es 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 heads of--1st, simple variability; 2nd, polymorphism; 3rd,
local forms; 4th, co-existing varieties; 5th, races or subspecies; and
6th, true species.

1. _Simple variability._--Under this head I include all those cases in
which the specific form is to some extent unstable. Throughout the whole
range of the species, and even in the progeny of individuals, there
occur continual and uncertain differences of form, analogous to that
variability which is so characteristic of domestic breeds. It is
impossible usefully to define any of these forms, because there are
indefinite gradations to each other form. Species which possess these
characteristics have always a wide range, and are more frequently the
inhabitants of continents than of islands, though such cases are always
exceptional, it being far more common for specific forms to be fixed
within very narrow limits of variation. The only good example of this
kind of variability which occurs among the Malayan Papilionidae is in
Papilio Severus, a species inhabiting all the islands of the Moluccas
and New Guinea, and exhibiting in each of them a greater amount of
individual difference than often serves to distinguish well-marked
species. Almost equally remarkable are the variations exhibited in most
of the species of Ornithoptera, which I have found in some cases to
extend even to the form of the wing and the arrangement of the nervures.
Closely allied, however, to these variable species are others which,
though differing slightly from them, are constant and confined to
limited areas. After satisfying oneself, by the examination of numerous
specimens captured in their native countries, that the one set of
individuals are variable and the others are not, it becomes evident that
by classing all alike as varieties of one species we shall be obscuring
an important fact in nature; and that the only way to exhibit that fact
in its true light is to treat the invariable local form as a distinct
species,