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Show 276 DARWINISM CIIAP. than half of the known species. It is ~his c~ass, therefore, that will afford us the best materials for a dtscusswn of the pro.blem, and that may perhaps lead us to ~ satisfactory explanatiOn of the causes to which sexual colour IS due. . The most fundamental characte~istic ~f birds, fro~ our presen t pom· t of view , is a greater mtend sitfy of colour m the 1 . ale. This is the case in hawks an a cons~ m m::t~1 Y tm h rus h es, warb l crs, and finches '· in l)i(or eons, partndg. es,h .n u1b 1, plovers, and many others. When the pl.umage IS Ig 1 y protective or of dull uniform tints, as m m~ny. of ~he t h rus h es an d war ·blcr·s , the sexes are almost or q. mte I.d entical m· co 1o ur ,. b u t when any rich marking. s or bnghht ft m· ts ar· e acquired, they are alm~st always wantmg or muc amtcr lll the female, as we see m the black-cap among warblers, and the chaffinch among finches. . It is in tropical regions, where from a vanety of causcg colour has been developed to its fullest extent, that we find the most remarkable examples of sexual divergence of col?nr. The most gorgeously coloured birds known arc t~e ~mb of paradise, the chatterers, the tanagers, the hummmg-bn·ds, and the pheasant-tribe, incl~di.ng the pe~cocks. In all .th?se the females are much less bnlhant, and, m the great maJonty of cases, exceptionally plain and dull coloured birds. Not only are the remark~ble plume.s, cr?sts, and gorgcts of the birds of paradise entirely wantmg m the females, but these latter are usually without any bright colour at all, and rank no higher than our thrushes in orna:nental plumage. Of the hummin0'-birds the same may be said, except that tho females are ofte~ green, and sometimes slightly metallic, bu~ from their small size and uniform tints arc never conspicuous. The glorious blues and purples, the pure white~ and intense crimsons of the male chatterers are represented m the females by olive-greens or dull browns, ~s arc the infinitely varied tints of the male tanagers. And m pheasants, the splendom of plumage which characterises the males is entirely absent in the females, which, though often ornamental, have alw<tys comparatively sober and protective tints. The same thi~1g occurs with many other groups. In the Eastern tropics are many brilliant birds belonging to the families of the warblers, flycatchers, shrikes, etc., but the female is always x COLOURS AND ORNAMENTS CIIARA 'TERISTIC OF SEX 277 much less brilliant than the male and often quito dull coloured. Cause of Dull Colours of Female Birds. The reason of this phenomenon is not difficult to find, if we consider the essential conditions of a bird's existence, and the most importa.nt function it ha.s to fulfil. In order that the species may be continued, young birds must be produced, and tho female birds have to sit assiduously on their eggs. While doing this they arc exposed to observation and attack by the numerous devomers of eggs and birds, and it is of vital importance tha.t they shouhl be protectively coloured in all those parts of the body which arc exposed during incubation. To secure this end all tho bright colours and showy ornaments which decorate the malo have not been acquired by the female, who often remains clothed in the sober hues which were probably once common to the whole order to which she belongs. The different amounts of colour acquired by the females have no doubt depended on peculiarities of habits and of environment, and on the powers of defence or of concealment possessed by tho species. Mr. Darwin has taught us that natural selection cannot produce absolute, but only relative perfection; and as a protective colour is only one out of many means by which tho female birds are able to provide for the safety of their young, those which are best endowed in other respects will have been allowed to acquire more colour than those with whom the struggle for existence is more severe. Relation of Sex Colou1· to Nesting Ilabits. This principle is strikingly illustrated by tho existence of considerable numbers of birds in which both sexes arc similarly and brilliantly colourod,-in some cases as brilliantly as tho males of many of the groups above referred to. Such arc tho extensive families of tho kingfi hers, the woodpeckers, tho toucans, the parrots, tho turacos, the hangnosts, tho starlings, and many other smaller groups, all the species of which arc conspicuou ·ly or brilliantly coloured, while in all of them the fema.les are either coloured exactly like tho males, or, when differently coloured, arc equally conspicuous. When |