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Show ilGO SEXUAL SELECTION: BIRDS. PART II. inheritance from the males any marked accession of brio-htness, would sooner or later be destroyed. But theb tendency in the males to continue for an indefinite period transmitting to their female offspring their own brightness, would have to be eliminated by a change in the form of inheritance; and this, as shewn by our previous illustration, would be extremely difficult. The more probable result of the long-continued destruction of the more brightly-coloured females, supposing the equal form of transmission to prevail, would be the lessening or annihilation of the bright colours of tho males, owing to their continually crossing with the duller females. It would be tedious to follow out all the other possible results; but I may remind tho reader, as shewn in the eighth chapter, that if sexuaUy-limited variations in brightness occurred in the female., even if they were not in the least injurious to them and consequently were not eliminated, yet they would not be favoured or selected, for the male usually accepts any female, and does not select the more attractive individuals; consequently these variations would be liable to be lost, and would have little influence on the character of the race ; and this will aid in accounting for the females being commonly less brightlycoloured than the males. In the chaptei· just ref,13rred to, instances were given, and any number might have been added, of variations -occuning at different ages, and inherited at the same nge. It was also shown that variations which occur late in lifo are commonly transmitted to the same sex in which they :first appeared; whilst variations occurring early in life are apt to be transmitted to both sexes; not that all the cases of sexnally-limited transmission can thus be accounted for. It was further shewn that if a male bird varied by becoming brighter whilst CHAP. XV. SEXUALLY LIMITED INHERITANCE. 161 yonng, such variations would be of no service until the age for reproduction had arrived, and there was competition between rival males. If we suppose that three-fourths of the young males of any species are on an average destroyed by various enemies; then the chances would be as three to one against any one individual more brightly-coloured than usual surviving to propagate its kind. But in the case of birds which live on the ground and which commonly need the protection of dull colours, bright tints would be far more dangerous to the young and inexperienced than to the adult males. Consequently the males which varied in brightness whilst young would suffer much destruction and be eliminated through natural selection ; on the other hand the males which varied in this manner when nearly mature, notwithstanding that they were exposed to some additional danger, might survive, and from being favoured through sexual selection, would procreate their kind. The brightly-coloured young males being destroyed and the mature ones being successful in their courtship, may account, on the principle of a relation existing between the period of variation and the form of transmission, for the males alone of mnny bi1·ds, having acquired and transmitted brilliant colours to their male offspring alone. But I by no means wish to maintain that the influence of age on the form of transmission is indirectly the sole cause of the great difference in brilliancy between the sexes of many birds. As with all birds in which the sexes differ in colour, it is an interesting question whether the males alone have been modified through sexual selection, the females being left, as far as this agency is concerned, unchanged or only partially changed; or whether the females have been specially modified through natural selection for the sn,ko of protection, I will discuss this question at con- VOL. II. M |