eviation from the usual arrests their
attention and incites them to closer investigation. It is obvious that
such detailed--we might almost say such subtle--deceptive resemblances
could only have come about in the course of long ages through the
acquirement from time to time of something new which heightened the
already existing resemblance.

In face of facts like these there can be no question of chance, and no
one has succeeded so far in finding any other explanation to replace
that by selection. For the rest, the apparent leaves are by no means
perfect copies of a leaf; many of them only represent the torn or
broken piece, or the half or two-thirds of a leaf, but then the leaves
themselves frequently do not present themselves to the eye as a whole,
but partially concealed among other leaves. Even those butterflies
which, like the species of Kallima and Anaea, represent the whole of
a leaf with stalk, ribs, apex, and the whole breadth, are not actual
copies which would satisfy a botanist; there is often much wanting.
In Kallima the lateral ribs of the leaf are never all included in the
markings; there are only two or three on the left side and at most four
or five on the right, and in many individuals these are rather obscure,
while in others they are comparatively distinct. This furnishes us with
fresh evidence in favour of their origin through processes of selection,
for a botanically perfect picture could not arise in this way; there
could only be a fixing of such details as heightened the deceptive
resemblance.

Our postulate of origin through selection also enables us to understand
why the leaf-imitation is on the lower surface of the wing in the
diurnal Lepidoptera, and on the upper surface in the nocturnal forms,
corresponding to the attitude of the wings in the resting position of
the two groups.

The strongest of all proofs of the theory, however, is afforded by
cases of true "mimicry," those adaptations discovered by Bates in 1861,
consisting in the imitation of one species by another, which becomes
more and more like its model. The model is always a species that enjoys
some special protection from enemies, whether because it is unpleasant
to taste, or because it is in some way dangerous.

It is chiefly among insects and especially among butterflies that we
find the greatest number of such cases. Several of these have been
minutely studied, and every detail has been investigated, so that it is
difficult