268 COOK 



The essential difference between a species and a subspecies 

 does not lie, as commonly supposed, in the nature or amount of 

 the differences as such. The practical question is whether two 

 groups are actually separate in nature or are still connected. 

 Subspecies may be more different than other completely segre- 

 gated species. On the other hand, groups which are really 

 segregated in nature and thus unable to interbreed, are by that 

 fact on the road to the acquisition of specific differences. That 

 they may not have become very different from each other does 

 not prove that they are not good species or that it is undesirable 

 to accord them recognition as such. 



It does not follow, as some have supposed, that subspecies 

 are always incipient species, or that there is any inherent force 

 or tendency which will insure a subsequent separation into 

 distinct species. The existence of these diverse local forms has 

 not been shown to be any disadvantage to a species, and may, 

 indeed, conduce to its greater vigor, since it tends, like heterism, 

 to insure a certain amount of desirable diversity of descent. 



If the habits of a species were to change in the direction of 

 an increase of its power of dissemination and wide interbreed- 

 ing, the local differences would tend to disappear, since new 

 variations could then spread more rapidly throughout the whole 

 group and render its evolutionary progress more uniform. 



Porrism corresponds, inside the species, to many of the dif- 

 ferences between species. It is true that when species of the 

 same genus live in different environments and have different 

 habits they usually have structural difference corresponding to 

 their respective needs. Examples of such adaptations are fre- 

 quent among the higher plants and animals, and their super- 

 ficial similarity to artism inside the species has been the basis of 

 the doctrine that evolution has been effected by environmental 

 causes. The best corrective of this misapprehension is a study 

 of one of the lower groups of plants and animals in which the 

 same family, order or class has the same habits and the same 

 place in the economy of nature. Many excellent examples will 

 be found among the mosses, liverworts and alga? among plants, 

 and among the myriapoda and lower insects where the number 

 and character of the diversity of the species is out of all imag- 



