64 HELEN DEAN KING 



during the adolescent period was nearly equal to that found 

 in stock albino rats that have been under domestication for a 

 long period of time. 



The growth rate in females during the suckling period was 

 more rapid than that in males, but at all subsequent age 

 periods males grew more rapidly than females. 



At the twenty-fifth generation adult rats of both sexes were, 

 on the average, about 20 per cent heavier than individuals of 

 the first generation (figs. 3 and 4). 



Eats attaining an adult weight much above the average for 

 all individuals of like sex in the same generation group ap- 

 peared in increasing numbers as the generations advanced 

 (table 4, fig. 5). The weight increase in these individuals is 

 ascribed, tentatively, to genetic factors that activated growth 

 mainly during adult life. Some of these large rats, especially 

 males, showed a tendency to maim or kill smaller individuals 

 of the same sex. 



In both sexes of gray rats body-weight variability decreased 

 as the generations advanced. In males of the last generation 

 variability during early life was about one-half that in males 

 of the first generation, and subsequently was still further 

 reduced (table 5, fig. 6). In females of this generation body- 

 weight variability was less than that in females of the first 

 generation at all age periods, but the decrease was significant 

 only for periods during adult life (table 6, fig. 6). This 

 marked decline in variability is attributed mainly to the 

 method of selecting breeding stock and to inbreeding. 



There was no sex difference at birth in the body-weight 

 variability of gray rats, but males were more variable than 

 females during early life and less variable during adult life. 

 As the increased variability in adult females was due, proba- 

 bly, to weight fluctuations resulting from pregnancy and lacta- 

 tion, it is concluded that, on the whole, body-weight variability 

 in males was slightly greater than that in females. 



At the twenty-fifth generation the average length of the 

 reproductive period was nearly 8 months longer than the 

 average for the first generation (table 7, fig. 8). This exten- 

 sion was caused by the earlier breeding of the rats and the 



