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It is very
noticed by all observers, and it seems to be to some extent proportioned to the degree of colour and ornament in the species. Thus Mr. Salvin observes of Eugenes fulgens, that it is "a most pugnacious bird,” and that “hardly any species shows itself more brilliantly on the wing.” Again of Campylopterus hemileucurus,-“ the pugnacity of this species is remarkable. seldom that two males meet without an aërial battle,”and “the large and showy tail of this humming-bird makes it one of the most conspicuous on the wing.” Again, the elegant frill-necked Lophornis ornatus “ is very pugnacious, erecting its crest, throwing out its whiskers and attacking every humming bird that may pass within its range of vision;" and of another species L. magnificus, it is said that “it is so bold that the sight of man creates no alarm." The beautifullycoloured Thaumastura Cora “rarely permits any other humming-bird to remain in its neighbourhood, but wages a continual and terrible war upon them.” The magnificent bar-tail, Cometes sparganurus, one of the most imposing of all the humming-birds, is extremely fierce and pugnacious, “the males chasing each other through the air with surprising perseverance and acrimony." These are all the species I find noticed as being especially pugnacious, and cvery one of them is exceptionally coloured or ornamented; while not one of the small, plain, and less ornamental species are so described, although many of them are common and well observed species. It is also to be noticed that the remarkable pugnacity of these birds is not confined to one season or even to birds of the same species, as is usual in sexual combats, but extends to any other
species that may be encountered, while they are said even to attack birds of prey that approach too closely to their nests. It must be admitted that these facts agree well with the theory that colour and ornament are due to surplus vital energy and a long course of uchecked development. We have also direct evidence that the males are more active and energetic than the females. Mr. Gosse says that the whirring made by the male Polytmus humming-bird is shriller than that produced by the female; and he also informs us that the male flies higher and frequents mountains while the female keeps to the lowlands.
Theory of Typical Colours.—The remaining kinds of animal colours, those which can neither be classed as protective, warning, or sexual, are for the most part readily explained on the general principles of the development of colour which we have now laid down. It is a most suggestive fact, that, in cases where colour is required only as a warning, as among the uncatable caterpillars, we find, not one or two glaring tints only, but every kind of colour disposed in elegant patterns, and exbibiting almost as much variety and beauty as among insects and birds. Yet here, not only is sexual selection out of the question, but the need for recognition and identification by others of the same species, seems cqually unnecessary. We can then only impute this variety to the normal production of colour in organic forms, when fully exposed to light and air and undergoing great and rapid developmental modification. Among more perfect animals, where the need for recognition has been added, we find intensity and variety of colour at its highest pitch among the South American
butterflies of the families Heliconidze and Danaide, as well as among the Nymphalidæ and Erycinidæ, many of which obtain the necessary protection in other ways. Among birds also, wherever the habits are such that no special protection is needed for the females, and where the species frequent the depths of tropical forests, and are thus naturally protected from the swoop of birds of prey, we find almost equally intense coloration ; as in the trogons, barbets, and gapers.
Local Causes of Colour-development.-Another real, though as yet inexplicable cause of diversity of colour, is to be found in the influence of locality. It is observed that species of totally distinct groups are coloured alike in one district, while in another district the allied species all undergo the same change of colour. Cases of this kind have been adduced by Mr. Bates, by Mr. Darwin, and by myself, and I have collected all the more curious and important examples in my Address to the Biological Section of the British Association, at Glasgow in 1876 (Chap. VII. of this volume). The most probable cause for these simultaneous variations would seem to be the presence of peculiar elements or chemical compounds in the soil, the water, or the atmosphere, or of special organic substances in the vegetation; and a wide field is thus offered for chemical investigation in connection with this interesting subject. Yet, however we may explain it the fact remains, of the same vivid colours in definite patterns being produced in quite unrelated
groups, which only agree, so far as we yet know, in inhabiting the same locality.
Summary on Colour-development in Animals. Let us now sum up the conclusion at which we have arrived,
as to the various modes in which colour is produced or modified in the animal kingdom.
The various causes of colour in the animal world are, molecular and chemical change of the substance of their integuments, or the action on it of heat, light or moisture. It is also produced by interference of light in superposed transparent lamelle, or by excessively fine surface-strix. These elementary conditions for the production of colour are found everywhere in the surfacestructures of animals, so that its presence must be looked upon as normal, its absence as exceptional.
Colours are fixed or modified in animals by natural selection for various purposes ; obscure or imitative colours for concealment; gaudy colours as a warning; and special markings, either for easy recognition by strayed individuals, females, or young, or to direct attack from a vital part, as in the large brilliantlymarked wings of some butterflies and moths.
Colours are produced or intensified by processes of development,-either where the integument or its appendages undergo great extension or modification, or where there is a surplus of vital energy, as in male animals generally, and more especially at the breeding
Colours are also more or less influenced by a variety of causes, such as the nature of the food, the photographic action of light, and also by some unknown local action probably dependent on chemical peculiarities in the soil or vegetation.
These various causes have acted and reacted in a variety of ways, and have been modified by conditions dependent on age or on sex, on competition with new
forms, or on geographical or climatic changes. In so complex a subject, for which experiment and systematic inquiry has done so little, we cannot expect to explain every individual case, or solve every difficulty ; but it is believed that all the great features of animal coloration and many of the details become explicable on the principles we have endeavoured to lay down.
It will perhaps be considered presumptuous to put forth this sketch of the sulject of colour in animals, as a substitute for one of Mr. Darwin's most highly elaborated theories—that of voluntary or perceptive sexual selection ; yet I venture to think that it is more in accordance with the whole of the facts, and with the theory of natural selection itself; and I would ask such of my readers as may be sufficiently interested in the subject, to read again Chapters XI. to XVI. of the Descent of Man, and consider the whole subject from the point of view here laid down. The explanation of almost all the ornaments and colours of birds and insects as having been produced by the perceptions and choice of the females, has, I believe, staggered many evolutionists, but has been provisionally accepted because it was the only theory that even attempted to explain the facts. It may perhaps be a relief to some of them, as it has been to myself, to find that the phenomena can be shown to depend on the general laws of development, and on the action of "natural selection," which theory will, I venture to think, be relieved from an abnormal excrescence and gain additional vitality, by the adoption of the views here imperfectly set forth.
Although we have arrived at the conclusion that