LIFE PROCESSES IN CAPTIVE GRAY RATS 11 



fact that in individuals of this group growth was retarded 

 by adverse conditions in the colony. That improved environ- 

 mental and nutritive conditions had a favorable influence on 

 body growth is shown by the rapid rise of graphs 2 and 3, 

 which run very close together during their entire length. The 

 difference in the level of the graphs during the latter part of 

 their course indicates that adult males of the last two genera- 

 tion groups weighed about 15 per cent more than did those of 

 the first group. The temporary check to body growth result- 

 ing from the transfer of the colony to a new location accounts 

 for the depression near the end of graph 3. 



Body weight changes with age in the three groups of fe- 

 males (tables 1 to 3) are shown graphically in figure 2. 



While the graphs in figure 2 are similar in form to those 

 in figure 1, they run at lower levels, as females tend to weigh 

 less than males at all age periods after weaning. 



Females of the last two generation groups responded to 

 better environmental conditions by growing at a more rapid 

 rate during early life, and attaining a larger size than did 

 females of earlier generations. They weighed, on the average, 

 about 13 per cent more during adult life than did females of 

 the first group, as the levels of the graphs in figure 2 indicate. 



According to Sorin ('32), litter size has a marked effect on 

 the body growth of guinea pigs. This factor could not have 

 influenced growth in gray rats, since throughout this investi- 

 gation the great majority of litters used for study contained 

 either five or six individuals: no litters of less than four or 

 more than seven young were ever reared. 



After twenty-five generations had been born in captivity, 

 both rate and extent of body growth in males had increased 

 greatly, as shown by the graphs in figure 3, which were con- 

 structed from body weight data given in table 5. 



In figure 3 graph (A) rises gradually from the beginning to 

 the end of its course, and shows no evidence of any pro- 

 nounced growth acceleration during the adolescent period in 

 males of the first generation. Body weight increase during 

 later life was very uniform, and at the final weighing these 





