LIFE PROCESSES IN CAPTIVE GRAY RATS 



17 



these individuals, growth data were compiled for twenty se- 

 lected pairs of rats of each sex belonging in the sixteenth to 

 the twenty-fifth generations. Each pair of rats of the same 

 sex, one large and one small, were members of the same litter. 

 All were individuals that continued to increase in weight 

 throughout adult life, and were apparently in good physical 



TABLE 4 



Increase in "body weight with age for twenty pairs of males and of females from 



the sixteenth to the twenty-fifth generation of captive gray rats. Each 



pair of the same sex (one large and one small) from the same litter 



condition at the end of the weighing period. Differences in 

 their body weights, therefore, cannot be attributed to disease. 

 The smaller individuals of the pairs were not runts, nor was 

 their growth restricted, as is shown by the fact that their 

 average body weights at any age period were not significantly 

 lower than corresponding weights for all rats in the genera- 

 tion groups from which they came (tables 2 and 3). Body 

 weight increase with age in these rats is given in table 4. 



