18 INBKEEDINO AND OUTBREEDING 



inbreeding was established. But this system appears to 

 have been deficient. Some evolutionary advantage asso- 

 ciated with separation of the sexes was lost. There is 

 reason for assuming that this advantage was connected 

 with cross-fertilization, for tertiary developments in each 

 kingdom brought about numerous mechanisms whereby 

 cross-fertilization was established in hermaphroditic or- 

 ganisms. Still, in spite of the obvious success of bisexual 

 and of cross-fertilized species, as shown by their fre- 

 quency, numerous self -fertilized species, and even species 

 reproducing exclusively by asexual methods have kept 

 their places in the struggle for existence. Both inbreed- 

 ing and outbreeding systems have developed side-by-side 

 under natural conditions. The data of comparative mor- 

 phology, therefore, seem as contradictory as those from 

 anthropology and agriculture. 



The puzzles presented by these general facts taken 

 from history, husbandry and biology have one common 

 feature. They centre on the problem of inheritance. For- 

 tunately, though less than two decades have passed since 

 the application of quantitative experimental methods to 

 biology became somewhat general, the mechanism of 

 heredity is no longer a riddle; and to-day the effect of 

 inbreeding and outbreeding on plants and animals can 

 be described in considerable detail and interpreted with 

 singular precision. Having this interpretation it may be 

 applied to the three fields of interest we have described. 



In the ensuing pages the important controlled experi- 

 ments in inbreeding and outbreeding necessary for an 

 orderly and consistent interpretation of the facts are dis- 

 cussed. Uncontrolled experiments, casual observations 

 of stock breeders, data on human marriages between near 



