ASPECTS OF KINETIC EVOLUTION 363 



all the members of the species are closely similar, but in others 

 they may be extremely unlike, as when the specializations of 

 sex and polymorphism have been developed. There is no need, 

 however, that we define species as a morphological term, since 

 species are not caused nor constituted by the likeness or unlike- 

 ness of the component organisms. Indeed, it is unlikeness rather 

 than likeness that conduces to the prosperity of the species. 



The species in nature is constituted by the fact that the com- 

 ponent individuals breed together. For evolutionary purposes 

 a species is a group of interbreeding organisms ; nothing more 

 is required, nothing less will suffice. Species are units of 

 organic evolution ; organisms continue to exist and to make evo- 

 lutionary progress only in large groups of freely interbreeding 

 individuals. Groups of organisms which do not interbreed are 

 no longer species ; they no longer have the typical and essential 

 evolutionary constitution of living matter. 



Whether the individuals are alike or different does not in the 

 least affect the specific unity of a group if the organisms are 

 associated in nature on a basis of free interbreeding. If the 

 groups have ceased to interbreed, Avhether by reason of geo- 

 graphical barriers, or of structural or instinctive incompatibility, 

 they are no longer a unit of evolution, no matter how close the 

 external similarity may appear. 



Natural species are not the only groups of organisms to which 

 the name is applied, but since all other so-called species are 

 mere parts or fragments of natural species, a recognition of 

 natural species must precede a true appreciation of the more or 

 less artificial subdivisions of species. 



These evolutionary facts are quite independent of the old 

 taxonomic idea that the limits of species could be determined 

 by ascertaming whether the animals or plants can interbreed. 

 The evolutionary question is whether they do interbreed. 

 Groups able to interbreed perfectly will still follow divergent 

 courses of evolution, if kept apart. On the other hand, the 

 failure of the extreme members of the same species to inter- 

 breed would not destroy the unity and coherence of the group. 1 



1 Cook, O. F., 1905. The Evolutionary Significance of Species. Smith- 

 sonian Report for 1904. 



