Contributions to the theory of natural selection. A series of essays ... By Alfred Russel Wallace ...

ILLUSTRATIVE OF NATURAL SELECTIOQA. t43
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, considerable 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 M5r. Darwin's principles. When
a species exists over a wide area, it must have had,
and probably still possesses, great powers of dispersion. 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, however, a species has a limited range, it indicates less
active powers of dispersion, and the process of modification 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 confounded. I shall proceed to consider these under the
leads of-lst, simple variability; 2nd, polymorphism;