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Show 23G. SEXUAL . SELECTION : BIUDS. PAnT n .. vVith the species, in which the sexes differ in colour, it is possible that at first there existed a tendency to transmit the successive variations equally to both sexes; and that the females were prevented from acquiring the bright colours of the males, on account of the daDger to which they would have been exposed during incubation. 13ut it would be, as far as I can see, an extremely difficult process to convert, by means of natural selection, one form of transmission into another. On the other hand there would not be the least difficulty in rendering a female dull-coloured, the male being still kept brightcoloured, by the selection of successive variations, which were from the first limited in their transmission to the same sex. Whether the females of many species have actually been thus modified, must at present remain doubtful. \Vhen, through the law of the equal transmission of characters to both sexes, the females have been rendered as conspicuously coloured as the males, their instincts have often been modified, and they have been ]eel to build domed or concealed nests. In one smaU and curious class of cases the characters and habits of the two sexes have been completely transposed, for the females are larger, stronger, more vociferous a~d brightly-coloured than their males. They have, also, become so quarrelsome that they often fight together like the males of the most pugnacious species. If, as seems probable, they habitually drive away rival females, and by the display of their bright colours OL' other charms endeavour to attract the males, we can understand bow it is that they have gradually been rendered, by means of sexual selection and sexually-limited transmission, more beautiful than the males-the latter being left unmodified or only slightly modified. Whenever the law of inheritance at corresponding ages prevails, but not that of sexually-limited trans- CHAP. XVI. SUMMARY. '237 mission, then if the parents vary late in life-and we know_ that thi~ constantly occurs with our poultry, and occasiOnally wtth other birds-the young will be left nnaffected, whilst the adults of both sexes will be n~odified. If both these laws of inheritance prevail and e1th~r sex varies late in life, that sex alone will be modified, the other sex and the young being left unaffect~ d. When variations in brightness or in other conspiCuous characters occur early in life, as no doubt often !lappen.s, they w~ll not be acted on through sexual selectiOn until the perwd of reproduction arrives; consequently they will be liable to be lost by the accidental deaths of the young, and if dangerous will be eliminated through natural selection. Thus we can understand how it is that variations arising late in life have chiefly been preserved for the ornamentation and arming of the males, the females and the young being left almost unaffe~ ted, a~d. therefore like each other. With species havm~ a di~tmct summer and winter plumage, the males of ~hwh either resemble or differ from the females durmg both seasons or during the summer alone the degrees and kinds of resemblance between the v~ung and ~he old are exceedingly complex; and this .. complexity apparent!~ depends on characters, first acquired by the male_s, .bemg transmitted in various ways and degrees, as hm1ted by age, sex, and season. As the young of so many species have been but little modified in colour and in other ornaments, we are enabled to for~ some judgment with respect to the plumage of their early progenitors; and we may infer that the beauty of our existing species, if we look to the whole .class, ha~ been largely increased since that period of whiCh the Immature plumage gives us an indirect record. Many birds, especially those which live much <>n the ground, have undoubtedly been obscurely co- |