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yellow with black heads. In North America we might have raccoons, squirrels, and opossums, in parti-coloured livery of white and black, so as exactly to resemble the skunk of the same country; while in South America they might be black with a yellow throat-patch, so as to resemble with equal closeness the tayra of the Brazilian forests. Were such resemblances to occur in anything like the number and with the wonderful accuracy of imitation met with among the Lepidoptera, they would certainly attract universal attention among naturalists, and would lead to the exhaustive study of the influence of local causes in producing such startling results.
One somewhat similar case does indeed occur among the Mammalia, two singular African animals, the Aard-wolf (Proteles) and the hyæna-dog (Lycaon), both strikingly resembling hyænas in their general form as well as in their spotted markings. Belonging as they all do to the Carnivora, though to three distinct families, it seems quite an analogous case to those we have imagined; but as the Aard-wolf and the hyæna-dog are both weak animals compared with the hyæna, the resemblance may be useful, and in that case would come under the head of mimicry. This seems the more probable because, as a rule, the colours of the Mammalia are protective, and are too little varied to allow of the influence of local causes producing any well-marked effects.
When we come to birds, however, the case is different, for although they do not exhibit such distinct marks of the influence of locality as do butterflies—probably because the causes which determine colour are in their case more complex
- yet there are distinct indications of some effect of the kind, and we must devote some little time to their consideration.
One of the most curious cases is that of the parrots of the West Indian islands and Central America, several of which have white heads or foreheads, occurring in two distinct genera,' while none of the more numerous parrots of South America are so coloured. In the small island of Dominica we have a very large and richly-coloured parrot (Chrysotis augusta) corresponding to the large and richly-coloured butterfly (Papilio homerus) of Jamaica. 1 Pionus albifrons and Chrysotis senilis (C. America), Chrysotis sallæi (Hayti). The Andaman islands are equally remarkable, at least six of the peculiar birds differing from their continental allies in being much lighter, and sometimes with a large quantity of pure white in the plumage, exactly corresponding to what occurs among the butterflies.
In the Philippines this is not so marked a feature; yet we have here the only known white-breasted kingcrow (Dicrurus mirabilis); the newly discovered Eurylæmus steerii, wholly white beneath; three species of Diceum, all white beneath; several species of Parus, largely white-spotted; while many of the pigeons have light ashy tints. The birds generally, however, have rich dark colours, similar to those which prevail among the butterflies.
In Celebes we have a swallow-shrike and a peculiar small crow allied to the jackdaw, whiter than any of their allies in the surrounding islands, but otherwise the colours of the birds call for no special remark.
In Timor and Flores we have white-headed pigeons, and a long-tailed flycatcher almost entirely white. *
In Duke-of-York island, east of New Guinea, we find that the four new species figured in the Proceedings of the Zoological Society for 1877 are all remarkable for the unusual quantity of white in their plumage. They consist of a flycatcher, a diceum, a wood-swallow, and a ground-pigeon, all equalling if not surpassing their nearest allies in whiteness, although some of these, from the Philippines, Moluccas, and Celebes, are sufficiently remarkable in this respect.
In the small Lord Howe's island we have the recently extinct white rail (Notornis alba), remarkably contrasting with its allies in the larger islands of New Zealand.
We cannot, however, lay any stress on isolated examples of white colour, since these occur in most of the great continents; but where we find a series of species of distinct genera all differing from their continental allies in a whiter coloration, as in the Andaman islands, Duke-of-York island,
1 Kittacincla albiventris, Geocichla albigularis, Sturnia andamanensis, Hyloterpe grisola var., Ianthænas palumboides, Osmotreron chloroptera.
Artamus monachus, Corvus advena. 3 Ptilopus cinctus, P. albocinctus.
4 Tchitrea affinis, var. 5 Monarcha verticalis, Diceum eximium, Artamus insignis, Phlogænas johannæ.
and the West Indies, and, among butterflies, in the smaller Moluccas, the Andamans, and Madagascar, we cannot avoid the conclusion that in these insular localities some general cause is at work.
There are other cases, however, in which local influences seem to favour the production or preservation of intense crimson or a very dark coloration. Thus in the Moluccas and New Guinea alone we have bright red parrots belonging to two distinct families, and which therefore most probably have been independently produced or preserved by some
Here, too, and in Australia we have black parrots and pigeons ;2 and it is a most curious and suggestive fact that in another insular sub-region—that of Madagascar and the Mascarene islands—these same colours reappear in the same two groups.3
Sense-perception influenced by Colour of the Integuments Some very curious physiological facts bearing upon the presence or absence of white colours in the higher animals have lately been adduced by Dr. Ogle. It has been found that a coloured or dark pigment in the olfactory region of the nostrils is essential to perfect smell, and this pigment is rarely deficient except when the whole animal is pure white. In these cases the creature is almost without smell or taste. This, Dr. Ogle believes, explains the curious case of the pigs in Virginia adduced by Mr. Darwin, white pigs being killed by a poisonous root which does not affect black pigs. Mr. Darwin imputed this to a constitutional difference accompanying the dark colour, which rendered what was poisonous to the white-coloured animals quite innocuous to the black. Dr. Ogle, however, observes that there is no proof that the black pigs eat the root, and he believes the more probable explanation to be that it is distasteful to them; while the white pigs, being deficient in smell and taste, eat it and are killed. Analogous facts occur in several distinct families. White sheep are killed in the Tarentino by eating Hypericum cris
1 Lorius, Eos (Trichoglossidæ), Eclectus (Palæornithidæ).
3 Coracopsis, Alectrænas.
pum, while black sheep escape ; white rhinoceroses are said to perish from eating Euphorbia candelabrum ; and white horses are said to suffer from poisonous food where coloured ones escape. Now it is very improbable that a constitutional immunity from poisoning by so many distinct plants should, in the case of such widely different animals, be always correlated with the same difference of colour ; but the facts are readily understood if the senses of smell and taste are dependent on the presence of a pigment which is deficient in wholly white animals. The explanation has, however, been carried a step further, by experiments showing that the absorption of odours by dead matter, such as clothing, is greatly affected by colour, black being the most powerful absorbent, then blue, red, yellow, and lastly white. We have here a physical cause for the sense-inferiority of totally white animals which may account for their rarity in nature, for few, if any, wild animals are wholly white. The head, the face, or at least the muzzle or the nose, are generally black; the ears and eyes are also often black; and there is reason to believe that dark pigment is essential to good hearing, as it certainly is to perfect vision. We can therefore understand why white cats with blue eyes are so often deaf, a peculiarity we notice more readily than their deficiency of smell or taste.
If, then, the prevalence of white coloration is generally associated with some deficiency in the acuteness of the most important senses, this colour becomes doubly dangerous, for it not only renders its possessor more conspicuous to its enemies, but at the same time makes it less ready in detecting the presence of danger. Hence, perhaps, the reason why white appears more frequently in islands, where competition is less severe and enemies less numerous and varied. Hence, also, a reason why albinoism, although freely occurring in captivity, never maintains itself in a wild state, while melanism does. The peculiarity of some islands in having all their inhabitants of dusky colours (as the Galapagos) may also perbaps be explained on the same principles, for poisonous fruits may there abound which weed out all white or light-coloured varieties owing to their deficiency of smell and taste. We can hardly believe,
however, that this would apply to white-coloured butterflies ; and this may be a reason why the effect of an insular habitat is more marked in these insects than in birds or mammals. 1
It is even possible that this relation of sense-acuteness with colour may have had some influence on the development of the higher human races. If light tints of the skin were generally accompanied by some deficiency in the senses of smell, hearing, and vision, the white could never compete with the darker races so long as man was in a very low or savage condition, and wholly dependent for existence on the acuteness of his senses. But as the mental faculties became more fully developed and more important to his welfare than mere sense-acuteness, the lighter tints of skin and hair and eyes would cease to be disadvantageous whenever they were accompanied by superior brain-power. Such variations would then be preserved; and thus may have arisen the Xanthochroic race of mankind, in which we find a high development of intellect accompanied by a slight deficiency in the acuteness of the senses as compared with the darker forms.
Summary on Colour-development in Animals Let us now sum up the conclusions 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 lamellæ, or by excessively fine surface-striæ. These elementary conditions for the production of colour are found everywhere in the surface-structures 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 divert attack from a vital part, as in
1 In Darwinism, pp. 229, 230, I have suggested an explanation of most of the facts of colour in islands as due to the lesser need of protection.