colour is greatly influenced by their need of protection from, or warning to, their numerous enemies; colours of other kinds must always, to a certain extent, be dangerous for the species. Is it probable, then, that, whilst gay colours occur only in the flowers of those plants to which they are of real use, conspicuous colours should occur in animals to which they are of real danger-merely because the females find them beautiful ?

Mr. Wallace, whose well-known criticism of Mr. Darwin's theory of sexual selection1 seems, in many points, to be conclusive, suggests that the very frequent superiority of the male bird or insect in brightness or intensity of colour is due to the greater vigour and activity and the higher vitality of the male. This intensity of coloration is therefore most manifest in the male during the breeding season, when the vitality is at a maximum. It would be further developed by the combats of the males for the possession of the females; and the most vigorous and energetic usually leaving the most numerous and most healthy offspring, natural selection would indirectly become a preserver and intensifier of colour. Mr. Wallace has made it very probable that there is some connection between vigour and colour, but another question is whether this connection, depending on some unknown physiological law, is so necessary that it takes place even when colour is positively disadvantageous to the species. Nothing of the kind is found in the vegetable kingdom. We know, as Mr. Wallace himself remarks, that colours which rarely or never appear in the species in a state of nature, continually occur among cultivated plants and domesticated animals-a fact which shows that the capacity to develop colour is ever present. Among wild plants such colour variations are never preserved except when they are useful. Is it not most reasonable to suppose that the like is the case with animals?

The truth seems to be that colour subserves the same purpose in both of the great kingdoms of the organic world. Just as flowers are coloured that insects may recognize where honey is to be found, and thus may be led to promote fertil

1 'The Colours of Plants and the Origin of the Colour-Sense,' in 'Tropical Nature,' pp. 221-248. 'Darwinism,' ch. x.

2 Wallace,' Tropical Nature,' pp. 193-195.

3 Ibid., p. 187.

ization, so the sexual colours of animals have been developed to make it easier for the sexes to find each other during the pairing time. Protective colours are useful so far as they conceal the animal from its enemies, but, at the same time, they conceal it from individuals of its own species. Sexual colours are therefore useful as well, because they make the animal more visible. It is quite in accordance with the theory of natural selection that, where such colours occur, the advantage from them should be greater than the disadvantage. We can see the reason for the brilliant colours of humming-birds, as these birds, on account of their great activity "are practically unmolested," and for the bright hues of the rose chafers, who are saved from attack by a combination of protecting characters. But generally there is danger in sexual colours, so that nature has given them with the utmost cautiousness. Usually they occur in males only, because of the females' greater need of protection.3 They are not developed till the age of reproduction, and they appear, in a great many species, only during the pairing season. The greatest advantage is won with the least possible peril.

It is a fact of great importance that sexual colours occur exactly in those species whose habits make these colours most visible. Thus the nocturnal moths, taken as a body, are much less gaily decorated than butterflies, all of which are diurnal in their habits, although, according to Mr. Wallace, the general influence of solar light and heat is no adequate cause for the variety, intensity, and complexity of the colours. The females of the ghost moth are yellow with darker markings, whereas the males are white, that they may be more easily seen by the females whilst flying about in the dusk ; and it is remarkable that, in the Shetland Islands, the male of this moth, instead of differing widely from the female, frequently resembles her closely in colour,-as Mr. Fraser suggests, because, at the season of the year when the ghost moth appears in these northern latitudes, the whiteness of the males is not needed to render them visible to the females

4

1 Wallace, 'Tropical Nature,' p. 213.

2 Idem,' Contributions to the Theory of Natural Selection,' pp. 73, et seq. 3 Ibid., pp. 259-261. 4 Fraser, in 'Nature,' vol. iii. p. 489.

in the twilight night. Both Mr. Darwin1 and Mr. Wallace 2 think that, in this case, colour may be a means of recognition.

Sexual colours occur chiefly in species which, because of their manner of living, are to be seen at a distance; they seldom occur in sedentary or slowly moving terrestrial animals. The members of the lowly organized order Thysanura are wingless and dull-coloured. The Hemiptera, which usually lurk about plants, and prey upon hapless insects, are not, as a rule, remarkable for conspicuous hues. The Orthoptera are all terrestrial in their habits, generally feeding upon plants, and, although some exotic locusts are beautifully ornamented, their bright tints, according to Mr. Darwin, do not seem to fall under the head of sexual coloration. On the other hand, the dragon-flies, which live in the open air,' possess splendid green, blue, yellow, and vermilion metallic tints, and the sexes often differ in their coloration. Every one has admired the extreme beauty of many butterflies, especially of the males. Amongst the Fishes, living in a medium through which bright colours may be observed at a distance, we often find, besides protective colours, conspicuous hues which are especially intense and visible during the pairing time. Among the Reptiles, the little lizards of the genus Draco especially deserve attention; they glide through the air on their rib-supported parachutes, and the beauty of their colours baffles description. Mammals, on the other hand, do not generally present the splendid tints so common among male birds; and the brighter colours of certain arboreal mammals serve chiefly as means of concealment.

These phenomena seem to show that sexual colours have been evolved for the purpose of being seen. They can scarcely be due merely to the fact that coloration is connected with the degree of vitality, since the Mammals, for instance, are certainly not less vigorous than any of the other Vertebrate orders. It may perhaps be

1 Darwin, 'The Descent of Man,' vol. i. p. 485.

2 Wallace, 'Darwinism,' p. 270.

3 The Gallinaceæ, however, form an exception; though almost wholly terrestrial, they have the most pronounced sexual colours. But they are

active and wander much.

suggested that, as flying animals more easily escape their enemies than terrestrial, they may with less danger be decorated with conspicuous hues. But here we have to observe the most important fact that animals which do not possess sexual colours generally have some other means of making themselves discoverable.

Flowers which need the help of insects for fertilization attract them, in some cases, not by bright colours, but by peculiar odours. And as we do not find conspicuous colours in plants fertilized by the wind, so flowers have no perfume except where it is of real use. The most brilliant flowers, as a rule, are those which possess least odour, whilst many of them have no scent at all. White or very pale flowers are generally the most odoriferous. M. Mongredien gives a list of about 160 species of hardy trees and shrubs with showy flowers, and another list of sixty species, with fragrant flowers; but only twenty of the latter are included among the showy species, and these are almost all white-flowered. Most of the white flowers are scented only at night, or their perfumes are most powerfully emitted at that time; the reason being that white flowers are fertilized chiefly by night-flying insects. We arrive thus at two conclusions: first, that powerful odours and conspicuous colours as guides to insect fertilizers are, as a rule, complementary to each other; secondly, that they occur alternately in the way most useful to the species.

In the animal kingdom various odours and sounds are closely connected with the reproduction of the species. During the season of love a musky odour is emitted by the submaxillary glands of the crocodile, and pervades its haunts. At the same period the anal scent-glands of snakes are in active function, and so are the corresponding glands of the lizards. Many mammals are odoriferous. In some cases the odour appears to serve as a defence or a protection, but in other species the glands are confined to the males, and almost always become more active during the rutting season. Again, a great many insects have the power of producing stridulous sounds. In two families of the Homoptera and in three of the Orthoptera, the males alone possess organs of sound in 1 Wallace, Tropical Nature,' pp. 230, et seq.

an efficient state, and these are used incessantly during the pairing season. Some male fishes have sound-producing instruments, and the fishermen of Rochelle assert that the males alone make the noise during the spawning-time. Of frogs and toads the males emit various sounds at the pairing time, as in the case of the croaking of our common frog. During the rutting season, and at no other time, the male of the huge tortoise of the Galapagos Islands utters a hoarse bellowing noise, which can be heard at a distance of more than a hundred yards. Professor Aughey states that on two occasions, being himself unseen, he watched from a little distance a rattle-snake coiled up with head erect, which continued to rattle at short intervals for half an hour; at last he saw another snake approach, and when they met they paired. Among Birds the power of song, or of giving forth strange cries, or even instrumental music, is exceedingly common, particularly in the males during the pairing season; and almost all male mammals use their voices much more during that period than at any other time. Some, as the giraffe and porcupine, are stated to be completely mute except during the rutting season.

The colours, odours, and sounds of animals, like the colours and odours of plants-so far as they may be assumed to be in some way connected with the reproductive functions-are, as a rule, complementary to each other. Stridulating insects are generally not conspicuously coloured. Among the Homoptera, there do not seem to be any well-marked cases of ornamental differences between the sexes. Among crickets, the Locustidæ, and grasshoppers, some species are beautifully coloured; but Mr. Darwin says, "It is not probable that they owe their bright tints to sexual selection. Conspicuous colours may be of use to these insects by giving notice that they are unpalatable." Other species have directly protective colours. The bright hues of stridulating beetles seem to be of use chiefly for protective and warning purposes; whereas species belonging to the orders Neuroptera and Lepidoptera, often extremely conspicuously coloured, are not remarkable for any stridulous sounds. Frogs and toads, which have an interesting sexual character in the musical powers possessed by the males, are