Definition of Species. 85 



"I find as a general rule that the constancy of species is in an inverse 

 ratio to their rano-e. Those which are confined to one or two islands 

 are generally very constant. When they extend to many islands, con- 

 siderable variability appears ; and when they have an extensive range 

 over a large part of the archipelago, the amount of unstable variation 

 is very large. These facts are explicable on Mr. Darwin's principles. 

 When a species exists over a wide area, it must have had, and prob- 

 ably still possesses, great powers of dispersion. Under the different 

 conditions of existence in various portions of its area, different varia- 

 tions from the type would be selected, and, were they completely iso- 

 lated, would soon become distinctly modified forms ; but this process 

 is checked by the dispersive powers of the wdiole species, which leads 

 to tlie more or less frequent intermixture of the incipient varieties, 

 which thus become irregular and unstable. Wliere, however, a spe- 

 cies lias a limited range, it indicates less active powers of dispersion, 

 and the process of modification under changed conditions is less inter- 

 fered with. The species will, therefore, exist under one or more per- 

 manent forms, according as portions of it have been isolated at a more 

 or less remote period." [Wallace on Natural Selection, p'age 141.] 



Mr. Darwin, in his summary to chap. 2, p. 67, of his "Origin of 

 Species," says : 



" Varieties can not be distinguished from species except— first, by the 

 discovery of intermediate linking forms ; and secondly, by a certain in- 

 definite amount of difference between them ; for two forms, if differing 

 very little, are generally ranked as varieties, notwithstanding that they 

 can not be closely connected ; but the amount of difference considered 

 necessary to give to any two forms the rank of species can not be de- 

 fined. In genera having more than the average number of species iu 

 any country, the species of these genera have more than the aver- 

 age number of varieties. In large genera species are apt to be closely, 

 but unequally, allied together, forming little clusters around certain 

 other species. Species very closely allied to other species apparently 

 have restricted ranges. In all these several respects the species of 

 large genera present a strong analogy with varieties. And we can 

 clearly understand tliese analogies .if species once existed as varieties 

 and thus originated ; whereas these analogies are utterly inexplicable 

 if spe«es are independent creations. 



It is the most flourishing or dominant species of the larger genera 

 within each class which on an average yield the greatest number of 

 varieties ; and varieties tend to become converted into new and distinct 

 species. Thus, the larger genera tend to become larger ; and through- 

 out nature the forms of life which are now dominant tend to become 



