EFFECTS OF INBREEDING ON FERTILITY AND VIGOR 375 



the inbred rats became sufficiently homozygous for vigor to 

 become fairly constant. Beyond the point, as the data show, 

 there was little variation in the fertility or in the longevity of the 

 animals up to the twenty-fifth generation. Selection and favor- 

 able environment kept the strain at a point of high productive- 

 ness, but, under the conditions of the experiment, they did not 

 increase vigor beyond the stage which was reached at the tenth 

 generation. As already stated, no attempt was made in the 

 course of these experiments to influence fertility by selecting 

 breeding animals from large or from small litters. Whether 

 selection can act in the rat, as it does in Drosophila, and produce 

 strains of high and of low productiveness within a line that has 

 been inbred for many generations is a problem for the future. 

 As the strain has been very fertile for many generations it seems 

 very improbable that any sudden loss in fertility will occur in the 

 future, unless sterility appears as a mutation which cannot be 

 eliminated by selection. 



While corresponding records for the two inbred series (A,B) 

 are in close agreement, there are, nevertheless, differences be- 

 tween the series that have persisted from the very beginning. 

 Female A, one of the two females with which the experiment 

 were started, showed a relatively high degree of fertility since she 

 gave birth to fi\'e litters, containing thirty-five young, before she 

 was killed at the age of one year: female B, a sister of female A, 

 cast only one litter of five young, although she was paired con- 

 tinuously for several months and appeared to be in good physical 

 condition. The two litter brothers with which these females 

 were paired showed no marked differences in size or in vigor. 

 The rats of the A series (which were descended from female A) 

 were, as the records show, somewhat more fertile than the rats 

 of the B series (the descendants of female B), and they also tended 

 to mature earlier and to live longer. The differences found were 

 not very marked in any case, and they might well be ignored were 

 it not for the fact that in all of the characters noted the animals 

 of the A series were superior to those of B series. Environment 

 cannot be held accountable for these differences, since the two 

 series of inbreds were kept constantly under similar conditions of 



