68 HELEN DEAN KING AND HENRY H. DONALDSON 



for experimental work requiring a pure strain of known 

 ancestry. 



SUMMAEY 



1. The analysis of various life processes in captive gray 

 rats, as given in part I of the present paper, is based on data 

 for the first ten generations of these animals that descended 

 from six wild Norway females brought into The Wistar In- 

 stitute Animal Colony in the spring of 1919. Data given com- 

 prise 1539 litters, containing 9505 individuals, 4680 males and 

 4825 females. 



2. Marked changes in the rate and in the extent of body 

 growth occurred in both sexes of captive Grays as the genera- 

 tions advanced (tables 1 to 3). At the tenth generation 

 growth during early life was much more rapid than that in 

 animals of the first generation, and was approaching the 

 albino type. Growth during adult life changed relatively 

 little, however, as the average body weight of adult males in 

 the tenth generation was only about 4 per cent greater than 

 that of males in the first generation (table 4, chart 4) ; in 

 adult females the body weight had increased about 8 per cent 

 (table 4, chart 5). 



3. The variability in body weight at birth was about the 

 same in both sexes of captive Grays, but the range of varia- 

 tion in the birth weights was slightly greater in males than in 

 females. 



4. Males of the tenth generation were significantly less 

 variable in body weight throughout adult life, but not during 

 early life, than were males of the first generation (table 4) ; 

 females of this generation showed a slight decrease in varia- 

 bility at all age periods (table 5). 



5. As the generations advanced, the trend of variability in 

 both sexes of captive Grays was toward that found in stock 

 Albinos. Males of the tenth generation were slightly more 

 variable in body weight during early life than were females, 

 but a reverse relation was found throughout adult life (tables 

 4 and 5). In both sexes variability in body weight was cor- 



