18 INTROGRESSIVE HYBRIDIZATION 



the same farm, was in a spot that had been less radically 

 disturbed, and it contained fewer individuals of obviously 

 hybrid ancestry. Throughout the entire site, as a matter of 

 fact, the degree of introgression was directly proportional to 

 the disturbance of ''natural conditions" by man and his do- 

 mestic animals. 



In general, therefore, the habitat exercises a tremendously 

 strong restriction upon hybridization between well-differ- 

 entiated entities. Recombinations resembling the parental 

 forms, and backcrosses resembling the parents, are at a strong 

 selective advantage. The production of hybrid swarms is 

 limited to particular times and places at which man or nature 

 may have ''hybridized the habitat." Even in many of these 

 cases, as the previous ecological balance is restored, recom- 

 binations closely resembling the original parents will be those 

 most likely to survive. The commonest end result of a hybrid 

 swarm will be the introduction of a comparatively few genes 

 from one species into the germplasm of another — in other 

 words, introgression. 



