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 



