LIFE PROCESSES IN CAPTIVE GRAY RATS 



27 



breeding was 266 days. Generation changes in the age of 

 females at the beginning and at the end of reproductive life 

 are indicated by the graphs in figure 7. 



The earlier breeding of females in late generations can be 

 ascribed, in part at least, to their more rapid growth during 

 adolescence. As a rule, rats that grow rapidly when young 

 are the ones that begin to breed at an early age; those that 



TABLE 7 



Length of Ihe reproductive period in the tenth to the twenty-fifth generation of 



captive gray rats 



grow slowly do not mature until much later. It is possible, 

 also, that in the rat, as in the domestic fowl (Warren, '34; 

 Hays, '36) age at sexual maturity is an inherited character 

 that can be influenced by selection. Assuming that some indi- 

 viduals in the foundation stock of this strain carried genes 

 tending to induce early maturity, such genes would have accu- 

 mulated in the stock after a time because of inbreeding. The 



