14 HELEN DEAN KING 



lactation on body weights could not be eliminated nor offset 

 in compiling the data. The course of these graphs indicates 

 that females of the twenty-fifth generation grew much more 

 rapidly during early life, and were heavier at all subsequent 

 age periods, then females of the first generation, their weight 

 excess at the end of the weighing period being about 19 per 

 cent. 



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

 growing during adolescence and early maturity at a rate 

 approaching that characteristic of albino rats maintained in 



Females 



Body weight in grams 



A Generation 1 

 B Generation 25 



Age in days 



50 100 150 200 250 300 350 400 450 500 550 600 



Fig. 4 Body weight in females of the first and of the twenty-fifth generation. 



captivity for a long period of time. The average body weights 

 of adult grays, however, were greater than those of albinos 

 of like ages. The effects of captivity on the rate and extent 

 of body growth in gray Norway rats are similar to those that 

 have occurred in various domesticated mammals, such as 

 cattle, horses, and sheep. These animals, as Darwin (1875) 

 has shown, grow more rapidly and attain a much larger size 

 than did their wild prototypes. This growth increase in 

 domesticated animals is due, in all probability, to selective 

 breeding and to the more favorable conditions of environment 

 and of nutrition that captivity entails. 



