change which blacks undergo will not seem surprising if we reflect that in Nature, as well as in Art, they generally consist of an intense olive or brown to which a deep blue or purple is superadded. The latter hues, being the more fugitive, fade first on exposure to light, and thus a dirty olive or a rusty brown must remain. These changes are in partial harmony with what we observe in the vegetable kingdom. A dull, dirty brown is the ultimate goal towards which leaves, flowers, and fruits, as well as insects, tend while fading; but those splendid intermediate changes which we find in autumnal foliage have nothing analogous in the decaying colours of insects. It is curious that in the manufacture of those artificial colours which now play so important a part in tinctorial operations a corresponding rule holds good. If these dyes, during their elaboration, are submitted to a heat too high or too prolonged, the product becomes dusky, and a dirty brownish grey is the final result. We must further note how, in the animal and vegetable kingdoms, pure and bright colours are connected with the highest vitality only. We plant the dusky seed in the earth amidst the dark remains of decomposing organic matter, and as it grows up we see it put on higher and higher colours, till, in the culminating moment of its life, in the act of inflorescence, prismatic hues are all but universal. Then begins the process of decay, attended by a degradation of colour. Similar changes may be traced in animals. Externally we need merely compare the dull-coloured larva with the brilliant imago, or the sombre-coated nestling with the brighter plumage of the mature bird. Internally we may contrast the intensely-vitalised scarlet arterial blood. with the darker-coloured and more contaminated venous blood, and still further with excrementitious matters. The great truth to which we are here calling attention has not altogether escaped the notice of Mr. Wallace, who writes"The very frequent superiority of the male bird or insect in brightness or intensity of colour, even when the general tints and colouration are the same, now seems to me to be due to the greater vigour and activity and the higher vitality of the male. The colours of an animal usually fade during disease or weakness, while robust health and vigour add to their intensity.* This intensity of colouration is most manifest in the male during the breeding season, when the *Those who are brought practically in contact with animals have long been familiar with the fact that a" dull coat" is indicative of disease, or at least of weakness. vitality is at a maximum." But we are not aware that either Mr. Wallace or any one else has fully grasped the principle laid down above, or traced its numerous applications, æsthetic as well as biological. But among the "pigment-colours" there is a very great diversity in permanence due to the nature of the colours themselves, or to that of the tissues in which they inhere. Dr. Hagen divides such colours into epidermal, placed in hair, in feathers, and in the chitinic exo-skeleton of insects, and hypodermal, situate in the softer internal layers of the skin. That the latter are the more easily affected by any external influence is natural. Alterations and degradations of colour similar to those above-mentioned may indeed, under certain circumstances, be produced even in the absence of light. But we have direct experimental evidence to show that, other things being equal, animal matters retain their colours most completely in the absence of light, and fade the more rapidly in proportion to the intensity of the illumination to which they are exposed. Hence we are compelled to recognise light as a destroyer of animal colouration. But light is generally regarded not merely as a colourdestroyer, but as a colour-producer, and it is with this its supposed function that we have now to deal. Those who take here the affirmative view rely mainly on two facts, or supposed facts, to which we have already briefly referred, the higher colouration and the superior brilliance of the tropical fauna, and the sombre hues of nocturnal and subterranean beings. At these facts we must look, and seek to ascertain their meaning. We must of course admit that Europe produces no humming-birds or trogons, no Belionota or Pachyrhynchi; but we must also remember that the total number of species of birds, of reptiles, and of insects found, say in South America, is far greater than the sum total existing in Britain or on the European continent. Hence, even if the tendency to produce a gay colouration were equal in either case, the probability is that South America would be the richer in gorgeous species. Again, travellers who visit tropical countries not unnaturally select the most showy forms, and their collections are therefore not a fair average. Naturalists, such as Mr. Wallace, who have taken the trouble to examine closely, find that even in New Guinea, Borneo, or Brazil dull-looking species exist in numbers. Had we catalogues of the insects of such countries as complete as those we possess for Britain, France, or Germany, our views as to the general character of a tropical fauna would be doubtless modified. As the insects of warm climates, also, are upon the whole larger than those of our hyperborean latitudes, they necessarily attract attention, and their beauty does not pass unseen; yet every entomologist knows that even in Britain we possess "tiny miracles of Nature" which, if viewed with a lens of low power, display a splendour little-if at all-inferior to the most richly attired tropical species. We will merely mention, as instances, Chryseis ignita, Chrysomela cerealis, Donacia proteus, Polydrusus micans and flavipes, Rhynchites betulæ and populi, Lampra rutilans, and Anthraxia salicis. Calosoma sycophanta, also, if very rare in Britain, is very common in certain parts of Central Europe, and may be fairly considered one of the most gorgeous species of the entire family of Carabidæ to be met with in any part of the world. The case, then, seems to stand thus :-We have in Britain certain species, small, and it may be rare, which display the very same shades of colour and the same brilliance as we find in the most admired forms of tropical life. This fact seems to us scarcely consistent with the theory that the more intense light of low latitudes is a prominent factor in the production of splendid colours. Were such the case gaily-coloured species in our climate would not merely be fewer and smaller; they would rather be altogether wanting. Again, different portions of the torrid zone differ very widely as regards the number, and even the beauty, of the richly-attired birds and insects they produce. Thus, as Mr. Wallace has pointed out, in New Guinea 50 per cent of the birds are brilliantly coloured, whilst in the Malay Islands and in the Valley of the Amazon the proportion does not exceed 33 per cent. Can this distinction be rationally ascribed to any excess of light enjoyed by New Guinea over and above the amount received by the Valley of the Amazon? Both these respective districts lie under the Equator; both are fruitful, plentifully supplied with moisture, well-wooded, and exposed-as far as we can perceive-tu very similar meteorological conditions. But if excess of light cannot be the cause of the superiority of New Guinea over equinoctial Brazil, why should it be put forward to explain the superiority of Brazil as compared with Britain? Why should the fauna of the Philippine Islands, as is remarked by Mr. Wallace in his invaluable "Glasgow Address," be so rich in species of exceptionally splendid colours? Can there be in those islands either any excess in the quantity or any peculiarity in the quality of the sunlight? That there is, no one has yet even attempted to show, and were such the case it would doubtless be traceable in a variety of phenomena not limited to the organic world. Another important point has been raised by Mr. Bates. He shows that whilst in many tropical butterflies the males are most splendidly coloured, the females-in numbers of cases at least-are sombre and insignificant in appearance, so much so that in former times they were often regarded as specifically distinct from their mates. If excess of light, therefore, be the producing cause of the splendour of the tropical Lepidoptera, why should not the effect appear alike in both sexes? To this argument, however, the reply has been made that in these very species the females are exceedingly sedentary in their habits, remaining generally concealed in shady thickets, whilst the males flutter about in the sunshine, and, being thus more exposed to light, experience modifications which-transmitted with constant accumulation from one generation to another-have produced the splendour now characteristic of their sex. To this question of the relative amount of exposure to light in different stages of existence we shall have to return. But the amount-or at least the intensity and clearnessof the sun does not necessarily vary with latitude alone. The air of some countries is more transparent, less obscured by fogs and clouds than that of others. More light evidently reaches the earth's surface on open plains or on table-lands and in deserts than in dense forests and in narrow valleys Do we find any corresponding variation in the prevalent hues of the animal population of these respective Îocalities? Mr. Wallace points out that the most brilliantly clad birds and insects are dwellers in the forests where the amount of light received is comparatively scanty. On the other hand, in the deserts, where-as we have already mentioned-light must attain its terrestrial maximum, the prevalent colouration, if not dark, is certainly neither light nor brilliant. As the Rev. H. Tristram remarks, in such regions the smaller Mammalia, the birds, the snakes, and lizards are alike sand-coloured, their hues having evidently more reference to concealment than to the influence of an intense illumination. There is indeed, if we wish to come to details, a curious want of harmony in the effects which light is expected to produce. We know that it bleaches in certain cases and darkens in others; but it is no easy task for us to predict when either of these opposite effects will be manifested. Still it is perfectly possible that light might have a bleaching power upon some living organisms, and a darkening effect upon others, according to their different molecular structure. There is, for instance, little doubt but that the air of Persia is, as a rule, exceedingly transparent; the climate is dry, mists and clouds comparatively rare, woodlands scanty, and the country generally open. We have even heard it stated that there the satellites of Jupiter are occasionally visible with the naked eye. Here, therefore, we have doubtless a case of light in its greatest intensity; but, according to Mr. Blanford, Persian specimens are generally paler than their nearest European representatives. Here, if light be directly concerned, its action must be of a bleaching character; yet we generally find in mammals, in birds and reptiles, as well as in insects, the upper surface, or portion most exposed to the sun, is darker than the under side, or than parts generally kept in the shade. An animal in whom the contrary arrangement prevails-such as the common badger-has much of the appearance of a caricature. This darkening of the superior surface of animals is again adduced as an instance of the chromogenic power of light, a view to which we shall afterwards take occasion to revert. As regards the comparison between the tropical and the extra-tropical faunæ the case may, perhaps, be fairly summed up thus:-There are certain cosmopolitan groups whose members, wherever found, are alike devoid of rich or brilliant colouration; there are other groups-such as the Ornithoptera, the Papiliones, the Buprestidæ, the Cetoniadæ, the trogons, humming-birds, birds of paradise, &c.which have a remarkable and hitherto-unexplained tendency to the development of splendid hues, and which, if not exclusively tropical, have their head-quarters and produce their largest representatives within the torrid zone. groups, again, attain their greatest splendour beyond the tropics, as, e.g., the ducks, the pheasants, and among insects the ground-beetles or Carabidae. It has, indeed, been suggested that if the colder regions of the earth are now inferior to the tropical districts in the beauty of their fauna, the cause may be sought in the ravages of the Glacial epoch. If the most magnificent species were forest-dwellers, as we now find it to be the case in warm climates, their destruction would be almost inevitably involved in the desolation of their haunts and the annihilation of their food. Perhaps, too, the very splendour of such supposed forms would render them more conspicuous to their enemies, and thus accelerate their extirpation. All such speculations, however, are little more than conjectural. We conclude indeed, judging from the fossil remains of insects discovered Other |