[[p. 749]] In Mr. Cunningham's review of Mr. Romanes' new work, "Darwin, and after Darwin,"* [*Natural Science, vol. i, p. 546] the writer gives an independent and thoughtful criticism of my views on that subject, and I should like to be permitted to make a few observations in reply thereto. Mr. Cunningham says:--"Mr. Wallace argues as though the superior male, facile princeps, in the competition for a living, could found a line of descendants inheriting his own health and vigour, without female assistance." And again: "A male that excels in the struggle for existence is a complete failure, so far as the species is concerned, unless he can succeed also in finding mates." These passages seem to me to involve suppositions against all probability and all evidence. They imply that the better organised male, in all respects, except in ornament, is rejected by the females in favour of the worse organised in every respect, except in ornament. There are here two improbable assumptions--the first, that the most ornament is usually, or frequently, dissociated from the best general organisation; and the second, that any such less perfectly organised male would be preferred by the female on account of his slightly superior ornament. For the difference, it must be always remembered, is slight. Out of a hundred male pheasants or peacocks of the same age, the difference in length of plumes or shade of colour is rarely very conspicuous or even perceptible, except by close comparison; and if whatever difference there is were not usually associated with vigour and health, then the two forms of sexual selection--by combat and by display of ornament--would lead to different results; and, as males with ornamental appendages usually do fight for the females, the most ornamented would not, in their case, be the parents of the next generation.

    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.