EFFECTS OF INBREEDING 



fifty-nine generations in succession without ob- 

 taining a diminution in either the vigor or the 

 fecundity of the race, which could with cer- 

 tainty be attributed to that cause. A slight 

 diminution was observed in some cases, but 

 this was wholly obviated when parents were 

 chosen from the more vigorous broods in each 

 generation. Nevertheless crossing of two in- 

 bred strains of Drosophila, both of which were 

 doing well under inbreeding, produced off- 

 spring superior in productiveness to either 

 inbred strain. Even in this case, therefore, 

 though inbreeding is tolerated, cross-breeding 

 has advantages. 



In the case of many domesticated animals, 

 it is the opinion of experienced breeders, sup- 

 ported by such scientific observations as we 

 possess, that decidedly bad effects follow con- 

 tinuous inbreeding. Bos ( '94) practiced con- 

 tinuous inbreeding with a family of rats for 

 six years. No ill-effects were observed during 

 the first half of the experiment, but after that 

 a rapid decline occurred in the vigor and fer- 

 tility of the race. The average-sized litter in 

 the first half of the experiment was about 7.5, 



147 



