Popular Science Monthly/Volume 34/December 1888/Evolving the Camel
| EVOLVING THE CAMEL. |
By GRANT ALLEN.
AS I sauntered to-day down the Rue d’Isly, on evolutionary thoughts intent, I met a caravan of camels, in long single file, coming in from the desert with their bales of merchandise. Poor, weary creatures they looked, in all conscience, their humps shrunken to mere bags of loose skin, and their patient faces bearing all too openly the marks of their long and toilsome journey across the hill country. At their head stalked a lordly Arab in a dirty white burnous; drivers and attendants of lesser station followed in the rear with a tread as stately and solemn as the camels' own. For, dejected and foot-sore though they all were, men and beasts had alike even so the free and firm step of the open desert. Little Moorish children from the dark shops ensconced in the wall ran out with childish delight and clapping of hands to see them pass; women with their faces muffled up to the eyes turned timidly to give them a casual glance; and even old Hamid Abder-Rahman himself, sitting cross-legged on his bench before his cup of coffee in the open bazaar, deigned to remove his pipe from his mouth one moment and remark to Omar on the divan beside him that prime dates were coming in from the oases very well this season.
As for me, standing there in my alien garb, I rejoiced in soul that I had seen a caravan, and could forthwith begin philosophizing on camels. "I could have played on any timbrel," says the poet at the Zoo, "For joy that I had seen a whimbrel." And I could have burst prosaic trammels, for joy that I had seen those camels. Everybody knows, of course, the famous story of the German student who evolved the camel from his own inner consciousness. Now, that mode of evolving a species I hold to be illegitimate; you should always draw your animal from the life, even though, like the Frenchman in the same old apologue, yon only go to the Jardin des Plantes for the model on which you base your rhapsodical portrait. But when a man has actually been to Africa itself, and seen a caravan in all its glory, headed by a real live Arab in a burnous of the dirtiest, fresh from the sands and siroccos of the desert—who, I should like to know, if not he, is entitled to speak with authority about camels? For here I am, on the borders of the desert, upon whose flats I can look down (at a safe distance) from yonder mountain-heights; and if ever there was a case of "adaptation to the environment," the camel has indeed adapted himself wholly and solely to the conditions of Sahara.
Deserts, in fact, are exacting in the matter of adaptation; you must obey them or die. No other environment (not even perhaps the arctic snows) demands so much in the way of adaptiveness from all that live in it. The plants are every one of them saline and alkaline; they must content themselves with sand instead of soil, and with brackish pools instead of fresh water. The animals are all peculiar to their habitat; bird and insect must assume alike the uniform gray sabelline tint of external nature everywhere around them. Only two higher types subsist at all among those great sand-wastes—two types specially fitted for their own exceptional mode of life, one plant and one animal—the date-palm and the camel. They make Sahara. Nobody ever saw a picture of the desert without a date-palm and a camel in the foreground. Those two inseparable elements of the Africa of our fancy shall not be parted even in this sober biological sketch. Nature, indeed, has joined them together, and science shall not be permitted here to put them asunder.
And yet, though the camel as we know him is peculiarly Saharan, a product of the great African, Indian, and Bactrian deserts, it is not to the Old World that we must look at all if we wish to evolve the camel historically, rather than to develop him by a priori process from the depths of our own inner consciousness. It is America that gives us geologically the earliest evidence of the camel's ancestors; and it is America that still contains the greater number of species of the camel family, in the persons of the llama, the alpaca, the guanaco, and their allies. Prof. Cope has drawn up the pedigree of the race for us in full detail. The Asiatic and African camels, in fact, are mere surviving Oriental members of a family American in origin and history, but stranded, as it were, in a remote corner of the Old World, where they have survived the competition of newer and higher types in virtue of their special minor adaptations to the peculiar circumstances of their strange habitat. Having early fitted themselves in certain outer points to desert conditions, they have been enabled to outlive all their younger and more highly developed competitors solely in virtue of their singular combination of desert-resisting qualities.
Now, it must at once strike everybody as a curious circumstance in the geography of animals that the existing cameloids should be so strangely distributed—one group of them in the desert region of Asia and Africa, the other group away across the world among the snowy slopes of the Andes of South America. What can be the meaning of so quaint a freak of distribution? Why should the two surviving cameloid tribes be thus separated from one another by half the earth's surface, and by many deep seas and shadowy mountains—one in the Old World and one in the New; one in the desert and one in the uplands; one in the northern hemisphere and one in the southern? Clearly, the answer suggested alike by geological facts and by analogies elsewhere, is simply this: we have here, as it were, two little surviving biological islands, colonies of an ancient race which once covered both worlds alike with its numerous members. Time was when the camels or their allies were of cosmopolitan distribution. They ranged, no doubt, the Eocene plains of all the great continents. But they are an ancient and in many respects an undeveloped ungulate form, which has become extinct elsewhere in the intermediate regions through the fierce competition of the higher ruminants, and has lingered on only under special circumstances in two remote corners of the world—in the deserts of Arabia and in the Andes of Peru.
The llamas and alpacas, as the lower and less specialized type of the two, explain best the true systematic position of the family. For South America, as everybody knows, is in many respects a very antique biological province. Less ancient in its life-forms than Australia, that world of living mesozoic fossils, it yet retains in many places the scattered remnants of its extremely old-fashioned fauna. There is reason to believe, indeed, that the circumpolar continent—Europe, Asia, and North America—was once for many ages continuous, while Australia, South Africa, and the South American peninsula formed separate islands in a wide and winding southern sea. Hence the higher life-forms developed rapidly in the broad and varied northern land-mass, while more antiquated types continued to live on, uninfluenced by their competition, in the three isolated southern provinces. Of these three, Australia alone still remains a great island; but South Africa has been joined to the Mediterranean world by a gradual upheaval of the Saharan area; while the Isthmus of Panama, still later in date, apparently, has formed a great natural bridge by which some of the North American land-animals have been able to invade the comparatively unpeopled tropical realms of the low southern species. In both cases, however, many of the low local types still hold out in extreme recesses or under special climatic and geographical conditions; and thus the llama and alpaca have been preserved to our time intact in the narrow belt of temperate slope between the snow-clad Andes and the Pacific shore.
I have said that the cameloids are a very ancient type of ruminants indeed: their skeleton abundantly proves this fact; but I will not dwell at length upon such dry points of anatomical detail, because I fancy I have noticed on various occasions that the general public does not wildly interest itself in questions of carpal and metatarsal bones. It is not frantically enthusiastic about distinctions of odd-toed or even-toed ungulates. What most of us really want to know, and what the comparative anatomists as a body still studiously neglect to tell us in plain language, is how each animal came to obtain, not its bones which we don't see, but its distinctive external shape and characteristics—its horns, its tusks, its hump, or its antlers. We would rather learn a few simple facts about the evolution of the elephant's trunk or the peacock's tail than a whole volume of learned memoirs on the cervical vertebræ and the carinate sternum. Those things are doubtless very convincing in their own way, but they are not of a sort to rouse our profound personal attention. There are, however, two other visible points about the camel-kind which clearly mark their true position as very early ruminants indeed, and which can yet be readily apprehended by the ordinary surface-loving, non-anatomical intelligence. One is, that the camels as a group antedate the development of horns or antlers; the other is that they still possess in full, like other animals, those canine and incisor teeth which are partly obsolete, partly altered in shape, in all the higher and later ruminants. Each of these peculiarities has a meaning of its own, and points back to certain interesting episodes in the development of the great ruminant order.
The vast mass of ruminants generally at the present day possess some form or other of horns or their equivalents. In the giraffe, which in a few points (mostly delusive) approaches the camels, the horns are merely blunt protuberances of bone, persistent through life, and covered with a continuous hairy skin. They show us the lowest surviving stage in the evolution of frontal weapons. In the deer tribe, they appear at first under much the same form, as little knobs or bosses of bone on the forehead, underlying a fold of skin technically known as the velvet; but when the horns are fully grown, the velvet is rubbed off, and the bone alone shows its naked material as the branching antlers with which we are all so familiar in the Scotch red deer. Horns of this type are shed annually, and reproduced in more and more complex forms (representing successive ancestral stages) with each renewal. Finally, in the great central group of the ruminants, represented in our day by the oxen, sheep, goats, and antelopes, the bony core or heart of the horn is protected by a sheath of agglutinated hair, which continues to increase by layers during life. This last form of horn is never shed, but persists through the whole of the animal's existence.
Historically, we know that the earliest ruminants, whose remains are preserved for us in Tertiary strata, were quite hornless; and the gradual evolution of horns and antlers, from the simplest to the most complex, has been traced out in full through successive geological ages by Gaudry, Boyd Dawkins, and other biologists. We can follow in detail the origin and rise of each tine and spike from the mere boss or knob on the forehead of the ancestral form to the branching horns of the reindeer, the wapiti, and the Irish elk. The camel, therefore, in its lack of horns represents for us an early undeveloped stage of the ruminant type, when the ruminants had as yet only just diverged from the common ancestors of the horses and pigs. Darwin has shown that horns and other familiar offensive weapons (especially when peculiar to the males alone, as is the case with the antlers of stags) have been developed in the struggle for mates, and are a necessary result of sexual selection. But all such ornaments belong to the higher and later stages of animal life, and are wholly wanting in the unarmed, undecorated, ugly camel. He is, in fact, a ruminant on which the higher types of selection have been little exercised, though, as we shall presently see, his special adaptations for a desert-life have been carried very far in particular directions, and so have enabled him to hold out bravely in his own narrow and restricted field against all more advanced and more highly specialized animals.
The teeth of the camels and of their allies the llamas tell the same tale in a somewhat different fashion. In all the higher ruminants—giraffes, deer, oxen, antelopes, and goats alike—the weapons of offense are the horns or antlers, and the teeth have almost or entirely ceased to be used in fighting. They have also undergone certain profound modifications of shape and arrangement (interesting only to the technical anatomists), which fit them for cropping grass or other low herbage, but get rid to a great extent of their tearing powers. On the other hand, there is one other group of ruminants besides the camels which is destitute of horns—the little group of musk-deer—and in these pretty, small creatures the canine teeth have been developed into long protruding tusks, which thus take the place of horns as offensive weapons, and are used by the males in their single combats for the possession of their mates. But, in the camels and llamas, no special fighting-weapon of any sort exists. When camels fight at all—which is very rarely—they fight merely by simple biting. They remain in this respect on the lowest mammalian level. Their teeth approximate rather to the type which occurs in horses and some other outlying ungulate groups than to the type which occurs in the true ruminants. They have always canines in both jaws; but these canines are not lengthened out into regular tusks, nor do they serve to any noticeable extent as weapons of warfare. In short, the camels by many points of their structure point back to a time when the ancestors of the ruminants had not diverged at all widely from the ancestors of the horse, the pig, or the hippopotamus, and they still retain in many particulars the early "generalized," or rather unspecialized, type of the common progenitor of the entire group.
The llamas and alpacas may be looked upon as the best living representatives of the camel tribe in its primitive state, before it had begun specially to assume its camel-stage. They do not possess the adaptive peculiarities which fit the camel for its desert existence; and, on the other hand, they exhibit to the full that awkward, ungainly, misshapen type which so often betrays Nature's first rough draught of an evolving order. They are, as it were, the sketchy outline only of the perfected ruminants. Compare for a moment the ugly, shambling, ungraceful alpaca with the red deer, flying over the open Scotch moorland; the gazelle, springing lightly along the Syrian plains; the antelope, careering across the South African veldt; or the chamois, leaping from crag to crag among the frozen Alps, and you will see at once what is meant by the difference between a specialized and a generalized type—the difference between Nature's early attempts in a given line, and her fully evolved and carefully molded final product.
The antelopes and deer, with their various allies, such as the gnu, the eland, the ibex, the buffalo, the bison, the sheep, the bighorn, and the musk-ox, represent for us the developed ruminant types, produced by fierce competition in the struggle for life in the great continents. Their fleetness of foot, their exquisite horns, their agility, their grace of movement, all depend upon the existence in their native countries of highly evolved beasts of prey, from whose fierce attacks they have had to save themselves by speed and acuteness. To the same cause they owe also the keenness of their senses, the slimness of their legs, and to some extent also the elegance and beauty of their entire bodies. The smaller kinds, like the gazelles, are remarkable for their vigilance, their timidity, and their alertness, the hereditary result of ages spent in avoiding the attacks of predatory enemies. Natural selection, in short, has given to the advanced ruminants generally their distinctive rapidity, lightness, and beauty of shape. To sexual selection, on the other hand, they owe their twisted horns or branching antlers, their dappled coats and exquisite markings, their ornamental manes and their proud and stately gait and carriage. All these points are wholly wanting in the clumsy llama and alpaca group. Stranded remnants, as it were, of the Eocene world, those antique creatures linger on among their mountain valleys a perpetual milestone by whose indications we may mark the progress since effected, under stress of selective agencies, in the main advancing body of the higher ruminants.
On such a simple original type, defenseless and ungainly, the camel is a specialized and adapted desert variation. The undeveloped llamas have no humps, and they have their two toes quite separated in a certain awkward, ungraceful, splay-footed fashion. In the true camels, on the other hand, the two toes are united below by a kind of horny sole, almost to their points, which terminate in a couple of small hoofs, and beneath the foot there is a soft cushion, by which the instep bears upon the sandy soil over whose expanses the creature is adapted to move. This padded sole is to the camel what the solid hoof is to the horse, it fits him exactly for the sort of ground over which his ancestors have stalked and shambled for countless generations. And it is interesting to note the similarities and differences which natural selection has brought about in the case of these two chief human beasts of burden.
In both the foot has become adapted for scouring the open plain only; firmness and sureness of tread have been the sole qualities that really told, and hence, in both, the toes as such have become practically extinct, and in their place one gets at last a single united broad-based foot, such as gives the animal the most secure foundation for his heavy body upon the level ground.
Compare for a moment these two types of practically toeless foot with the grasping hand of the forestine monkeys, the sharp claws of the tree-haunting squirrels, the light paw of the leaping hare, or even the slender and delicate ungulate feet of the gazelles and the chamois, and you will see how wholly they have been specialized for their work as trotters only. In the ruminants generally, as in all the great division of hoofed mammals, the extremities are calculated for support alone; but in the horse and in the camel this restriction of function reaches its highest practical point, and the feet and legs exist merely as adequate and extremely stable props for the heavy framework. In the horse the solid hoof remains as the sole surviving toe out of the original five possessed by his primitive ancestors in the American Eocene (though the "splint-bones," well known to the veterinaries, are the last functionless relics of two other toes); in the camel the same result is practically attained by the union of the two toes which it still possesses through the medium of a single horny sole, as well as by throwing the main weight of the body on the padded cushion underneath the instep.
On the other hand, in the horse, adapted as he is by nature for scouring open, grassy plains or hill-sides, natural selection has favored the development of a particularly hard and solid hoof, whose native qualities man still further exaggerates by shoeing him with a clanking ring of iron; while in the camel, the direct product of desert conditions, a singular softness and pliability of foot has rather been encouraged by the soft and shifting nature of Saharan or Bactrian sands. For this reason, it is found practically that the horse and the camel are in any given country mutually exclusive; where the one thrives the other languishes. Here, in northern Africa, outside the Atlas, camels can not be profitably employed as beasts of burden; the few that come here in caravans from the desert arrive with a weary, foot-sore, dejected look, tired of tramping with their soft-padded feet over the hard and smooth macadamized roads which the French engineers have substituted for the narrow, paved Moorish packways, where mules and Arabs once transacted in their slow and lumbering fashion all the business of Algeria and Tunis. But beyond the shallow belt between the mountains and the sea the horse is of no avail: his hard and unyielding hoof sinks deep into the shifting sand of the desert, and he struggles and shuffles in helpless despair where the light dromedary, with his loose, shambling gait, his long trot, and his padded sole, fitting itself accurately to the sand beneath, accomplishes with ease his hundred miles a day for a week together. On hills or rocks the camel is nowhere, on open sandy plains he can hold his own against all comers.
It is interesting to note, indeed, how much alike in many adaptive particulars, but especially in their awkward gait, their tall necks, their long, shambling swing, and the powerful flanks which bring it about, are the three chief inhabitants of the desert or its outskirts—the camel, the giraffe, and the African ostrich. In the last-named case, the likeness is all the more curious and striking because one would almost have said beforehand that to adapt a bird and a ruminant mammal to the same environment, and to turn them out at last with many striking external resemblances of shape and gait, would be simply impossible; and yet Nature has accomplished this strange feat so perfectly that Linnæus, struck by the singular analogy between the two creatures, gave the ostrich the scientific name, which it still bears, of Struthio camelus. Even the reduction in the number of the toes to two, and their provision with a soft pad underneath, have been accurately reproduced in the great bird. As to the giraffe, its old name of camelopard sufficiently attests the popular appreciation of its outer similitude to the ship of the desert. The fact is, no large animal can be properly adapted for Saharan conditions (liability to attack from lions and other great beasts of prey included), unless it combines these three attributes of a soft tread, a swift, swinging gait, and a long neck, enabling it to reach its food above or below, as necessitated by the height of its legs and body. Ostriches, giraffes, and camels alike, all feed to a considerable extent indeed on foliage of trees.
Of all these animals, however, the most purely desert-haunting is the camel itself, and it exhibits, therefore, a few special peculiarities not equally well developed in any other creature. In the first place, desert journeys imply continued privation, or even at times complete absence of food. Now, whenever in the animal kingdom such a necessity frequently arises in the ordinary life-history of a species, natural selection has provided against it by favoring the survival of those individuals which can lay up spare material against the period of famine in their own tissues. A starving sheep, Prof. Huxley well remarks, is as much a carnivore as the lion that would devour it; it subsists strictly upon its own fat and its own muscle, which it slowly unbuilds to use up in the needful action of its heart, its lungs, its limbs, and its internal organs generally. Hence, in hard times, those animals which have the largest reserve-fund of fat at their disposal will survive longest, and species which often encounter hard times learn organically by hereditary experience to supply such a reserve-fund against possible contingencies. Thus the bear and the dormouse go to sleep sleek and plump for the annual hibernation, and wake up mere loose and baggy masses of skin and bone. The zebu and other tropical oxen gather a huge hump of fat between the shoulders in the wet months while grass is plentiful, to serve them as a store of food during the dry season. But in the camel and dromedary this special provision against famine reaches the highest point, and produces the hump or humps on the back—one in the Arabian or African, two in the Bactrian or Indian variety.
Structurally, of course, the humps are nothing—mere lumps of fat, collected under a convenient fold of the skin, and utterly unprovided for in the framework of the skeleton. When the animal is at its best and well fed, they are full and plump, standing up on his back firm and upright; but on a long journey they are gradually absorbed to keep up the fires that work the heart and legs, and in the caravan camels which arrive at the coast, the skin hangs over, an empty bag, upon the creature's flanks, bearing witness to the scarcity of external food during the course of his long, forced march from the interior. A starved small camel in this state of health far more closely resembles a Peruvian llama than any one who has only seen the fine, well-kept beasts in European menageries or zoölogical gardens could readily imagine.
But water is even scantier in the desert than food; and against want of water, therefore, the camel has had to provide himself, functionally at least, if not structurally, quite as much as against want of herbage. His stomach has accordingly acquired the power of acting as an internal reservoir, and he can take in as much water at the Bahrs or Wadys, where he rests for a while on his toilsome march, as will supply his needs for four or five days together. There are some differences in this respect, however, between the two chief varieties of the camel. The African kind is most abstemious, and best adapted to sandy deserts; the Bactrian, a product of more varied and better-watered country, is larger and stronger, but less patient of hunger and thirst, while at the same time it can manage to subsist and to make its way into somewhat rockier and more rugged country.
One other adaptive peculiarity the camel possesses which marks it out essentially as a desert-born animal, modified for generations by the baking expanse of Sahara or the Arabian sand-wastes. On those scorching surfaces a horse could not lie down to rest in the eye of the sun without scalding or excoriating his tender skin. But all the parts of the camel's body which touch the sweltering sand in his ordinary patient kneeling position are provided with callosities of thickened hide, which enable him to rest on the burning surface without danger or discomfort. The only other desert-haunting ruminant of similar habits, the giraffe, has analogous callosities for the same purpose on his breast and knees. Such adaptive characters, however, are never a key to real relationship; they necessarily result from mere exposure to the same circumstances; and hence we get the seemingly paradoxical principle, so well enunciated by Mr. A. R. Wallace, that the more useful any organ or point of structure is to its possessor, the less is its value as a test of systematic position. Unseen little bones and internal organs, which fail to strike the imagination of the outside observer, are rightly used as the keys to underlying relationship by the systematic biologist. The real affinities of the camel are closest, indeed, not with the giraffe which so strongly resembles it, but with the llama and alpaca, so remotely connected in outer seeming, and so widely separated from it in space by an entire hemisphere.
Camels, llamas, and alpacas alike, then—to sum up the conclusion to which we have all along been tending—represent a very simple and early ruminant type, unmarked by any of those higher features induced in the ruminants of the open plains of the great continents by the necessity for protection from the advanced carnivores. They recall for us in their main points of structure, as well as in their low and undeveloped grade of intelligence, the general characteristics of the ruminant race at the dawn of its existence in the Eocene period. They have no horns or tusks or weapons of offense, such as grow up in the savage battles of the males among dominant races; and their very docility and gentleness of demeanor result in the last resort from this undeveloped character of their entire class; for non-fighting animals are always timid, patient, and inoffensive, though often obstinate and self-willed to a noteworthy degree, as the camel can be whenever he chooses. Their virtues themselves thus tell against them; they betray the stupidity and the archaic, unprogressive character of the whole type. The camelidæ, as a group, in short, are surviving specimens of the raw material from which, by natural and sexual selection, the higher ruminants, in diverging lines, have been slowly evolved through innumerable ages.
But of this antique and unspecialized type, the camel itself is in certain ways a highly modified and peculiarly adapted desert offshoot. Retaining still in its internal structure the marks of its early undeveloped character, it nevertheless presents in external configuration and functional peculiarities a remarkable instance of special adaptation to a restricted environment. While as a ruminant it is extremely low, as a desert animal it is at the very top of the tree. And it is this early adaptation to a very unusual mode of life that has enabled the camel, lowly as it is in general organization and in intellectual grade, to hold its own successfully against all later comers, and to preserve for us still in the great central Eurasiafrican continent a type of life otherwise extinct save in a single outlying and practically insulated district of the old South American life-region.—Longman's Magazine.
