have no doubt acted and re-acted on each other; and when
conditions have changed one of these characters may often have become
modified, while the other, though useless, may continue by hereditary
descent an apparent exception to what otherwise seems a very general
rule. The facts presented by the sexual differences of colour in birds
and their mode of nesting, are on the whole in perfect harmony with that
law of protective adaptation of colour and form, which appears to have
checked to some extent the powerful action of sexual selection, and to
have materially influenced the colouring of female birds, as it has
undoubtedly done that of female insects.

_Use of the gaudy Colours of many Caterpillars._

Since this essay was first published a very curious difficulty has been
cleared up by the application of the general principle of protective
colouring. Great numbers of caterpillars are so brilliantly marked and
coloured as to be very conspicuous even at a considerable distance, and
it has been noticed that such caterpillars seldom hide themselves. Other
species, however, are green or brown, closely resembling the colours of
the substances on which they feed, while others again imitate sticks,
and stretch themselves out motionless from a twig so as to look like one
of its branches. Now, as caterpillars form so large a part of the food
of birds, it was not easy to understand why any of them should have such
bright colours and markings as to make them specially visible. Mr.
Darwin had put the case to me as a difficulty from another point of
view, for he had arrived at the conclusion that brilliant colouration in
the animal kingdom is mainly due to sexual selection, and this could not
have acted in the case of sexless larvae. Applying here the analogy of
other insects, I reasoned, that since some caterpillars were evidently
protected by their imitative colouring, and others by their spiny or
hairy bodies, the bright colours of the rest must also be in some way
useful to them. I further thought that as some butterflies and moths
were greedily eaten by birds while others were distasteful to them, and
these latter were mostly of conspicuous colours, so probably these
brilliantly coloured caterpillars were distasteful, and therefore never
eaten by birds. Distastefulness alone would however be of little service
to caterpillars, because their soft and juicy bodies are so delicate,
that if seized and afterwards rejected by a bird