EVOLUTION AS SEEN IN THE CLASSIFICATION OF ANIMALS 317 



(1940) has concluded that there is no evidence that the two species breed 

 together and produce hybrids — this despite the fact that when mice of the 

 two species are kept together in laboratory cages they readily produce 

 healthy and fertile hybrid offspring. Why do they not hybridize under natu- 

 ral conditions? It does not seem too anthropomorphic to conclude that when 

 given a choice each prefers members of its own species to members of the 

 other. Accordingly we conclude that where morphological or physiological 

 barriers to interbreeding do not exist, psychological barriers may operate 

 to the same end (see pp. 471-473). 



We readily appreciate that this failure of species to interbreed (reproduc- 

 tive isolation) is important for the maintenance of the species as a discrete 

 unit. Without this tendency not to hybridize, whenever two species came 

 into contact interbreeding would tend to combine the two species into a 

 varied hybrid population, an amalgamation in which the species concerned 

 would become inextricably combined, losing identity and individuality. Al- 

 though this process doubtless occurs at times, the fact that species in general 

 maintain their individuality indicates that such interbreeding is not the usual 

 situation. 



As intimated above, the statement that members of different species do 

 not interbreed is subject to exceptions. When hybrids between species are 

 produced, however, they commonly, but not invariably, have lowered fer- 

 tility or are completely sterile. Thus, despite the production of hybrids, the 

 reproductive isolation of the species is maintained. Obviously a sterile 

 hybrid population could not perpetuate itself and become a "melting pot" 

 into which the two parent species would sink. 



At times, on the other hand, hybridization may play an important role in 

 evolution just as it does in the breeding programs by which man improves 

 his domesticated animals and cultivated plants. Hybrids possess new com- 

 binations of characteristics arising from their mixed ancestry. In cases in 

 which they are viable and fertile, such hybrids may under some circum- 

 stances enjoy advantages not possessed by either parent species, and hence 

 be favored by natural selection (see further discussion on pp. 477-481). 



In one way or another, the integrity of species is maintained by reproduc- 

 tive isolation. Without the latter there would be no species. Hence the justifi- 

 cation for the primacy given reproductive isolation in Mayr's definition of 

 species quoted above. Further discussion of the importance of reproductive 

 isolation and of the mechanisms by which it arises will be found in Chapters 

 20 and 21. 



We should note in passing, however, that the definition of "actually or 

 potentially interbreeding populations, which are reproductively isolated from 



