n other countries, and likewise, judging from Brachiopod
shells, at former periods of time. These facts are very perplexing, for
they seem to show that this kind of variability is independent of the
conditions of life. I am inclined to suspect that we see, at least in
some of these polymorphic genera, variations which are of no service or
disservice to the species, and which consequently have not been seized
on and rendered definite by natural selection, as hereafter to be
explained.

Individuals of the same species often present, as is known to every one,
great differences of structure, independently of variation, as in the
two sexes of various animals, in the two or three castes of sterile
females or workers among insects, and in the immature and larval states
of many of the lower animals. There are, also, cases of dimorphism and
trimorphism, both with animals and plants. Thus, Mr. Wallace, who has
lately called attention to the subject, has shown that the females of
certain species of butterflies, in the Malayan Archipelago, regularly
appear under two or even three conspicuously distinct forms, not
connected by intermediate varieties. Fritz Muller has described
analogous but more extraordinary cases with the males of certain
Brazilian Crustaceans: thus, the male of a Tanais regularly occurs
under two distinct forms; one of these has strong and differently shaped
pincers, and the other has antennae much more abundantly furnished with
smelling-hairs. Although in most of these cases, the two or three forms,
both with animals and plants, are not now connected by intermediate
gradations, it is possible that they were once thus connected. Mr.
Wallace, for instance, describes a certain butterfly which presents in
the same island a great range of varieties connected by intermediate
links, and the extreme links of the chain closely resemble the two forms
of an allied dimorphic species inhabiting another part of the Malay
Archipelago. Thus also with ants, the several worker-castes are
generally quite distinct; but in some cases, as we shall hereafter see,
the castes are connected together by finely graduated varieties. So it
is, as I have myself observed, with some dimorphic plants. It certainly
at first appears a highly remarkable fact that the same female butterfly
should have the power of producing at the same time three distinct
female forms and a male; and that an hermaphrodite plant should produce
from the same seed-caps