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 
