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

ILLUSTRATIVE OF NATURAL SELECTIOR02 145
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, even though it does not offer
better distinguishing characters than do the extreme
forms of the variable species. Cases of this kind are
the Ornithoptera Priamus, which is confined to the
islands of Ceram and Amnboyna, and is very constant
in both sexes, while the allied species inhabiting New
Guinea and the Papuan Islands is exceedingly variable;
and in the island of Celebes is a species closely allied to
the variable P. Severus, but which, being exceedingly
constant, I have described as a distinct species under
the name of Papilio Pertinax.
2. Polymorphism or dimorpAism.-By this term I
understand the co-existence in the same locality of two
or more distinct forms, not connected by intermediate
gradations, and all of which are occasionally produced
from common parents. These distinct forms generally
occur in the female sex only, and their offspring, instead of being hybrids, or like the two parents, appear
to reproduce all the distinct forms in varying proportions. I believe it will be found that a considerable
number of what have been classed as varieties are really
cases of polymorphism. Albinoism and melanism are
of this character, as well as most of those cases in
which well-marked varieties occur in company with the
parent species, but without any intermediate forms. If
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