III PROTECTIVE RESEMBLANCES AMONG ANIMALS 83 
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 under- 
stand 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 asa difficulty from another point of 
view, for he had arrived at the conclusion that brilliant 
coloration in the animal kingdom is mainly due to sexual 
selection, and this could not have acted in the case of sexless 
larve. 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 dis- 
tasteful to them, and these latter were mostly of conspicuous 
colours, so probably these brilliantly coloured caterpillars were 
distasteful, and therefore never eaten by birds. Distasteful- 
ness 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, they would almost 
certainly be killed. Some constant and easily perceived 
signal was therefore necessary to serve as a warning to birds 
never to touch these uneatable kinds, and a very gaudy 
and conspicuous colouring with the habit of fully exposing 
themselves to view becomes such a signal, being in strong 
contrast with the green or brown tints and retiring habits 
of the eatable kinds. The subject was brought by me 
before the Entomological Society (see Proceedings, 4th March 
1867), in order that those members having opportunities 
for making observations might do so in the following 
summer; and I also wrote a letter to the Field news- 
paper, begging that some of its readers would co-operate 
in making observations on what insects were rejected by 
birds, at the same time fully explaining the great interest 
and scientific importance of the problem. It is a curious 
example of how few of the country readers of that paper are 
at all interested in questions of simple natural history, that I 
