EFFECTS OF IXBREEDING ON FERTILITY AND VIGOR 369 



During the first three, years, as table 13 shows, there was little 

 diminution in the average size of the litters produced. In the 

 three following years, however, litter size decreased considerably, 

 and at the end of the investigation the litters averaged less than 

 one-half the size of those obtained in the beginning. These 

 results certainly justify Ritzema-Bos' conclusion that: ''Die 

 fortgesetze Zucht in engster Verwandtschaft vermindert das 

 Fortpflanzungsvermogen, kann sogar schliesslich voUkommene 

 Unfruchtbarkeit verursachen." Lloyd ('12) has suggested that 

 the deterioration in Ritzema-Bos' stock might have been due to 

 overcrowding, since many varieties of rats will not breed in close 

 confinement. 



TABLE 13 



Showing Ritzema-Bos' data for the average size of the litters and for infertile matings 



in a series of inbred rats 



Von Guaita obtained a number of white mice from a strain 

 that had been inbred by August Weismann for twenty-nine gener- 

 ations. How these mice were inbred I do not know, since I have 

 not been able to find any account of the details of this experiment. 

 Von Guaita crossed these white mice with Japanese waltzing 

 mice, and then inbred their descendants for five generations. 

 The data regarding the average size of the litters obtained in these 

 two sets of investigations are shown in table 14. 



Weismann's data, given in table 14, show that the average size 

 of the litters decreased directly as the inbreeding advanced, and 

 so appear to indicate that inbreeding lessened the fertility of the 

 mice. In this experiment there seems to have been a very great 

 difference in the number of litters that were produced in the vari- 

 ous generations. In the first two generations there was an aver- 



