372 HELEN DEAN KING 



the tendency to sterility that is seemingly caused by hybridiza- 

 tion. Such a selection was not attempted, apparently, in 

 any of the series of experiments cited above, nor was it done 

 in my own work with hybrid stock. The experiments of Crampe, 

 of Ritzema-Bos, and of von Guaita show unquestionably that 

 fertility in hybrid rats is diminished by random inbreeding, but 

 they cannot legitimately be used to give evidence regarding the 

 effects of inbreeding on the fertility of a pure race. 



Other series of inbreeding experiments made on pure strains of 

 rodents show that inbreeding does not necessarily lead to a 

 marked decrease in fertility. Neither Schultze ('03) nor Cope- 

 man and Parson ('09) found inbred mice less productive than the 

 outbred strain; Castle ('16) did not find any great decrease in 

 fertility in various races of rats inbred for seventeen generations. 

 In the inbreeding experiment with guinea-pigs that has been 

 carried on for several years at the Bureau of Animal Industry 

 in Washington, there is, to quote Popenoe ('17): ''no general 

 deterioration. While a few strains have run out, others are 

 nearly as vigorous as are the control families." 



Results comparable to the above have been obtained with 

 other animals. It is well known that inbreeding has been used 

 extensively, and with very favorable results, in the building up 

 of various strains of thoroughbred horses and cattle (Wriedt, '16), 

 and the productiveness of these strains has not been greatly 

 lessened. In the extensive series of inbreeding experiments with 

 Drosophila, made by Castle et al. ('06), it was found that ''in- 

 breeding probably reduces very slightly the productiveness of 

 Drosophila, but the productiveness may be fully maintained 

 under constant inbreeding (brother and sister) if selection is made 



from the more productive families Selection has a 



much greater influence on fertility than inbreeding, so that 

 selection from the most productive pairs is able to more than 

 offset the effects of inbreeding." The effectiveness of selection 

 in increasing the fertility within an inbred strain is shown with 

 great clearness in Moenkhaus' ('11) experiments with Drosophila. 

 Moenkhaus was able to establish two distinct strains, one of high 

 and one of low fecundity, by selecting, from among the variable 



