RESEMBLANCES AMONG ANIMALS. 117 
of birds. These have no doubt acted and re-acted 
on each other; and when conditions have changed 
one of these characters may often have become modi- 
fied, while the other, though useless, may continue 
by hereditary descent an apparent exception to what 
otherwise seems a very general rule. The facts pre- 
sented 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 consi- 
derable 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 mark 
