Page:Territory in Bird Life by Henry Eliot Howard (London, John Murray edition).djvu/224

166 are degrees of suggestiveness, which well there may be, some males will mate sooner than others and some will remain mateless—this is the theory of sexual selection. The question to be decided here is whether the biological emphasis is on loudness, or specific distinctness, or pitch, or modulation, or the manner in which the phrases are combined—that is, on some qualities in preference to others—or whether the emphasis is on the whole. We have already seen, and it is well known, that there is much variation in the voices of different individuals of the same species, and thus the first condition of the theory is fulfilled. Now the conditions which lead to variation are threefold—immaturity, seasonal sexual development, and isolation. Of the three, the variation in the case of the immature bird is the most instructive; the tone is not so pure, the combination of phrases is incomplete, and elaboration is imperfect, and yet, notwithstanding all these imperfections, we can observe that the bird pairs as readily as does the adult. But even if we lacked this demonstrative evidence, we should still be justified in assuming that such must be the case, for we know from experience in the preservation of game, where there is no surer way of reducing the stock than by leaving too high a percentage of old cocks, that for the young bird to be at any disadvantage in competition with the adult is detrimental, if not disastrous, to the species. So that while there is plenty of evidence of variation in those particular qualities which appeal to our æsthetic