82 NATURAL SELECTION HI 



build in holes in banks. Bee-eaters, trogons, motmots, and 

 toucans all build in holes, and in none is there any difference 

 in the sexes, although they are, without exception, showy 

 birds. Parrots build in holes in trees, and in the majority of 

 cases they present no marked sexual difference tending to 

 concealment of the female. Woodpeckers are in the same 

 category, since, though the sexes often differ in colour, the 

 female is not generally less conspicuous than the male. 

 Wagtails and titmice build concealed nests, and the females 

 are nearly as gay as their mates. The female of the pretty 

 Australian bird, Pardalotus punctatus, is very conspicuously 

 spotted on the upper surface, and it builds in a hole in the 

 ground. The gay -coloured hang-nests (Icterinae) and the 

 equally brilliant tanagers may be well contrasted; for the 

 former, concealed in their covered nests, present little or no 

 sexual difference of colour while the open -nested tanagers 

 have the females dull-coloured and sometimes with almost 

 protective tints. No doubt there are many individual 

 exceptions to the rule here indicated, because many and 

 various causes have combined to determine both the colora- 

 tion and the habits of birds. These have no doubt acted and 

 reacted 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 differ- 

 ences 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, 



