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. 



