PDF - Wallace Online

82 NATURAL SELECTION HIbuild in holes in banks. Bee-eaters, trogons, motmots, andtoucans all build in holes, and in none is there any differencein the sexes, although they are, without exception, showybirds. Parrots build in holes in trees, and in the majority ofcases they present no marked sexual difference tending toconcealment of the female. Woodpeckers are in the samecategory, since, though the sexes often differ in colour, thefemale is not generally less conspicuous than the male.Wagtails and titmice build concealed nests, and the femalesare nearly as gay as their mates. The female of the prettyAustralian bird, Pardalotus punctatus, is very conspicuouslyspotted on the upper surface, and it builds in a hole in theground. The gay -coloured hang-nests (Icterinae) and theequally brilliant tanagers may be well contrasted; for theformer, concealed in their covered nests, present little or nosexual difference of colour while the open -nested tanagershave the females dull-coloured and sometimes with almostprotective tints. No doubt there are many individualexceptions to the rule here indicated, because many andvarious causes have combined to determine both the colorationand the habits of birds. These have no doubt acted andreacted on each other ;and when conditions have changedone of these characters may often have become modified,while the other, though useless, may continue by hereditarydescent an apparent exception to what otherwise seems avery general rule. The facts presented by the sexual differencesof colour in birds and their mode of nesting are onthe whole in perfect harmony with that law of protectiveadaptation of colour and form, which appears to have checkedto some extent the powerful action of sexual selection, and tohave materially influenced the colouring of female birds, as ithas undoubtedly done that of female insects.Use of the gaudy Colours of many CaterpillarsSince this essay was first published a very curious difficultyhas been cleared up by the application of the general principleof protective colouring. Great numbers of caterpillars are sobrilliantly marked and coloured as to be very conspicuous evenat a considerable distance, and it has been noticed that suchcaterpillars seldom hide themselves. Other species, however,