Moreover, as appears from what has just been said, the sexual colours, the perceptible scents and sounds of animals are complementary to each other in the way that is best suited to make the animals easily discoverable. As bright colours would be of no advantage to flowers fertilized by night-flying insects, so they would be of comparatively little advantage to animals living among grass and plants, in woods and bushes; whereas sounds and scents make the animal recognizable at a considerable distance. We have also seen that it is among flying and aquatic animals that sexual colours chiefly occur, whereas terrestrial animals excel in sound and scents. Thus most of the stridulating insects are terrestrial. Whilst brightly-coloured lizards, living on trees or running from stone to stone, must attract attention by the brilliance of their covering, crocodiles inhabiting rivers and jungles, and frogs crawling among the grass, allure their mates, the former by emitting musky odours, the latter by producing loud sounds. The odour of the Australian musk-duck, which depends for its food and for its preservation from danger upon its powers of diving rather than upon those of flying, is, as Mr. Gould observes, often perceptible long before the animal can be seen.[1506] Mr. Darwin remarks, as regards birds, “Bright colours and the power of song seem to replace each other. We can perceive that, if the plumage did not vary in brightness, or if bright colours were dangerous to the species, other means would be employed to charm the females; and melody of voice offers one such means.”[1507] But if we accept Mr. Darwin’s theory of sexual selection, we are compelled to suppose that that inexplicable æsthetic sense of the females has been developed in the way most dangerous to the species. Conspicuous colours are admired by the females of those animals which, by means of such colours, are most easily discovered by their enemies, and sounds and odours are appreciated exactly in those species to which they are most perilous. If, on the contrary, we accept the explanation that, although sexual colours, odours, and sounds are in some ways hurtful to the species, they are upon the whole advantageous, inasmuch as they make it easier for the sexes to find each other, we have a theory in accordance with all known facts, as well as with the great principle of natural selection. It may be objected that it is not the females but the males that are the seekers, whilst the secondary sexual characters generally occur in the males only. But we have no reason to think that the females are entirely passive during the pairing season; and several of the statements collected by Mr. Darwin directly indicate that females are attracted by the sounds of their future partners. If Burdach is correct in saying that the male sex generally possesses more acute senses than the female,[1508] it is obvious that secondary sexual characters would be of less use to females than to males, as it