y may doubt whether many of the
insignificant details of the marking can really be of advantage to the
insect. Such details are for instance the apparent holes and splits
in the apparently dry or half-rotten leaf, which are usually due to
the fact that the scales are absent on a circular or oval patch so
that the colourless wing-membrane lies bare, and one can look through
the spot as through a window. Whether the bird which is seeking or
pursuing the butterflies takes these holes for dewdrops, or for the
work of a devouring insect, does not affect the question; the
mirror-like spot undoubtedly increases the general deceptiveness, for
the same thing occurs in many leaf-butterflies, though not in all, and
in some cases it is replaced in quite a peculiar manner. In one
species of Anaea (_A. divina_), the resting butterfly looks exactly
like a leaf out of the outer edge of which a large semi-circular piece
has been eaten, possibly by a caterpillar; but if we look more closely
it is obvious that there is no part of the wing absent, and that the
semi-circular piece is of a clear, pale yellow colour, while the rest
of the wing is of a strongly contrasted dark brown.

But the deceptive resemblance may be caused in quite a different
manner. I have often speculated as to what advantage the brilliant
white C could give to the otherwise dusky-coloured "Comma butterfly"
(_Grapta C. album_). Poulton's recent observations[46] have shown that
this represents the imitation of a crack such as is often seen in dry
leaves, and is very conspicuous because the light shines through it.

The utility obviously lies in presenting to the bird the very familiar
picture of a broken leaf with a clear shining slit, and we may
conclude, from the imitation of such small details, that the birds are
very sharp observers and that the smallest deviation 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