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are the invariable accompaniments of health and energy. The same rule applies to the feathers of birds, whose colours are only seen in their purity during perfect health ; and a similar phenomenon occurs even among insccts, for the bright hues of caterpillars begin to fade as soon as they become inactive preparatory to undergoing their transformation. Even in the vegetable kingilom we see the same thing; for the tints of foliage are deepest, and the colours of flowers and fruits richest, on those plants which are in the most healthy and vigorous condition.
This intensity of coloration becomes most developed in the male during the breeding season, when the vitality is at a maximum. It is also very general in those cases in which the male is smaller than the female, as in the hawks and in most butterflies and moths. The same phenomena occur, though in a less markeıl degree, among mammalia.
Whenever there is a difference of colour between the sexes the male is the darker or more strongly marked, and the difference of intensity is most visible during the breeding season (Descent of Man, p. 533). Numerous cases among domestic animals also prove, that there is an inherent tendency in the male to special developments of dermal appendages and colour, quite independently of sexual or any other form of selection. Thus,—“the hump on the male zebu cattle of India, the tail of fat-tailed rams, the arched outline of the forehead in the males of several breeds of sheep, and the mane, the long hairs on the hind legs, and the dewlap of the male of the Berbura goat,”—are all adduced by Mr. Darwin as instances of characters peculiar to the male, yet not derived from any parent ancestral form.
Among domestic pigeons the character of the different breeds is often most strongly manifested in the male birds; the wattles of the carriers and the eye-wattles of the barbs are largest in the males, and male pouters distend their crops to a much greater extent than do the females, while the cock fantails often have a greater number of tail-feathers than the females. There are also some varieties of pigeons of which the males are striped or spotted with black while the females are never so spotted (Inimals and Plants under Domestication, I. 161); yet in the parent stock of these pigeons there are no differences between the sexes either of plumage or colour, and artificial selection has not been applied to produce them.
The greater intensity of coloration in the malewhich may be termed the normal sexual difference, would be further developed by the combats of the males for the possession of the females. The most vigorous and energetic usually being able to rear most offspring, intensity of colour, if dependent on, or correlated with vigour, would tend to increase. But as differences of colour depend upon minute chemical or structural differences in the organism, increasing vigour acting unequally on different portions of the integument, and often producing at the same time abnormal developments of hair, horns, scales, feathers, &c., would almost necessarily lead also to variable distribution of colour, and thus to the production of new tints and markings. These acquired colours would, as Mr. Darwin has shown, be transmitted to both sexes or to one only, according as they first appeared at an carly age, or in adults of one sex; and thus we may account for some of the most
marked differences in this respect. With the exception of butterflies, the sexes are almost alike in the great majority of insects. The same is the case in mammals and reptiles; while the chief departure from the rule occurs in birds, though even here in very many cases the law of sexual likeness prevails. But in all cases where the increasing development of colour became disadvantageous to the female, it would be checked by natural selection; and thus produce those numerous instances of protective colouring in the female only, which occur in these two groups, birds and butterflies.
Colour as a Jeans of Recognition. There is also, I believe, a very important purpose and use of the varied colours of the higher animals, in the facility it affords for recognition by the sexes or by the young of the same species; and it is this use which probably fixes and determines the coloration in many cases.
When differences in size and form are very slight, colour affords the only means of recognition at a distance, or while in motion; and such a distinctive character must therefore le of especial value to flying inscets which are continually in motion, and encounter cach other, as it were, by accident. This view offers us an explanation of the curious fact, that among butterflies the females of closely-allied species in the same locality sometimes differ considerably, while the males are much alike for, as the males are the swiftest and by far the highest fliers, and seek out the females, it would evidently be advantageous for them to be able to recognise their true partners at some distance off. This peculiarity occurs with many species of Papilio, Diadema, Adolias, and Coļius; and these are all genera, the males of which are
strong on the wing and mount high in the air.
In birds such marked differences of colour are not required, owing to their higher organization and more perfect senses, which render recognition casy by means of a combination of very slight differential characters.
This principle may perhaps, however, account for some anomalies of coloration among the higher animals. Thus, while aılmitting that the hare and the rabbit are coloured protectively, I r. Darwin remarks that the latter while running to its burrow, is made conspicuous to the sportsman, and no doubt to all beasts of prey, by its upturned white tail. But this very conspicuousuess while running away, may be useful as a signal and guille to the young, who are thus enabled to escape danger by following the older rabbits, directly and without hesitation, to the safety of the burrow; and this may be the more important from the semi-nocturnal habits of the animal. If this explanation is correct, and it certainly seems probable, it may serve as a warning of how impossible it is, without exact knowledge of the habits of an animal and a full consideration of all the circumstances, to decide that any particular coloration cannot be protective or in any way useful. Vr. Darwin himself is not free from such assumptions. Thus, he says :—" The zebra is conspicuously striped, and stripes cannot afford any protection on the open plains of South Africa." But the zebra is a very swift animal, and, when in herds, by no means void of means of defence. The stripes therefore may be of use by enabling stragglers to distinguish their fellows at a distance, and they may be even protective when the animal is at rest among herbagethe only time when it would need protective colouring.
Until the habits of the zebra have been observed with special reference to these points, it is surely somewhat hasty to declare that the stripes “cannot afford any protection.”
Colour Proportionate to Integumentary Development. -The wouderful display and endless variety of colour in which butterflies and birds so far exceed all other animals, seems primarily due to the excessive development and endless variations of the integumentary structures. No insects have such widely-expanded wings iu proportion to their bodies as butterflies and moths; in none do the wings vary so much in size and form, and in none are they clothed with such a beautiful and highly-organized coating of scales. According to the general principles of the production of colour already explained, these long-continued expansions of membranes and developments of surface structures, must have led to numerous colour-changes; which have been sometimes checked, sometimes fixed and utilised, sometimes intensified, by natural selection, according to the needs of the animal. In birds, too, we have the wonderful clothing of plumage—the most highly organized, the most varied, and the most expanded of all dermal appendages. The endless processes of growth and change during the development of feathers, and the enormous extent of this delicately-organized surface, must have been highly favourable to the production of varied colour-effects ; which, when not injurious, have been merely fixed for purposes of specific identification, but have often been modified or suppressed whenever different tints were needed for purposes of protection.
Selection by Females not a Cause of Colour.To