African Nature Notes and Reminiscences. Frederick Courteney Selous
of detection by the human eye amongst their natural surroundings. Every collecting entomologist knows how closely certain species of butterflies when resting with closed wings in shady forests resemble dead leaves, or moths the bark of trees. Birds too, especially those which nest on the ground, often harmonise with their surroundings in a most marvellous way.
In the open treeless regions within the Arctic Circle, as well as on bare mountain ranges, nearly all the resident species of animals and birds turn white in winter, when their whole visible world is covered with an unbroken mantle of pure white snow, and become brown or grey during the short period of summer.
In treeless deserts again within the tropics, where the rainfall is very scanty and the climate excessively hot and dry, with intense sunlight throughout the year, all resident living organisms, mammals, birds, reptiles, and insects, are found to be of a dull coloration which harmonises in the most wonderful way with the sandy or stony soil on which they live. It is also very often the case that animals which live in forests where the foliage is not too dense to allow the sun to penetrate are spotted or striped, whilst those which live in really thick jungle or amongst deep gloomy ravines are of a uniform dark coloration.
Now a most interesting question arises as to the true causes which have brought about the extraordinary variations of colour to be seen in living organisms inhabiting different parts of the world.
It is, I believe, the general opinion of modern naturalists that, putting aside cases where brilliant colours may have been produced amongst birds and insects by the action of the law of sexual selection, the coloration of all living organisms is protective, "serving," as that distinguished naturalist Mr. Alfred Russel Wallace puts it, when discussing the subject of the coloration of mammals, "to conceal herbivorous species from their enemies, and enabling carnivorous animals to approach their prey unperceived."
Many very striking facts can be adduced in support of this theory, and no doubt it is of advantage to most species of mammals, birds, reptiles, and insects to harmonise in colour with their surroundings; but there are many instances in nature, especially amongst birds and insects, where a very striking and conspicuous coloration does not appear to have been prejudicial to the life of a species.
The highly decorative but very conspicuously coloured plumage to be seen in the males of many species of birds, especially during the breeding season, was considered by the immortal Darwin to be due to the influence of sexual selection, and whatever may be urged against the correctness of this theory, it is supported by a long array of indisputable facts.
Great, however, as is the divergence between the plumage of the males and females in many species of birds, not only during the breeding season, but in a great number of cases at all times of year, and however gaudy and conspicuous the coloration of the former may be compared with that of the latter, such conspicuous coloration never appears to be prejudicial to the life of a species, though in some cases the brighter coloured male assists the female in incubation, and it would thus appear that in all such cases the sombre coloured plumage of the female was not absolutely necessary for purposes of protection against enemies.
I therefore think that if it is admitted that bright and conspicuous colours have been evolved in living organisms through the action of the law of sexual selection, without detriment to the life of the species in which such conspicuous colours are shown, it must be conceded that a coloration harmonising with its surroundings is not a necessity of existence in all cases to all species of mammals, birds, reptiles, and insects, and that it is therefore quite possible that where living organisms agree very closely in colour with their surroundings, such harmonious coloration may have been produced by some other agency than the need for protection by colour, and I would suggest that in addition to the influence exerted in the evolution of colour in living organisms by the action of sexual selection, and the necessity for protection against enemies, a third factor has also been at work, which I will call the influence of environment.
It is worthy of remark, I think, that in hot, dry deserts, where the climatic conditions are stable, and where the general colour of the landscape is therefore very much the same all the year round, all the resident species of mammals, birds, reptiles, and insects are what is called protectively coloured, that is to say, they are all of a dull brown or greyish coloration,[1] which harmonises beautifully with their parched, dull-coloured environment. In the leucoryx, the Saharan representative of the gemsbuck of South-Western Africa, all the black markings which are so conspicuous in the latter animal have disappeared or become pale brown, whilst the general colour of the body has been bleached to a dirty white. Now, no one can persuade me that if the leucoryx were coloured exactly like its near relative the gemsbuck, it would suffer one iota more, in the open country in which it lives, from the attacks of carnivorous animals than it does at present, and I therefore believe that the faded colour of the leucoryx, as compared with that of the gemsbuck or the beisa antelope, has not been brought about in order to serve as a protection against enemies, but is directly due to the influence of its desert environment, and constant exposure to strong sunlight on treeless plains. Again, from the point of view of a carnivorous animal hunting for food by daylight and by sight, no two countries could be more alike than the open karoos of the Cape Colony and the plains in the neighbourhood of Lakes Nakuru and Elmenteita in British East Africa, where the grass is always kept very short by the large herds of game, as well as by the cattle, sheep, and goats belonging to the Masai, which pasture there. Before the advent of Europeans, the carnivorous animals inhabiting the Cape Colony were exactly the same as those found to-day in East Africa, viz. lions, leopards, chetahs, wild dogs, and hyænas. In both districts lions were once numerous, and in both zebras formed the principal food of these carnivora. But whereas Equus granti, the form of zebra found on the plains near Lake Nakuru, is the most brilliantly coloured representative of the genus to which it belongs, with jet black stripes on a pure white ground, the now extinct form of zebra—Equus quagga—which once abounded on the plains of the Cape Colony, was of a dull grey brown in ground colour, with darker brown stripes on the head, neck, and fore-part of the body alone. Now, these two races of zebras, both living on bare, open plains, could not both have been coloured in the best possible way to escape being seen by the lions which constantly preyed upon them. If, as has been contended, the juxtaposition of the black and white stripes in Grant's zebras renders these animals not only inconspicuous, but almost invisible under strong sunlight on an open plain, and is, in fact, the supreme triumph of protective coloration in large mammals, why had the quaggas of the Cape Colony become dull brown, for they also lived on open plains in strong sunlight, and needed protection from the lions every bit as much as their congeners of East Africa? Moreover, I think all naturalists and embryologists are agreed that Equus quagga was the descendant of boldly striped ancestors.
[1] The cock ostrich is, I think, the only exception to this rule, and in the case of this remarkable bird the influence of sexual selection has probably been more potent than that of a dull-coloured, monotonous environment.
To my mind the loss of stripes in the quagga was entirely due to the environment in which this species had lived for long ages; for on the karoos of the Cape Colony everything is of one dull brown colour, whether on hill or plain, and no shade is to be found anywhere, for the whole country is without trees. The air, too, is intensely hot and dry, and the rainfall scanty. In these semi-deserts of South-Western Africa, not only did the quaggas lose their black stripes, but the elands also lost the white stripes of their immediate ancestors, whilst the blesboks had already lost much of the white to be seen in the body colouring of the bonteboks, from which they are descended, and had become of a much duller colour generally. In East Africa, however, the plains are surrounded by well-wooded hills, which give some colour to the landscape, whilst the rainfall every year is heavy. If it is not the influence of their several environments which has brought about the differences between the well-striped elands and zebras of East Africa and their dull-coloured relatives that once lived in the karoos of the Cape Colony, the theory of protective coloration must be equally at fault, for in spite of the fact that in both countries both races of these animals have been hunted by lions from time immemorial on open plains, and under precisely similar conditions, they developed