EFFECTS OF INBREEDING ON FERTILITY AND VIGOR 353 



individuals. If young rats are fed exclusively on a meat diet, 

 puberty is considerably delayed (Watson, '06) ; the same effect is 

 produced by underfeeding (Osborne, Mendel and Ferry, '17). 

 According to my observations, the time of year in which the ani- 

 mals are born affects their subsequent growth and also the time of 

 their maturing. Rats born in the winter and early spring grow 

 rapidly, and usually breed at about three months of age; those 

 born in the summer and autumn grow more slowly and compara- 

 tively few of the females cast litters before they are four months 

 old, many not breeding until spring, which is the season of the 

 most pronounced sexual activity for the rat. Convincing evi- 

 dence that age alone does not determine the beginning or the end 

 of the reproductive life of the rat is given by Osborne and Mendel 

 ('15, '17), who found that Albino females, stunted at an early 

 age by underfeeding, were completely sterile until they were 

 properly nourished, when they grew rapidly, attained a normal 

 size, and were able to breed long after the age at which the meno- 

 pause usually appears. 



According to Darwin ('75) and others, favorable environment 

 tends to delay sexual maturity, though not necessarily to decrease 

 fertility. Since these inbred rats were reared, for the most part, 

 under environmental conditions that seemed well adapted to 

 their needs, and since they lacked the stimulus to reproductive 

 vigor which is said to come from outcrossing, it might be expected 

 that they would tend to mature much later than stock Albinos 

 which were not inbred. 



Table 9 shows the approximate age at which the breeding 

 females belonging to various generation groups of the two inbred 

 series cast their first litter. 



The records for the first generation group, given in table 9, 

 confirm Osborne and Mendel's findings that underfeeding tends 

 to retard sexual maturity, since they show that about one-half 

 of the breeding females in this group did not cast their first litter 

 until they were four months old. In subsequent generations, 

 when the animals were adequately nourished, they began breed- 

 ing at a much earlier age. Under the conditions of this experi- 

 ment, inbreeding seemingly hastened the onset of puberty, for 



