32 HELEN DEAN KING AND HENRY H. DONALDSON 



In the rat, as in the mouse and many other mammals, the 

 period of most rapid growth ends at puberty, or very shortly 

 afterward. The age at which sexual maturity was attained 

 by gray females was not determined in any case by the exact 

 method of Long and Evans ('22). The average age at which 

 females of the first generation cast their first litters was 266 

 days; that of females of the ninth generation was but 200 

 days. In these rats, therefore, the beginning of reproductive 

 life was advanced two months in the course of nine genera- 

 tions. Whether the more rapid growth of animals in the 

 later generations induced earlier maturity or whether both 

 the increase in the rate of growth and earlier maturity de- 

 pended upon other factors is not known. 



Robertson ('16) states: 



The fact that the syntheses which constitute the growth of an 

 organism are self-accelerated or autocatalyzed implies the existence 

 of substances capable of acting as catalysers of growth. We know 

 that many of the glands of internal secretion, anterior lobe of the 

 pituitary, the thyroid, and thymus are capable of exerting important 

 influences upon the processes of growth influences the lack of which 

 become apparent when one of the glands is removed. 



It seems not improbable that the changes which life in 

 captivity induced in the rate and in the extent of body growth 

 in gray rats, and the effects of captivity upon breeding, were 

 not related as cause and effect, but that both depended upon 

 some alteration in the endocrine system which tended to 

 speed up the growth processes and to induce earlier breed- 

 ing as the generations advanced. It is interesting, in this 

 connection, to note that the one endocrine gland in these rats 

 most affected by life in captivity w r as the hypophysis which, 

 as shown by Evans and Long ('22), is capable of influencing 

 body growth in the rat. This gland is somewhat larger in 

 both sexes of captive gray rats than it is in the wild form, 

 and has tended to approach the albino type, as will be shown 

 in a subsequent section (part II, p. 87). 



It may be argued that the course of body growth in cap- 

 tive gray rats of the first generation is not representative 



