I. LIFE PROCESSES 33 



of the growth of wild rats in their natural habitat, and that 

 conditions incident to captivity, the lack of exercise, changed 

 nutrition, excessive fear, etc., greatly retarded early growth, 

 which is probably very rapid in the wild form. On this as- 

 sumption, the more rapid growth of individuals in the later 

 generations would indicate merely that the animals had 

 become somewhat adapted to their changed environment and 

 were, therefore, resuming their normal rate of growth. Such 

 an argument cannot be refuted at present, or probably at any 

 time, since there seem to be insurmountable obstacles in the 

 way of a scientific study of growth processes in wild Norway 

 rats. 



Aside from restricting activity somewhat, the conditions 

 under which captive Grays lived would seem to be far more 

 favorable to growth and to reproduction than are those to 

 which wild rats are usually subjected. The element of fear 

 could certainly not have influenced the growth of caged rats 

 more than it must affect that of wild Xorways which are 

 constantly harassed by enemies of many kinds. The food 

 given caged animals was more abundant and more nutritious 

 than that wild rats obtain by foraging, and temperature con- 

 ditions in the colony room were not as extreme as those in 

 nature. Lack of exercise may possibly have influenced the 

 later growth of caged Grays, but it could hardly have changed 

 growth processes in young rats that spend the greater part 

 of their time in the nest. Taking all of these various factors 

 into consideration, it seems probable that the course of body 

 growth in the first generation of captive Grays is fairly 

 representative of that in the wild forms, and that it can he 

 used legitimately to indicate the effect of captivity on the 

 body growth of individuals in the later generations. 



Since the albino rat is a mutant variety of tin- irray Norway. 

 one may assume, perhaps, in light of the findings for captive 

 (I rays, that the rapid growth of Albinos during early life is 

 a result of their long domestication. They have seemingly 

 become completely adapted to life and conditions of captivity, 

 and they have lost all fear of man, which is a dominant trait 



MEMOIR 14 



