ILLUSTRATIVE ‘OF NATURAL SELECTION. 148 
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—Ist, simple variability ; 2nd, polymorphism ; 
