less numerous,
because their larvae are often protectively coloured and therefore
edible, while the larvae of the Heliconidae are adorned with warning
colours, spines, or tubercles, and are uneatable. It seems probable that
the larvae and pupae of the Heliconidae were the first to acquire the
protective distastefulness, both because in this stage they are more
defenceless and more liable to fatal injury, and also because we now
find many instances in which the larvae are distasteful while the
perfect insects are eatable, but I believe none in which the reverse is
the case. The larvae of the Pieridae are now beginning to acquire
offensive juices, but have not yet obtained the corresponding
conspicuous colours; while the perfect insects remain eatable, except
perhaps in some Eastern groups, the under sides of whose wings are
brilliantly coloured although this is the part which is exposed when at
rest.

It is clear that if a large majority of the larvae of Lepidoptera, as
well as the perfect insects, acquired these distasteful properties, so
as seriously to diminish the food supply of insectivorous and nestling
birds, these latter would be forced by necessity to acquire
corresponding tastes, and to eat with pleasure what some of them now eat
only under pressure of hunger; and variation and natural selection would
soon bring about this change.

Many writers have denied the possibility of such wonderful resemblances
being produced by the accumulation of fortuitous variations, but if the
reader will call to mind the large amount of variability that has been
shown to exist in all organisms, the exceptional power of rapid increase
possessed by insects, and the tremendous struggle for existence always
going on, the difficulty will vanish, especially when we remember that
nature has the same fundamental groundwork to act upon in the two
groups, general similarity of forms, wings of similar texture and
outline, and probably some original similarity of colour and marking.
Yet there is evidently considerable difficulty in the process, or with
these great resources at her command nature would have produced more of
these mimicking forms than she has done. One reason of this deficiency
probably is, that the imitators, being always fewer in number, have not
been able to keep pace with the variations of the much more numerous
imitated form; another reason may be the ever-increasing acuteness of
the enemies, which have again and again d