ASPECTS OF KINETIC EVOLUTION 267 



Individuals from neighboring localities may maintain the usual 

 amount of similarity, but if specimens from remote parts of the 

 geographic range of the species be compared they may prove 

 notably different. If the climatic or other conditions of the two 

 localities are unlike it is very natural to infer that this is the 

 cause of the differences between their organic inhabitants. 1 



That this explanation may prove, in some cases, to be correct, 

 does not justify us, however, in neglecting to perceive that the 

 remote members of a species may have opportunities to accu- 

 mulate diverse characteristics, much as though they belonged 

 to two distinct species. The extent to which they can do this 

 will depend upon the habits of the particular plant or animal. 

 Sedentary species of animals or plants which have no means of 

 securing wide dissemination of seeds or pollen, tend to manifest 

 local divergencies. The cause of this is, apparently, that new 

 characteristics appear in different parts of the range of the 

 species more rapidly than they can be distributed through the 

 whole interbreeding group. Thus the quail, or Virginia par- 

 tridge, a nonmigratory bird extending from New England to 

 Central America, shows a large number of appreciably different 

 local varieties or subspecies, which might not exist if the bird 

 were migatory and there were a more general intermingling of 

 the members of the species. The differences which charac- 

 terize such local subspecies may be quite the same, both in 

 character and amount, as those which distinguish completely 

 segregated species, but they are treated as subspecies because 

 the distribution of the whole group still remains continuous, and 

 provides a complete series of connecting links between the local 

 forms which happen to be described as subspecies. 



1 Engler, A., 1904. Plants of the Northern Temperate Zone in their Transi- 

 tion to the High Mountains of Tropical Africa. Annals of Botany, iS : 539. 



" I am convinced that in such cases the somewhat different climate is the 

 cause of all or at least of a part of the modifications. Sometimes in connection 

 with these new variations are also to be observed (cf. Cerastium ccespitosum), 

 which may become the beginning of other new forms. The constancv of such 

 climatical adaptations may be a different one and often become fixed through a 

 geological period. I may add that systematic studies have also convinced me 

 that many of the xerophytes, and that a good deal (I do not say all) of the quali- 

 ties of xerophytes, which are usually called adaptations for protection against a 

 dry climate, are caused by the climate itself." 



