f the larvae
or some other cause, possessed disagreeable juices that caused them to
be disliked by the usual enemies of their kind, they were in all
probability not very different either in form or coloration from many
other butterflies. They would at that time be subject to repeated
attacks by insect-eaters, and, even if finally rejected, would often
receive a fatal injury. Hence arose the necessity for some
distinguishing mark, by which the devourers of butterflies in general
might learn that these particular butterflies were uneatable; and every
variation leading to such distinction, whether by form, colour, or mode
of flight, was preserved and accumulated by natural selection, till the
ancestral Heliconoids became well distinguished from eatable
butterflies, and thenceforth comparatively free from persecution. Then
they had a good time of it. They acquired lazy habits, and flew about
slowly. They increased abundantly and spread all over the country, their
larvae feeding on many plants and acquiring different habits; while the
butterflies themselves varied greatly, and colour being useful rather
than injurious to them, gradually diverged into the many coloured and
beautifully varied forms we now behold.

But, during the early stages of this process, some of the Pieridae,
inhabiting the same district, happened to be sufficiently like some of
the Heliconidae to be occasionally mistaken for them. These, of course,
survived while their companions were devoured. Those among their
descendants that were still more like Heliconidae again survived, and at
length the imitation would become tolerably perfect. Thereafter, as the
protected group diverged into distinct species of many different
colours, the imitative group would occasionally be able to follow it
with similar variations,--a process that is going on now, for Mr. Bates
informs us that in each fresh district he visited he found closely
allied representative species or varieties of Heliconidae, and along
with them species of Leptalis (Pieridae), which had varied in the same
way so as still to be exact imitations. But this process of imitation
would be subject to check by the increasing acuteness of birds and other
animals which, whenever the eatable Leptalis became numerous, would
surely find them out, and would then probably attack both these and
their friends the Heliconidae in order to devour the former and reject
the latter. The Pieridae would, however, usually be