SPECIES

the distinctive characteristics of the family, and its position in the zoological system, are given in the articles equidae and perissodactyla. here attention is concentrated on the leading features of the horse as contrasted with the other members of the same family, and subsequently on the anatomical structure of the former animal. the evolution of the existing representatives of the family from primitive extinct animals is summarized in the article equidae. _horse_, _wild horse_, _pony_.--the horse (_equus caballus_) is distinguished from the others by the long hairs of the tail being more abundant and growing quite or nearly from the base as well as the end and sides, and also by possessing a small bare callosity on the inner side of the hind leg, just below the "hock" or heel joint, in addition to the one on the inner side of the fore-arm above the carpus or "knee," common to all the genus. the mane is also longer and more flowing, and the ears are shorter, the limbs longer, and the head smaller. though existing horses are usually not marked in any definite manner, or only irregularly dappled, or spotted with light surrounded by a darker ring, many examples are met with showing a dark median dorsal streak like that found in all the other members of the genus, and even with dark stripes on the shoulders and legs. two distinct types of horse, in many instances largely modified by interbreeding, appear to exist. (1) the northern, or dun type, represented by the dun ponies of norway (_equus caballus typicus_), the closely allied celtic pony (_e. c. celticus_) of iceland, the hebrides, &c., and the wild pony of mongolia (_e. c. przewalskii_), with which the now extinct tarpan of the russian steppes appears to have been identical. the prevalent colour is yellow-dun, with dark brown or black mane, tail and legs; in the wild forms the muzzle is often white and the root of the tail short-haired; while the head is relatively large and heavy. no depression exists in the skull in front of the eye. most of the ordinary horses of n.w. europe are descended from the dun type, with more or less admixture of barb blood. (2) the southern, or barb type, represented by barbs, arabs, thoroughbreds, &c. (_e. c. asiaticus_ or _libycus_), in which the typical colour is bay with black "points" and often a white star on the forehead, and the mane and tail are long and full. the skull generally shows a slight depression in front of the socket of the eye, which, although now serving as the attachment for the muscle running to the nostril, may represent the face-gland of the extinct _hipparion_. many of the dark-coloured horses of europe have barb or arab blood in their veins, this being markedly the case with the old english black or shire horse, the skull of which shows a distinct depression in front of the eye-socket. this depression is still more marked in the extinct indian _e. sivalensis_, which may have been the ancestral form. in europe wild horses were abundant in the prehistoric neolithic or polished-stone period. judging from the quantity of their remains found associated with those of the men of that time, the chase of these animals must have been among man's chief occupations, and horses must have furnished him with one of his most important food-supplies. the characters of the bones preserved, and certain rude but graphic representations carved on bones or reindeers' antlers, enable us to know that they were rather small in size and heavy in build, with large heads and rough shaggy manes and tails, much like, in fact, the recently extinct tarpans or wild horses of the steppes of the south of russia, and the still-surviving mongolian wild pony or "przewalski's horse." these horses were domesticated by the inhabitants of europe before the dawn of history. horses are now diffused by the agency of man throughout almost the whole of the inhabited parts of the globe, and the great modifications they have undergone in consequence of domestication, crossing, and selective breeding are well exemplified by comparing such extreme forms as the shetland pony, dwarfed by uncongenial climate, the thoroughbred racer, and the london dray-horse. in australia, as in america, horses imported by european settlers have escaped into unreclaimed lands and multiplied to a prodigious extent, roaming in vast herds over the wide and uncultivated plains. _ass_, _zebra_, _quagga_.--the next group is formed by the asiatic wild asses, or kiangs and onagers, as they might well be called, in order to distinguish them from the wild asses of africa. these asses have moderate ears, the tail rather long, and the back-stripe dark brown and running from head to tail. on the neck and withers this stripe is formed by the mane. there are two species of asiatic wild ass, with several varieties. the first and largest has two races, the chigetai (_equus hemionus_) of mongolia, and the kiang (_e. h. kiang_) of tibet, which is a redder animal. the onager (_e. onager_), of which there are several races, is smaller, with a broader dorsal stripe, bordered with white; the colour varying from sandy to greyish. this species ranges from baluchistan and n.w. india to persia, syria and arabia. these asses inhabit desert plains or open table-land; the kiang dwelling at elevations of about 14,000 ft. they are generally found in herds of from twenty to forty, although occasionally in larger numbers. all are fleet, and traverse rough ground with speed. on the lowlands they feed on dry grasses, and in tibet on small woody plants. in india and persia they are difficult to approach, although this is not the case in tibet. their sandy or chestnut colouring assimilates them to the horse, and separates them widely from the african wild asses, which are grey. the kiang has also larger and more horse-like hoofs, and the tail is haired higher up, thus approximating to _equus caballus przewalskii_. among the striped species, or zebras and quaggas of africa, the large grevy's zebra (_equus grevyi_) of somaliland and abyssinia stands apart from the rest by the number and narrowness of its stripes, which have an altogether peculiar arrangement on the hind-quarters, the small size of the callosities on the fore-legs, the mane extending on to the withers and enormous rounded ears, thickly haired internally. the large size of the ears and the narrow stripes are in some degree at any rate adaptations to a life on scrub-clad plains. next comes the closely allied species with small pointed ears, of which the true quagga (_e. quagga_) of south africa is now extinct. this animal has the dark stripes limited to the head, neck and shoulders, upon a brown ground. in the typical form, now also extinct, of the bonte-quagga, dauw, or burchell's zebra (_e. burchelli_), the ground-colour is white, and the stripes cover the body and upper part of the limbs. this was the commonest species in the great plains of south africa, where it roamed in large herds, often in company with the quagga and numerous antelopes. the species ranges from the orange river to the confines of abyssinia, but its more northern representatives show a gradual increase in the striping of the legs, culminating in the north-east african _e. burchelli granti_, in which the stripes extend to the hoofs. the markings, too, are alternately black and white, in place of brown and creamy, with intermediate "shadow stripes," as in the southern races. lastly, there is the true or mountain zebra (_e. zebra_), typically from the mountain ranges of cape colony, where it is now specially protected, but represented by _e. zebra penricei_ in south-west africa. in its relatively long ears and general build it approaches the african wild asses, from which it chiefly differs by the striping (which is markedly different from that of the quagga-group) and the reversal of the direction of the hairs along the spine. the african wild ass (_e. asinus_) is the parent of the domesticated breed, and is a long-eared grey animal, with no forelock, and either a shoulder-stripe or dark barrings on the legs. there are two races, of which the nubian _e. a. africanus_ is the smaller, and has a continuous dorsal stripe and a shoulder-stripe but no bars on the legs. the somali race (_e. a somaliensis_), on the other hand, is a larger and greyer animal, with an interrupted dorsal and no shoulder-stripe, but distinct leg-barrings. _hybrids._--there are thus eight modifications of the horse-type at present existing, sufficiently distinct to be reckoned as species by most zoologists, and easily recognizable by their external characters. they are, however, all so closely allied that each will, at least in a state of domestication or captivity, breed with any of the others. cases of fertile union are recorded between the horse and the quagga, the horse and the bonte-quagga or burchell's zebra, the horse and the onager and kiang or asiatic wild asses, the common ass and the zebra, the ass and bonte-quagga, the ass and the onager, the onager and the zebra, and the onager and the bonte-quagga. the two species which are farthest removed in structure, the horse and the ass, produce, as is well known, hybrids or mules, which in certain qualities useful to man excel both their progenitors, and in some countries and for certain kinds of work are in greater requisition than either. although occasional more or less doubtful instances have been recorded of female mules breeding with the males of one or other of the pure species, it is more than doubtful if any case has occurred of their breeding _inter se_, although the opportunities of doing so must have been great, as mules have been reared in immense numbers for at least several thousands of years. we may therefore consider it settled that the different species of the group are now in that degree of physiological differentiation which enables them to produce offspring with each other, but does not permit of the progeny continuing the race, at all events unless reinforced by the aid of one of the pure forms. the several members of the group show mental differences quite as striking as those exhibited by their external form, and more than perhaps might be expected from the similarity of their brains. the patience of the ass, the high spirit of the horse, the obstinacy of the mule, have long been proverbial. it is very remarkable that, out of so many species, two only should have shown any aptitude for domestication, and that these should have been from time immemorial the universal and most useful companions and servants of man, while all the others remain in their native freedom to this day. it is, however, still a question whether this really arises from a different mental constitution causing a natural capacity for entering into relations with man, or whether it may not be owing to their having been brought gradually into this condition by long-continued and persevering efforts when the need of their services was felt. it is possible that one reason why most of the attempts to add new species to the list of our domestic animals in modern times have ended in failure is that it does not answer to do so in cases in which existing species supply all the principal purposes to which the new ones might be put. it can hardly be expected that zebras and bonte-quaggas fresh from their native mountains and plains can be brought into competition as beasts of burden and draught with horses and asses, whose useful qualities have been augmented by the training of thousands of generations of progenitors. not infrequently instances occur of domestic horses being produced with a small additional toe with complete hoof, usually on the inside of the principal toe, and, though far more rarely, three or more toes may be present. these malformations are often cited as instances of reversion to the condition of some of the earlier forms of equine animals previously mentioned. in some instances, however, the feet of such polydactyle horses bear little resemblance to those of the extinct _hipparion_ or _anchitherium_, but look rather as if due to that tendency to reduplication of parts which occurs so frequently as a monstrous condition, especially among domesticated animals, and which, whatever its origin, certainly cannot in many instances, as the cases of entire limbs superadded, or of six digits in man, be attributed to reversion.