EVOLUTION AS SEEN IN THE CLASSIFICATION OF ANIMALS 327 



(Fig. 14.5). Race A interbreeds with race B where their ranges come into 

 contact. Race B interbreeds with race C, as does race C with race D. But 

 race D does not interbreed with race A where their ranges come into con- 

 tact. Are race D and race A to be considered separate species because they 

 are thus reproductively isolated from each other? Perhaps not so long as 

 the circle of interbreeding forms (races B and C) connecting them exists. 

 But if races B and C were not known (had become extinct before biologists 

 investigated the situation) probably race A and race D would be considered 

 separate species. We mention this matter ( 1 ) to illustrate the fact that sub- 

 species are not in all cases clearly distinguishable from species, and (2) to 

 indicate one way in which species may arise from subspecies. (For further 

 examples and discussion see Rensch, 1960, p. 23 ff.; Goldschmidt, 1940, 

 p. 117 ff.; Lack, 1947, Fig. 23, p. 127, and accompanying text.) 



We do not wish to give the impression that all subspecies are on the way 

 to becoming species. For most subspecies, conditions will probably never 

 be favorable for further development. Opportunity will not knock on their 

 doors. But for a minority, conditions will favor further evolution leading 

 to the formation of new species, and even perhaps eventually to new genera, 

 families, and so on. 



In subsequent chapters we shall discuss in more detail the processes in- 

 volved in subspecies and species formation, as well as in the production of 

 the major adaptive changes which usually distinguish members of different 

 higher categories (e.g., different orders). 



Conclusion 



Let us return for a moment to the question of the manner in which classifi- 

 cation supports the idea of creation by evolution, as contrasted with the idea 

 of special creation. We have seen that classification is based upon similarity, 

 primarily morphological similarity. To most biologists it seems more reason- 

 able to explain fundamental similarities as based upon inheritance from 

 common ancestry than as being due to the fact that separately created ani- 

 mals were created to be similar, or created according to similar patterns. 

 Again, the difficulty of defining a species, of separating one species from 

 another, and of telling whether a certain group is a species or a subspecies 

 seems to indicate a web of interrelationships in nature, interrelationships 

 most readily explained as arising by divergence from common ancestry. If 

 each species were separately created it should be much easier than it is to 

 draw sharp lines between them, and to draw sharp lines between species 

 and subspecies. 



