194 INBEEEDING AND OUTBEEEDING 



effect of crossing various distinct species, has led many- 

 writers to identify the phenomena. Further, several 

 critics have maintained that a theory which purports to 

 interpret sterility in the one case, should interpret it in 

 the other. Now this is a point of view which is obviously 

 incorrect even with our present meagre knowledge of the 

 facts. The sterility often accompanying inbreeding is 

 not the same thing as the sterility resulting from hybrid- 

 ization. The resemblance is superficial in the extreme. 

 In the one case there is the differentiation of distinct 

 strains differing anatomically and physiologically in their 

 ability to perform the act of reproduction. It is a phe- 

 nomenon of Mendelian heredity which stands out in the 

 clear-cut manner it does, because the progenitors of the 

 individuals thus characterized have gone through with the 

 mechanical process which segregates factors, in the pre- 

 cise manner necessary to accomplish the purpose. In the 

 other case, the individuals are sterile because they cannot 

 go through this same process in the exact and proper way 

 required, on account of the incompatibility of the unit- 

 ing cells. 



