PROLOGUE Xlll 



The albino rat thrives when inbred, provided there is selec- 

 tion for vigor. In a revolving cage it runs long distances volun- 

 tarily much to the advantage of its general health. To train- 

 ing, it responds readily. 



If the life span of three years in the rat is taken as equivalent 

 to 90 years in man, it is found that the growth changes in the 

 nervous system occur within the same fraction of the life span 

 (i.e., at the equivalent ages) in the two forms. What is true 

 for the nervous system is also probably true for some of the 

 other systems and this makes possible the cross reference of the 

 results from the two species, with a high degree of precision. 



It is now appreciated that the mammals associated with 

 man, of which group the rat is a good example, are very re- 

 sponsive to environmental changes. The sensitiveness which 

 they show and the speed with which the responses occur, re- 

 quire to be kept in view in all work observational or experi- 

 mental on the living animal. This is a matter on which it is 

 hardly possible to lay too great emphasis. 



Such responsiveness is an advantage when it is sought to 

 induce changes, but a distinct disadvantage when the constancy 

 of the control animals is desired. 



The critical periods in development are well marked, and 

 birth, weaning and puberty, though somewhat differently spaced 

 in the span of life, bring with them the same changes that appear 

 in man. 



Finally, in the case of the rat, the wild form, the Norway, is 

 generally available for comparison with the domesticated Al- 

 bino, and thus the modifications induced by captivity and 

 domestication can be readily observed. 



Some of the effects of domestication are already recognized, 

 but the matter calls for further study. In this connection it is 

 not without interest to note that the Albino rat has probably 

 been under domestication for less than a century, whereas, by 

 contrast, most of the common domesticated animals have been 

 associated with man for hundreds or even thousands of years, 

 and in most instances the wild types from which they came are 

 either doubtful or hard to obtain. 



