Russel Wallace : Alfred Russell Wallace (sic)
We are, therefore, forced to conclude that the two qualities--general vigour and ornament--are not independent of each other, but are developed pari passu, and the problem then becomes, does the female determine her choice by the latter rather than the former? I quite agree with Mr. Cunningham when he says: "The sexual desire of the female has a hereditary association with certain sensory [[p. 750]] stimuli, and the means of furnishing these stimuli are constantly reproduced and improved by inheritance in the males." I further admit that the display of ornament by the male is one of the means of exciting this desire; but mainly because it is an indication of sex, of sexual maturity, and of sexual vigour, probably not at all on account of details of colour or pattern.
There is, however, another consideration which Mr. Cunningham appears to have overlooked, and that is, the necessary weakness, comparatively, of female selection, owing to the very limited range of her choice. The law of survival of the fittest has such enormous selecting power because of the overwhelming odds against the less fit. A species which has two or three broods a year, or one large brood, and which lives, say, ten or twenty years, as do many of the vertebrata, produces from 50 to 100 successors of each pair, from which one or two only are selected to take the place of their parents. But in the case of sexual selection, it is a question of probably not more than two or three to one in most species, and in many even less, for there is no evidence and little probability that the number of healthy and competent males that fail to find mates bears any large proportion to those that do find them. Much of the success of particular males must depend on early chance encounters with a mate, while the competition can only be among small groups in each locality. If we add to this the consideration that in almost every case combat, or agility, or bodily vigour must have great influence, the part that remains to be played by ornament alone will be very small, even if it were proved, which it is not, that a slight superiority in ornament alone usually determines the choice of a mate.
This, however, is a matter that admits of experiment, and I would suggest that either some Zoological Society or any person having the means, should try such experiments. A dozen male birds of the same age--domestic fowls, common pheasants, or gold pheasants, for instance--should be chosen, all known to be acceptable to the hen birds. Half of these should have one or two tail plumes cut off, or the neck plumes a little shortened, just enough to produce such a difference as occurs by variation in nature, but not enough to disfigure the bird, and then observe whether the hens take any notice of the deficiency, and whether they uniformly reject the less ornamented males. Such experiments, carefully made and judiciously varied for a few seasons, would give most valuable information on this interesting question. Till this is done, suppositions as to what determines the choice of the female can have but little value.