in 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 as a 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 

 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 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 



