t cases so constant in large series of specimens,
and so easily separable from each other, that I know not on what
principle we can refuse to give them the name and rank of species. One
of the best and most orthodox definitions is that of Pritchard, the
great ethnologist, who says, that "_separate origin and distinctness of
race, evinced by a constant transmission of some characteristic
peculiarity of organization_," constitutes a species. Now leaving out
the question of "origin," which we cannot determine, and taking only the
proof of separate origin, "_the constant transmission of some
characteristic peculiarity of organization_," we have a definition which
will compel us to neglect altogether the _amount_ of difference between
any two forms, and to consider only whether the differences that present
themselves are _permanent_. The rule, therefore, I have endeavoured to
adopt is, that when the difference between two forms inhabiting separate
areas seems quite constant, when it can be defined in words, and when it
is not confined to a single peculiarity only, I have considered such
forms to be species. When, however, the individuals of each locality
vary among themselves, so as to cause the distinctions between the two
forms to become inconsiderable and indefinite, or where the differences,
though constant, are confined to one particular only, such as size,
tint, or a single point of difference in marking or in outline, I class
one of the forms as a variety of the other.

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 Mr. 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 speci