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 



