FERTILITY 195 



effects of recombination of the different sorts of qualities 

 contained in the parental strains. His attack on the 

 hypothesis of rejuvenation that was so generally held 

 at that time did very great service in exposing the 

 mystical nature of such an imagined effect of cross- 

 fertilization. In particular, Weismann's endeavor to 

 connect the theory of recombination with the facts 

 of maturation of the egg and sperm has opened our 

 eyes to possibilities that had never been realized before. 

 His work has led directly to the third advance that 

 has been made in very recent years, when the results 

 of Mendelian segregation have been applied directly 

 to the study of fertility and sterility. 



As I have said, Darwin's work showed that cross- 

 fertilization is generally beneficial. The converse 

 proposition has long been held that continued inbreed- 

 ing leads to degeneration and to sterility. This opinion 

 rests largely on the statements of breeders of domesti- 

 cated animals and plants, but there is also a small 

 amount of accurate data that seems to support this 

 view. I propose first to examine this question, and 

 then consider what cross-fertilization is supposed to do, 

 in the light of the most recent work. 



Weismann inbred white mice for 29 generations, 

 and Ritzema-Bos bred rats for 30 generations. In 

 each case the number of young per litter decreased 

 in successive generations, more individuals were sterile 

 and many individuals became weakened. This evi- 

 dence falls in line with the general opinion of breeders. 



On the other hand, we have Castle's evidence on 

 inbreeding the fruit fly through 59 generations. He 

 found some evidence of the occurrence of sterile pairs 



