ILLUSTRATIVE OF NATURAL SELECTION. 143 



I find as a general rule that the constancy of species 

 is in an inverse ratio to their range. 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 probably still possesses, great powers of disper- 

 sion. Under the different conditions of existence in 

 various portions of its area, different variations from 

 the type would be selected, and, were they completely 

 isolated, would soon become distinctly modified forms ; 

 but this process is checked by the dispersive powers 

 of the whole species, which leads to the more or less 

 frequent intermixture of the incipient varieties, which 

 thus become irregular and unstable. Where, how- 

 ever, a species has a limited range, it indicates less 

 active powers of dispersion, and the process of modi- 

 fication under changed conditions is less interfered 

 with. The species will therefore exist under one or 

 more permanent forms according as portions of it have 

 been isolated at a more or less remote period. 



Laws and Modes of Variation. 



What is commonly called variation consists of several 

 distinct phenomena which have been too often con- 

 founded. I shall proceed to consider these under the 

 heads of 1st, simple variability ; 2nd, polymorphism ; 



