this experiment was the same as in the two preceding, bread soaked in milk. Cumulative time was used in recording the earlier trials as in the previous experiment. When the rats began to succeed in entering the food box within the half hour, each one was required to open and enter the food box three times each day. At the end of the third trial it was allowed to feed in the box for five minutes, but was permitted no more food until the completion of the next day's experiment. Each rat was used daily until it had learned the problem perfectly, the criterion of perfection being three perfect trials for each of three successive days. A perfect trial consisted in running frbm the entrance to the point of operation on the plane at the rear of the food box, opening the door, run- HABIT FORMATION IN ALBINO RATS 31 V O 43 O to 32 GARDNER CHENEY BASSET ning around and entering the box, all within four seconds; but, if the time consumed in opening the box after passing the entrance was more than two seconds, or if the time consumed in entering the box after having opened the door was more than two seconds, the trial was considered a failure. Thus it was possible for a rat to have a perfect trial in as long a total time as four seconds, or a failure in a less total time. Those rats failing to learn within one hundred days (three hundred trials) were no longer used for experimentation. Those rats learning the inclined plane were, at the conclusion of the experiment, fed for sixty days in the runway. At the end of this period they were tested for absolute retention and relearning. Three of the rats formed the habit of lifting the plane at the end nearest the food box and thus formed the contact, but this method apparently affected neither the rapidity of each trial nor the number of days required for perfect learning. One of the normal rats placed his nose between the electrical contacts and received a shock, but other than one squeal and a vigorous rubbing of the nose, he showed no evidence of harm and had apparently forgotten the experience the following day. Some of the rats jumped to the point of operation from a distance; some placed the fore paws on the end of the plane and pressed down; and still others ran slowly around to the plane, halting an instant on the point of operation, and then continued the run around to the door. As a rule, the last made the best time. As in the maze experiment, many of the inbred rats were subject to errors which persisted throughout the experiment. In particular may be mentioned one rat that invariably formed a loop in the course from the entrance to the point of operation. The shortest period of time required by an inbred rat to learn the inclined plane perfectly was twelve days ; by a normal con- trol rat, nine days. Eleven inbred rats and one control failed to learn the inclined plane within the one hundred days allowed. In Table V is presented a comparative summary consisting of the daily averages of the entire inbred group and, directly beneath, the corresponding daily averages of the entire normal control group. The inbred rats required, on the average, 73.70 + days to learn the inclined plane; the controls but 45. 9 74- day s. The absolute retention of the inbreds was, on the aver- age, 31.842 seconds; of the controls, but 22.587 seconds. All HABIT FORMATION IN ALBINO RATS 33 the inbreds had relearned at the end of the twenty-fourth day; but all the controls had relearned at the end of the seventeenth day. The inbred rats required, on the average, 6.74 days to re- learn; the controls but 4.68 days. In all these criteria of ability, learning, absolute retention and relearning, the rats of the normal control series are shown, on the average, to be superior to those of the inbred series. The body length of the inbred rats used in the inclined plane experiment is, on the average, a trifle greater than that of the controls; the body weight is slightly less. The average actual brain weight of the inbreds is less than that of the controls. The relative brain weight (in reference to body length) of the inbreds is 5.89% less than that of the controls. The relative brain weight (in reference to body weight) of the inbreds is 2.38% less than that of the controls. Although the inbred rats were killed, on the average, at a more advanced age than the normal controls, the percentage of water in brain and cord is higher. The figures presented in Table V support the hypothesis that a less than normal average brain weight in a strain of rats is ac- companied by an average lesser ability to form habits. TABLE Va THE INCLINED PLANE DAILY LEARNING AVERAGES OF INBRED AND NORMAL CONTROL RATS Inbred Average . . Control Average. Inbred Average Control Average Inbred Average . . Control Average . Inbred Average Control Average Inbred Average . Control Average. Inbred Average . Control Average. (Time Day 1 4673.131 2769.953 in seconds) Day 2 1218.976 1072.722 Day 3 166.997 133.287 Day 4 56.576 61.600 Day 5 22.926 23.995 Day 6 36.878 25.874 Day 7 12.422 13.478 Day 8 11.061 11.704 Day 9 10.751 11.280 Day 10 8.136 6.961 Day 11 9.383 6.759 Day 12 7.876 6.858 Day 13 8.625 6.347 Day 14 7.188 7.045 Day 15 9.586 6.383 Day 16 8.710 5.069 Day 17 8.069 5.400 Day 18 7.364 6.158 Day 19 9.191 5.376 Day 20 6.717 5.352 Day 21 6.919 5.284 Day 22 6.363 4.378 Day 23 6.951 5.173 Day 24 6.458 5.280 Day 25 7.329 5.025 Day 26 5.674 4.978 Day 27 6.262 5.139 Day 28 6.627 5.276 Day 29 5.514 4.302 Day 30 27.802 4.303 34 GARDNER CHENEY BASSET T Inbred Average Control Average 'ABLE Va (Continue Day 31 Day 32 7.440 6.707 4.354 5.075 Day 36 Day 37 '6.957 5.334 4.121 4.457 Day 41 Day 42 4.898 4.938 3.748 4.033 Day 46 Day 47 4.011 5.076 3.367 3.091 Day 51 Day 52 4.762 4.408 3.044 3.160 Day 56 Day 57 4.072 3.916 3.000 3.392 . Day 61 Day 62 4.172 5.026 3.173 3.251 Day 66 Day 67 4.160 3.955 3.098 3.109 Day 71 Day 72 4.383 3.869 2.976 3.072 Day 76 Day 77 3.726 3.669 2.667 2.799 Day 81 Day 82 3.359 3.465 3.094 2.663 Day 86 Day 87 3.729 3.388 2.763 2.834 Day 91 Day 92 3.501 3.393 2.683 2.665 Day 96 Day 97 2.999 3.460 2.772 2.657 Failed to Learn 11 4) Day 33 Day 34 6.416 6.775 3.583 3.868 Day 38 Day 39 5.689 5.479 3.958 4.905 Day 43 Day 44 5.093 4.762 3.446 3.729 Day 48 Day 49 3.831 4.991 3.159 3.050 Day 53 Day 54 4.693 5.006 2.841 2.924 Day 58 Day 59 5.909 6.235 3.047 ,2.991 Day 63 Day 64 4.437 3.317 2.848 3.951 Day 68 Day 69 5.858 4.806 3.155 3.138 Day 73 Day 74 3.461 3.713 3.163 2.640 Day 78 Day 79 3.646 4.043 2.770 2.750 Day 83 Day 84 3.558 3.322 2.665 2.652 Day 88 Day 89 3.686 3.639 2.669 2.681 Day 93 Day 94 4.124 3.463 2.623 2.732 Day 98 Day 99 3.604 2.982 2.608 2.946 Days Required to Learn 73.70 + 45.97+ Day35 6.957 4.024 Day 40 5.289 4.096 Day 45 4.553 3.857 Day 50 5.105 3.113 Day 55 4.191 3.759 Day 60 4.953 3.375 Day 65 3.741 3.035 Day 70 4.260 2.949 Day 75 3.649 3.129 Day 80 3.648 2.839 Day 85 3.471 2.823 Day 90 4.170 2.663 Day 95 3.457 2.846 Day 100 3.510 2.637 Inbred Average Control Average Inbred Average Control Average Inbred Average Control Average Inbred Average Control Average Inbred Average Control Average Inbred Average Control Average Inbred Average Control Average Inbred Average Control Average Inbred Average Control Average Inbred Average Control Average Inbred Average . , Control Average Inbred Average Control Average Inbred Average Control Average Inbred Average. Control Average. 2 HABIT FORMATION IN ALBINO RATS 35 TABLE Vb THE INCLINED PLANE AVERAGE ABSOLUTE RETENTION OF INBRED AND NORMAL CONTROL RATS Absolute Retention After 60 Days' Rest Inbred Average 31 . 842 seconds Control Average 22.587 seconds TABLE Vc THE INCLINED PLANE DAILY RELEARNING AVERAGES OF INBRED AND NORMAL CONTROL RATS (Time in seconds) Inbred Average . . Control Average . Inbred Average . . Control Average . Inbred Average . . Control Average. Inbred Average . . Control Average. Inbred Average . Control Average . Day 1 41.789 22.598 Day 2 7.301 6.198 Day 3 5.436 4.279 Day 4 4.783 3.985 Day 6 5.175 3.057 Day 7 4.239 3.569 Day 8 3.404 3.598 Day 9 3.186 3.021 Day 11 3.508 2.929 Day 12 2.712 3.676 Day 13 2.796 3.200 Day 14 2.845 2.633 Day 16 2.610 2.776 Day 17 2.603 2.586 Day 18 2.596 2.586 Day 19 2.733 2.586 Day 21 Day 22 2.607 2.586 Inbred Average. . Control Average . 2.729 2.586 Failed to Relearn Day 23 2.596 2.586 Day 24 2.554 2.586 Days Required to Relearn 6.74 4.68 Day 5 5.067 5.021 Day 10 2.828 3.293 Day 15 2.807 2.664 Day 20 2.677 2.586 Day 25 to 50 2.554 2.586 TABLE Vd THE INCLINED PLANE ANATOMICAL DATA OF INBRED AND NORMAL CONTROL RATS Body Weight in grms. 184.37 189.18 Body Length in mm. Inbred Average 195 . 93 Control Average 194 . 43 Brain Weight in grms. 1.72083 1.81840 Cord Weight in grms. .53787 .53922 Water in Brain per cent, 78.363 78.319 Water in Cord, per cent Inbred Average 71 .437 Control Average 71.223 Per cent Brain Weight in Relation to Body Length .87972 .93474 Per cent Brain Weight in Relation to Body Weight .97889 1.00275 Age killed, Days 220 194 36 GARDNER CHENEY BASSET In Plate VIII is shown the curve of learning (below) and of relearning (above) of the inbred rats compared with those of the normal control. These curves are constructed from figures given in Table V. The curve of the inbred rats is indicated by the solid line, that of the normal control by the broken line. The ordinates give the average daily time in seconds for each group, and the abscissae the number of the day in which such time was made. As in the other learning curves, the time required by both inbred and control rats for the first four days was so long that it is represented here by figures and does not appear in the curve. The descent in time of both inbred and control rats for the first ten days is quite rapid, although both show retardation on the sixth day. From the forty-first day the curve of the controls lies entirely below the four second mark. The inbred curve, throughout, shows great irregularities, especially on the thirtieth day, when it rises to an average of nearly twenty-eight seconds. The inbred curve of relearning is very similar to that of the control, and from the twenty-third day coincides with it. But 'again, in relearning, we are dealing with selected groups, the eleven inbreds and two controls that failed to learn not being included. The inbreds of this selected group had all relearned at the end of the twenty- fourth day; the control at the end of the seventeenth day. In Plate IX may be seen the distribution curves of learning of both the inbred and control series for the inclined plane experi- ment. The time is given in days in groups of five for learning, singly for relearning. It is very apparent that the advantage lies wholly in favor of the normal control series. Of the inbred rats used in the inclined plane experiment, fifteen were from the seventh generation, fourteen from the eighth, and one from the ninth. In Table VI is presented a comparative summary consisting of the daily averages of the seventh and eighth generation rats used in the inclined plane experiment. With the rats of the eighth generation may be included the one from the ninth, as her record was not far from the average of the eighth. The table shows that four of the seventh generation and seven of the eighth generation failed to learn the inclined plane. The seventh generation required, on the average, 59.60+ days to learn; the eighth generation, 86.53+ days. The absolute retention of the seventh genera- HABIT FORMATION IN ALBINO RATS 37 tion was, on the average, 44.945 seconds; of the eighth'genera- tion, 13.825 seconds. All the seventh generation had relearned at the end of the twenty-fourth day; but all the eighth genera- tion had relearned at the end of the eighth day. The seventh generation required, on the average, 8.00 days to relearn; the eighth generation but 5.00 days. Vla.it Effi. The Inclined Pi ane, Ittlrr.dStra.ia Mot mal Control Da< - lifted - Ho^tnat I J 1kl3:l3l 3.7 kl. If 3 3. ILIt.m i /**.y?7 In these criteria of ability the seventh generation excelled in learning, the eighth in absolute retention and relearning. But, again, in absolute retention and relearning, we are dealing with selected groups as the seven eighth generation and four seventh generation rats that failed to learn were not used. There seems, on the whole, to be but little difference between the abilities of 38 GARDNER CHENEY BASSET - ,40 en c E C ooi - % ff>- it, ?(, -98 tL - iL - '1 '9 - IS s - is of -fft fr - Itr h - f r - it -17 7 - IT t - f( X- O K - If V -II' f - If of - fr rr-/r r -9/ yi - a HABIT FORMATION IN ALBINO RATS 39 the seventh and eighth generations except that the former excelled in learning. The body length and body weight of the seventh generation average greater than those of the eighth. The relative brain weight (in reference to body length) of the seventh generation is 5.20% less than that of the eighth. The relative brain weight (in reference to body weight) of the seventh generation is 13.93% less than that of the eighth. The actual brain weight of the seventh generation, however, is greater than that of the eighth. The percentage of water in brain and cord of the seventh generation is greater than in the eighth. TABLE VI THE INCLINED PLANE DAILY LEARNING AVERAGES OF SEVENTH AND EIGHTH GENERATION INBRED RATS (Time in seconds) Day 1 Day 2 Day 3 Day 4 Day 5 Seventh Average . . ... 5656.546 1463.707 123.213 53.583 17.159 Eighth Average . . . 3689.716 974.245 210.781 58.369 28.693 Day 6 Day 7 Day 8 Day 9 Day 10 Seventh Average . . 11.142 11.373 8.160 8.845 5.853 Eighth Average 62.613 13.471 13.961 12.657 10.419 Day 11 Day 12 Day 13 Day 14 Dav 15 Seventh Average . . 10.855 5.459 6.037 5.197 6.367 Eighth Average . . 7.911 10.294 11.213 9.178 12.805 Day 16 Day 17 Day 18 Day 19 Day 20 Seventh Average. . 5.821 4.732 6.714 6.351 5.310 Eighth Average . . . 11.599 11.407 8.013 12.030 8.124 Day 21 Day 22 Day 23 Day 24 Day 25 Seventh Average. . 5.592 5.538 4.934 6.116 5.481 Eighth Average . . . 8,245 7.187 8.969 6.801 9.177 Day 26 Day 27 Day 28 Day 29 Day 30 Seventh Average. . 6.204 5.636 5.809 3.965 4.875 Eighth Average 5.143 6.889 7.445 7.063 50.729 5 Day 31 Day 32 Day 33 Day 34 Day3 Seventh Average . . Eighth Average. . . , 4.085 10.795 5.884 7.529 4.733 8.099 4.195 9.355 4.526 9.388 Day 36 Day 37 Day 38 Day 39 Day 40 Seventh Average . . Eighth Average . . . 4.481 9.433 3.921 6.747 4.192 7.186 4.467 6.491 4.561 6.017 Day 41 Day 42 Day 43 Day 44 Day 45 Seventh Average . . Eighth Average . 4.667 5.130 4.462 5.413 4.396 5.791 4.338 5.187 3.734 5.373 40 GARDNER CHENEY BASSET T, Seventh Average <\BLE VI (Continue Day 46 Day 47 3.435 4.121 4.587 6.031 Day 51 Day 52 4.289 3.591 5.235 5.226 Day 56 Day 57 3.251 3.584 4.893 4.247 Day 61 Day 62 4.295 5.753 4.049 4.299 Day 66 Day 67 3.331 3.571 4.988 4.339 Day 71 Day 72 4.415 3.717 4.350 4.021 Day 76 Day 77 3.760 3.601 3.691 3.736 Day 81 Day 82 3.319 3.039 3.398 3.891 Day 86 Day 87 3.251 3.037 4.207 3.739 Day 91 Day 92 3.211 3.157 3.792 3.629 Day 96 Day 97 2.725 2.650 3.273 4.270 Failed to Learn 4 d Day 48 Day 49 3.636 3.867 4.027 6.116 Day 53 Day 54 4.219 4.969 5.168 5.044 Day 58 Day 59 4.406 7.917 7.412 4.553 Day 63 Day 64 4.731 3.006 4.143 3.627 Day 68 Day 69 4.317 5.175 7.398 4.437 Day 73 Day 74 3.388 3.233 3.533 4.193 Day 78 Day 79 4.130 4.705 3.162 3.381 Day 83 Day 84 3.322 3.135 3.794 3.509 Day 88 Day 89 3.144 3.361 4.228 3.916 Day 93 Day 94 3.273 2.913 4.975 4.014 Day 98 Day 99 3.085 3.201 4.123 2.763 Days Required to Learn 59.60 + 86.53 + Day EO 4.277 5.933 Day 55 3.627 4.755 Day 60 4.325 5.581 Day 65 3.331 4.151 Day 70 4.565 3.954 Day 75 3.620 3.678 Day 80 3.623 3.673 Day 85 3.424 3.519 Day 90 3.837 4.503 Day 95 2.678 4.237 Day 100 3.067 3.954 Eighth Average Seventh Average Eighth Average Seventh Average Eighth Average Seventh Average Eighth Average Seventh Average Eighth Average Seventh Average Eighth Average Seventh Average Eighth Average Seventh Average Eighth Average Seventh Average Eighth Average Seventh Average Eighth Average Seventh Average . . . Eighth Average Seventh Average. Eighth Average . . 7 TABLE VIb THE INCLINED PLANE AVERAGE ABSOLUTE RETENTION OF SEVENTH AND EIGHTH GENERATION INBRED RATS Absolute Retention After 60 Days' Rest * Seventh Average 44 .945 seconds Eighth Average 13 . 825 seconds HABIT FORMATION IN ALBINO RATS 41 TABLE Vic THE INCLINED PLANE DAILY RELEARNING AVERAGES OF SEVENTH AND EIGHTH GENERATION INBRED RATS Seventh Average. Eighth Average. . Seventh Average'. Eighth Average. . Seventh Average. Eighth Average . . Seventh Average. Eighth Average . . Seventh Average . Eighth Average . . (Time in seconds) Day 1 Day 2 66.322 9.265 8.058 4.438 Day 6 6.297 3.633 Day 11 4.285 2.441 Day 16 2.733 2.441 Day 21 2.727 2.441 Seventh Average. . . Eighth Average .... Day 7 4.467 3.925 Day 12 2.909 2.441 Day 17 2.721 2.441 Day 22 2.939 2.441 Failed to Relearn Day 3 6.322 4.218 Day 8 5.879 2.441 Day 13 3.055 2.441 Day 18 2.709 2. Ml Day 23 2.709 2.441 Day 4 5.758 3.443 Day 9 3.728 2.441 Day 14 3.139 2.441 Day 19 2.945 2.441 Day 24 2.636 2.441 Days Required to Relearn 8.00 5.00 Day 5 6.279 3.400 Day 10 3.109 2.441 Day 15 3.073 2.441 Day 20 2.848 2.441 Day 25 to 50 2.636 2.441 TABLE VId THE INCLINED PLANE ANATOMICAL DATA OF SEVENTH AND EIGHTH GENERATION INBRED RATS Body Length in mm. Seventh Average 202. 13 Eighth Average 189 . 73 Water in Cord, per cent Seventh Average 71 . 569 Eighth Average 71 .304 Body Weight in grms. 201.44 167.33 Brain Weight in grms. 1.72868 1.71299 Cord Weight in grms. .54607 .52967 Water in Brain, per cent 78.542 78.185 Per cent Brain Weight in Relation to Body Length .85622 .90323 Per cent Brain Weight in Relation to Body Weight .90560 1.05218 Age killed, Days 223 217 In Plate X is shown the curve of learning (below) and of re- learning (above) of the seventh generation of inbred rats compared with those of the eighth. These curves are constructed from figures given in Table VI. The curve of the seventh generation rats is indicated by the broken line, that of the eighth generation 42 GARDNER 'CHENEY BASSET by the solid line. The ordinates show the average daily time in seconds for each group, and the abscissae the number of the day in which such time was made. Both curves in the learning series are very irregular, especially so that of the eighth genera- tion rats. Although irregular, the seventh generation curve lies fi.St) JB.Jil Plate X The Inclined Plane below that of the eighth except in a few instances. From the first, the relearning curves are similar and very regular, although the eighth generation curve remains below that of the seventh all the way. But both of the relearning groups are selected, four of the seventh and seven of the eighth generation having failed to learn the inclined plane. HABIT FORMATION IN ALBINO RATS 43 VI. SUMMARY AND CONCLUSIONS During a series of experiments in inbreeding conducted at the Wistar Institute of Anatomy and Biology, a strain of albino rats was produced, the relative brain weights of which averaged considerably less than normal. Whether such a condition was induced by the inbreeding or was due to environmental factors can not be stated with certitude at the present time. Inbreeding, per se, may not be, necessarily, productive of deleterious results if the parent stock be perfect in every respect ; but it is impossible, by any means at our command, to determine physical perfection in any organism. An environmental factor that may have had some bearing on the lesser relative brain weight condition of the two strains of rats (A and B) used in these experiments was, that after four generations of inbreeding the rats did not appear to thrive ; at that time a change of diet took place, after which they seemed in better health. The writer spent two years in the task of attempting to ascer- tain whether or not the less than normal relative brain weight was accompanied by a corresponding lesser ability to form habits, and, also, if such ability was progressively less from one generation of inbreeding to the next. There were used in all the experiments one hundred and twenty-four rats: sixty-two inbreds and sixty-two normal controls. An equal number of males and females from inbreds and controls were used in each experiment. Plate XI shows the distribution of relative brain weights (with reference to body length) of the inbred rats and of the normal control series. The inbred distribution is represented by the lower curve, that of the normal control by the upper. The greatest frequency in the inbred curve occurs at .88%; in the normal curve at .92%. The entire inbred distribution is from .70% to .95%; that of the normal controls from .84% to 1.05%. The average relative brain weight (with reference to body length) of the sixty-two normal control rats is .93351%; that of the inbreds is .87335%, or 6.44% less than that of the normal control. In order to compare the ability of the rats of the lesser brain weight strain (inbred rats) with a normal control series, three experiments were used: 1. The Maze, in which all the rats used were given five trials daily until they had learned perfectly, or, failing to learn, had 44 GARDNER CHENEY BASSET t * r >/> Q n LL HABIT FORMATION IN ALBINO RATS 45 worked one hundred days (500 trials). At the expiration of sixty days after perfect learning the rats, except those failing to learn, were tested for absolute retention and relearning until relearning was perfect, or, failing relearning, for fifty days (250 trials). 2. The Preliminary Inclined Plane, in which all the rats used were given five trials daily for twenty days (100 trials); at the expiration of sixty days after this period they were all tested for absolute retention and relearning for a period of five days (25 trials). 3. The Inclined Plane, in which all the rats used were given three trials daily until they had learned perfectly, or, failing to learn, had worked one hundred days (300 trials). At the ex- piration of sixty days after perfect learning the rats, except those failing to learn, were tested for absolute retention and relearning until relearning was perfect. In all these experiments the strain of rats of lesser relative brain weight (the inbreds) learned less well, on the average, than the normal control series. In the maze and inclined plane ex- periments the average number of days required to learn and re- learn, and the time of absolute retention, was far greater in the case of the inbred rats than in that of the normal control series. In the maze experiment, two inbreds and one control failed to learn; two inbreds failed to relearn. In the inclined plane ex- periment, eleven inbreds and two controls failed to learn. The similarity of behavior of the control rats containing blood of the B strain to that of the inbreds suggests the importance of crossing a strain of inbred rats of lesser brain weight with normal rats, and carrying out a series of tests such as have been presented in this paper, with two controls : one of normal rats, and one of rats of lesser relative brain weight. In the maze experiment the inbred rats of the seventh genera- tion did a little less well than those of the sixth. In the inclined plane experiment the rats of the eighth generation did a little less well than those of the seventh. It would seem (although lessening of relative brain weight had ceased after the fourth generation of inbreds) that the ability to form habits lessened pro- gressively with successive generations of inbreeding. The writer had intended to attempt a correlation (if any existed) between the number of days required to learn a habit 46 GARDNER CHENEY BASSET and the number of days required to relearn after sixty days' rest. But most of the rats relearned very quickly without reference to trie number of days required for learning ; in numbers, too, the rats were too few for such mathematical consideration. An investigation along such a line should consist of but one relatively simple experiment; several hundred rats of one sex only should be used; and the period of time between the corii- pletion of learning and the beginning of relearning should be lengthened to, at the least, ninety days. The general results of the experiments here set forth may be summed up as follows: On the average, the strain of inbred rats having a less than normal relative brain weight did less well in learning to form habits than did the normal control series. From these results the following may be formulated: A less than normal brain weight in a strain of rats is accompanied by a less than normal ability to form habits. ADDENDUM The tables of individual daily averages, from which the tables of group averages contained in this monograph are derived, are so extensive as to preclude publication. If, however, any may be interested in them, the original copy is deposited with the library of the Wistar Institute of Anatomy and Biology, Philadelphia, Pennsylvania; and a duplicate is in the private library of the author'. VITA Gardner Cheney Basset was born in Boston, Massachusetts, June 17, 1873. He received his elementary and secondary edu- cation in the public schools of Boston and Newton, Massachu- setts. From the year 1890 to 1908 he was in the wholesale shoe business, serving successively as receiving clerk, buyer, traveling salesman and superintendent. He entered the Col- legiate Department of Clark University in 1908; was assistant in Biology during the year 1909-1910; assistant in Psychology during the year 1910-1911; and in 1911 he received the degree of Bachelor of Arts with highest honor. He spent the summer of 1910 at Cornell University in the study of Experimental Psychology. In October, 1911, he entered the Johns Hopkins University, and was appointed University Fellow in that insti- tution for the year 1912-1913. While at the Johns Hopkins University Mr. Basset studied Psychology under Drs. Watson and Dunlap, Psychiatry under Dr. Meyer, Genetics under Dr. Jennings, and Neurology under Drs. Mall and Sabin. UNIVERSITY OF CALIFORNIA LIBRARY BERKELEY THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW ./Books not returned on time are subject to a fine of 5@e per volume after the third day overdue, increasing tf^$1.00 per volume after the sixth day. Books not in demand may be renewed if application is made before expiration of loan period. 3 JAH29T960 Fe3'60JM NOV 1 3 196<& 50m-7,*16 ^ " s UNIVERSITY OF CALIFORNIA LIBRARY