I. LIFE PROCESSES 17 



'18, etc.; Slonaker and Card, '23; Evans and Bishop, '22, 

 '22 a, '23; Kennedy, '26) have shown conclusively that rats 

 require a well-balanced, highly nutritious diet for normal 

 growth and for reproduction and that these animals respond 

 very quickly to changes in nutritional regime. Males, seem- 

 ingly, are more influenced by unfavorable conditions than are 

 females, especially during fetal and early postnatal life, when 

 the mortality among them is much greater (King, '21). It 

 seems probable, therefore, that the slow growth of the males 

 during early postnatal life in Donaldson's first series of 

 Albinos was due to inadequate nutrition. 



The growth relation of the sexes during early life in the 

 first generation of captive Grays cannot be attributed to in- 

 adequate nourishment, since these animals received an abun- 

 dance of highly nutritious food. Moreover, in the various 

 strains of albino rats in the colony on the same nutritional 

 regime, the growth of the males was more rapid than that 

 of the females at all age periods. It must be assumed, thcre- 

 fore, either that the growth relations of the sexes during 

 early postnatal life in the first generation of captive Grays 

 was that normal for the race in its wild state or that lack <>t' 

 adjustment to conditions of life in captivity adversely af- 

 fected the wild mothers of these rats and, through her, in- 

 fluenced the vigor of the young, particularly that of the males, 

 so that they did not grow at a normal rate. Whichever of 

 these alternatives one chooses to accept, it is certain that 

 after one generation in captivity gray rats became so well 

 adjusted to their new habitat that the course of body Lrruwth 

 in their offspring showed the same sex relations at all age 

 periods as those usually found in albino rats. 



No marked changes occurred in the growth of the indi- 

 viduals in any of the first ten generations of captive (J ray- 

 that made one generation stand out conspicuously from the 

 rest. It seemed advisable, therefore, to combine the data 

 for each three succeeding generations, beginning with the 

 second, thus making three groups of data for nine gem-r- 

 ations. Data for the first generation stand alone, to be used 



MEMOIR 14 



