CONSANGUINEOUS MARRIAGES 241 



fertility could be increased as well as decreased by selective breed- 

 ing. Some of the lines were inbred (brother and sister) for 75 

 generations without loss of fertility or vigor. The work of 

 Shultze and of Copeman and Parsons on mice, of Castle on rats 

 (bred for 17 generations), and the observations on guinea pigs 

 reported by Popenoe revealed no evidence of a decline of fer- 

 tility as a result of inbreeding. 



The most thorough investigation of inbreeding has been 

 carried on by Miss H. D. King with the albino rat. The work of 

 Miss King revealed several sources of error that have to be 

 guarded against in a study of inbreeding and which not improb- 

 ably misled some previous investigators of the subject. Without 

 describing the methods and precautions followed by Miss King, 

 it may be stated that 25 generations of such close inbreeding as 

 brother and sister matings did not produce any loss in the vigor, 

 growth, or fertility of the inbred strains as compared with the 

 controls. 



The rediscovery of Mendel's law in 1900 stimulated renewed 

 interest in the problems of inbreeding and cross breeding, and led 

 to attempts to interpret the varied results in terms of this illum- 

 inating principle. The usual explanation given is that inbreeding 

 is injurious only when it brings out unfavorable characteristics 

 that have been latent in the stock. Naturally, inbreeding affords 

 an opportunity for recessive characters to make their appearance. 

 If, for instance, such a recessive trait as albinism is present in a 

 stock, it may be brought out by inbreeding. Davenport remarks 

 that "Albino communities of which there are several in the 

 United States are inbred communities, but not all inbred com- 

 munities contain albinos." 



Many strains contain recessive characteristics of an undesirable 

 kind. So long as these are kept from appearing by the presence of 

 corresponding dominant characteristics all goes well. But when 

 two organisms are crossed in each of which the recessive trait 

 occurs, we should expect the trait to appear in one-fourth of the 

 offspring. In the different varieties of corn there are probably 

 many factors upon which size, vigor and fertility depend. Most 



