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Show 158 SEXUAL SELECTION: BIRDS. PAR'!' II. not all, of the successive steps might be transferred to both sexes, and tho female would then closely resemble the male. There can hardly be a doubt that this iR the cause of the male pouter pigeon having a somewhat larger crop, and of the male carrier pigeon having somewhat larger wattles, than their respective females ; for fanciers have not selected one sex more than the other, and have bad no wish that these characters should be more strongly displayed in the male than in the female, yet this is the case with both breeds. The same process would have to be followed, and the same difficulties would be encountered, if it were desired to make a breed with the females alone of some new colour. Lastly, our fancier might wish to make a breed with the two sexes differing from each other, and both from the parent-species. Here the difficulty would be extreme, unless the successive variations were from the first sexually limited on both sides, and then there would be no difficulty. We see this with the fowl; thus the two sexes of the pencilled Hamburghs differ greatly from each other, and from the two sexes of the aboriginal C:allus bankiva; and both are now kept constant to their standard of excellence by continued selection which would be impossible unless the distinctive charac~ ters of both were limited in their transmission. The Spanish fow 1 offers a more curious case · the male bas an immense comb, but some of the succe~sive variations by the accumulation of which it was acquired, appea~ to have been transferred to the female; for she has a comb many times larger than that of the females of the parent-species. But the comb of the female differs in one respect ~ro~ that of the male, for it is apt to lop over; and w1thm a recent period it has been ordered by the fancy that this should always be the case, and G([AP. XV. SEXUALLY-UMITED INIIElUTANCE. 159 success has quickly followed the order. Now the lopping of the comb must be sexually limited in its transmission, otherwise it would prevent the comb of tho male from being perfectly upright, which would be abhorrent to every fancier. On the other hand the uprightness of the comb in the male must likewise be a sexually-limited character, otherwise it would prevent the comb of the female from lopping over. ] rom the foregoing illustrations, we sec that even with almost unlimited time at command, it would be an extremely difficult and complex process, though perhaps not impossible, to change through selection one form of transmission into the other. Therefore, without distinct evidence in each case, I am unwillino· to admit that this has often been effected with natural species. On the other hand by means of successive variations, which were from the first sexually limited in their transmission, there would not be the least difficulty in rendering a male bird widely different in colour or in any other character from the female; the latter being left unaltered, or slightly altered, or specially modified for the sake of protection. As bright colours are of service to the males in their rivalry with other males, such colours would be selected, whether or not they were transmitted exclusively to the same sex. Consequently the females might be expected often to partake of the brightness of the males to a greater or less degree; and this occurs with a host of species. If all the successive variations were transmitted equally to both sexes, the females would be undistinguishable from the males ; and this likewise occurs with many birds. If, however, dull colours were of high importance for the safety of the female durinoincubation, as with many ground birds, the female~ which varied in brightness, or which received through |
