quired by the horse varies according to the character of his food; but, roundly stated, about eight quarts a day will be a fair average. When resting, water should be given three times a day; when at work, more frequently. The very prevalent impression that when a horse is warm he should not be allowed to drink is very erroneous. No matter how warm a horse may be, it is always safe to allow him from 6 to 10 swallows of water. The danger is not in the water, but in the excessive quantity that the animal will take when warm, if not restrained. It should not be given when ice cold. The total number of horses in the United States continues to increase despite the fact that much of the service formerly rendered by horses is now performed by the automobile. The number on farms in 1914 was estimated by the Department of Agriculture at 20,962,000 against 16,736,059 in 1904; the value, in 1914, $2,291,638,000 against $1,136,940,000 in 1904. The number in cities was, in 1910, 3,182,789 against 2,936,781 in 1900.

Bibliography. Flower, The Horse: A Study in Natural History (London, 1891); Schwarznecker, Rassen, Zuchtung, und Haltung des Pferdes (Berlin, 1894); Simonoff and Moerder, Les races chevalines, avec une étude spéciale sur les chevaux russes (Paris, 1894); Rommel, Market Classes of Horses (Washington, 1902); Cook, History of the English Turf (London, 1903); Parlin, The American Trotter (Boston, 1905); Speed, The Horse in America (New York, 1905); Tozer, The Horse in History (London, 1908); Axe, The Horse (ib., 1908); Meysey-Thompson, The Horse: Its Origin, and Development Combined with Stated Practice (ib., 1911); Richardson, The New Book of the Horse (New York, 1911).

VARIETIES: Du Hays, The Percheron Horse (trans., New York, 1886); Des Farges, Race Horses (trans., London, 1890); Bruce, The Thoroughbred Horse (New York, 1892); Tweedie, The Arabian Horse: His Country and People (Edinburgh, 1893); Wallace, The Horse of Amer ica in his Derivation, History, and Development (New York, 1897); Hayes, Among Horses in Russia (London, 1900); Wasser, The Horses of the World's Armies, United Service, vol. i (3d series, New York, 1902); Ewart, The Multiple Origin of Horses and Ponies (Washington, 1904); Trevanthan, The American Thoroughbred (New York, 1905); Ridgeway, The Origin and Influence of the Thoroughbred Horse (Cambridge, 1905); Rommel, The Preservation of our Native Types of Horses (Washington, 1908); Wrangel, Die Rassen der Pferdes (Stuttgart, 1909); Lydekker, The Horse and its Relatives (London, 1912).

POINTS: Goubaux and Barrier, The Exterior of the Horse (Philadelphia, 1904); Rueff, Das Aeussere des Pferdes und seine Fehler (Stuttgart, 1885); Müller, Lehre vom Exterieur des Pferdes (5th ed., Vienna, 1895); Hayes, Points of the Horse (London, 1904).

BREEDING, CARE, AND TRAINING: Leisering and Hartmann, Der Fuss des Pferdes in Rücksicht auf Bau, Verrichtungen und Hufbeschlag (Dresden, 1870); Clarke, Horses' Teeth (New York, 1886); Hayes, Illustrated Horse-Breaking (London, 1889); Merwin, Road, Track, and Stable (Boston, 1892); Marvin, Training the Trotting Horse (1893); Johnstone, The Horse Book (Chicago, 1908); Galvayne, The XXth Century Book on the Horse (London, 1912); Harper, Management and Breeding of Horses (New York, 1913);

Gay, Productive Horse Husbandry (Philadelphia, 1914). See CURB; ROARING.

HORSE. A miner's term, applied to any intruded material which is the apparent cause of a sudden interruption in the continuity of a mineral that is being quarried, as when a dike of igneous rock cuts across an ore body. In vein mining a detached mass of rock which fills the vein is called a horse, while colliers apply the term to the shale which replaces the coal bed, as well as to such interruptions as seem to have been the channels of small streams, and which were subsequently filled up by the clay that formed the roof of the coal.

HORSE. On shipboard, an iron jackstay or rail used as a guide for a traveler or as a securing bar, usually called a horse rail. Also the old name for a footrope on a yard. The secondary footrope at the end of a yard is still called the Flemish horse.

HORSE, FOSSIL. Remains of horses, often of extinct species, have been found in the cave beds, river gravels, bone licks, and loess deposits of the Quaternary period, or Age of Man, in almost all parts of the world. In the more ancient deposits of the Tertiary period, or Age of Mammals, have been found remains of a series of ancestors of the horse which illustrate the evolution of this race through this entire geological period, a time probably of some millions of years. Fossil horses of the Age of

Man are much like the existing species and are included for the most part in the same genus (Equus). They have been found in Europe, Asia, Africa, North and South America, but none in Australia or in the Oceanic islands, except in those which, like the British Isles, were joined to the continental mainland during the early part of the Quaternary period. With these exceptions the animal was of world-wide distribution and inhabited especially the open grassy plains and high plateaus of the interior of the great continents, at the beginning of the Age of Man. All these races were at first wild.

In the New World they apparently became extinct, for when the Spaniards invaded America, they found no horses, wild or domesticated. The Indians, who had domesticated the llama, the alpaca, and the dog, knew nothing of the horse and were astonished and terrified at the sight of the strange and unfamiliar animals which the newcomers rode. Yet, when introduced by the white races, the horse ran wild and flourished and increased greatly in the same regions where its native cousins had formerly lived, showing how well the country was

suited to its needs.

In Central Asia two wild races, Przewalsky's horse and the Asiatic wild ass, or kiang (q.v.), persist to the present day; others were domesticated by man in the earliest times, and their use in Chaldæa and Egypt for draft and riding is depicted in the ancient mural paintings. In Africa the larger species became extinct, but the smaller zebras still survive in the southern part of the continent and the African wild ass in the northern part (Somaliland).

The wild species of Europe, a small race, short-legged and shaggy-haired, was domesticated by man, for it is represented in rude prehistoric drawings scratched on bone or ivory by men of the Neolithic or Polished-Stone age.

The domesticated horses now in use are chiefly derived from the Asiatic race, but it is probable that in some breeds there is a considerable

strain of this European species or variety, and it is possible also that African races may have been domesticated and to some extent mixed with the Asiatic species. The existing wild horses of North and South America, the bronchos and mustangs, are descendants of the animals brought over by the Spaniards; but it is possible that in South America some survivors of the native races still existed at the time of the discovery of the continent, and mixed with the introduced species when it ran wild.

In general, only fragmentary specimens, parts of skulls, bones, or teeth of these extinct horses have been found fossil, so that their characters are very imperfectly known. A number of complete skeletons were found in Texas in 1899, one of which was placed in the American Museum of Natural History, New York. This species, Equus scotti, was of the size of a trotting horse, but in proportions more like a zebra, with deep jaw, short neck and legs, and small feet. Another extinct species, the Hippidium of Argentina and Patagonia, was large-headed, with extremely short and stumpy legs and feet, exaggerating some of the peculiarities of the Shetland pony, although of larger size.

Tertiary Ancestors of the Horse. The history of the evolution of the horse through the Tertiary period affords the best-known illustration of the doctrine of evolution by means of natural selection (q.v.) and the adaptation of a race of animals to their environment. The ancestry of this family has been traced nearly as far back as the beginning of the Tertiary without a single important break. During this long period of time, estimated at nearly 3,000,000 years, these animals passed through important changes in all parts of the body, but especially in the teeth and in the feet, by which they became adapted more and more perfectly to that particular environment, viz., the open plains of a great plateau region with their scanty stunted herbage, which are the natural habitat of the modern horse.

Equine Characteristics. The horse (including under his name the asses and zebras, as well as the true horses-see EQUIDE) is unique among modern animals in the fact that it walks on the extreme tip of the central digit of the foot, corresponding to the middle finger nail of man, and that all the remaining toes have completely disappeared. The feet are greatly elongated, so as to equal the other segments of the limbs in length, and to raise the animal much higher above the ground than if he walked, as does a man or a bear, on the sole of the foot. In each foot the first and fifth digits (thumb and little finger) have completely disappeared and left no trace, while of the second and fourth digits only a small slender rudiment exists which represents the metapodial, or bone of the palm, and is called a splint. These two splint bones lie closely against the cannon bone, or metapodial of the central digit, and are not indicated on the surface of the foot. The modern horse is therefore one-toed.

The teeth are equally peculiar. There are six grinding teeth in a closely set row on each side of each jaw; the crowns of these are very much elongated, so that they can be pushed up in their sockets as fast as they wear away at the grinding surface. The grinding surface displays a complicated pattern caused by infoldings of the enamel of the tooth, the enamel edges being supported on one side by the dentine of the

tooth, on the other by a similar substance called cement, deposited on the outer surface of the unworn enamel before the teeth are extruded from the gums. This arrangement secures

at all stages of wear a series of hard enamel ridges projecting a little above the surface of the softer dentine and cement, and makes a remarkably efficient grinder for the hard, dry grasses which are the natural food of the horse (q.v.). In the series of animals which lead up to the modern horse we can trace every step in the evolution of these marked peculiarities of teeth and feet, from an ancestor so little suggesting the horse that when first found it was named by Richard Owen, the greatest comparative anatomist of his time, Hyracotherium (coney-like beast). Its relation to the horse was not at all suspected and was recognized by Huxley and Marsh only when the series of intermediate stages between Hyracotherium and the modern horse was discovered. This first ancestor of the horse line is very much more like the contemporary ancestors of other lines of descent and indicates how all the modern quadrupeds have diverged from a single type, each becoming adapted to its especial mode of life.

The

Stages in the Evolution Series. The Hyracotherium (renamed Eohippus) of the Lower Eocene was a small animal, no larger than the domestic cat, with four complete toes on each forefoot and three on each hind foot. There is reason to believe that the still more ancient ancestors of this and all other mammals had five toes on each foot, and in the Hyracotherium forefoot we find a splint bone representing the missing first digit, or thumb, while in its hind foot there is a splint bone representing the missing outer (fifth) digit, but here no trace is left of the innermost (first) digit. The proportions of the skull, the short neck and arched back, and the limbs of moderate length were very little horselike, recalling some modern carnivorous animals, especially the civets. teeth, short-crowned, covered with low rounded knobs of enamel, suggested those of monkeys and of pigs, but not at all the long-crowned complicated grinders of the horse. Beginning with this small and primitive animal, 11 stages have been recognized from as many successive formations, showing the gradual evolution of the race into its modern form. Each stage is characteristic of its particular geological horizon. Some have been found in several parts of the world, but by far the most complete and bestknown series comes from the Tertiary Bad Lands of the Western States. Besides the main line of descent, which has led into the modern horses, asses, and zebras, there were also collateral branches, which have left no descendants. Only the more important stages can be mentioned here.

The successors to the Hyracotherium were the Protorohippus and Orohippus of the Middle Eocene. A complete skeleton of the former animal, from the Wind River valley, Wyoming, is in the American Museum of Natural History in New York City. It is about the size of the kit fox, and much like its predecessor, the Hyracotherium, except that the splint representing the fifth digit in the hind foot has disappeared.

In the Upper Eocene the Epihippus occurs, but only fragmentary specimens have been found. These show that the middle toe was

becoming more prominent, and the side toes, especially the outer toe of the forefoot, were quite slender. Contemporary with this animal was the very much larger Palæotherium of Europe, related to the horses, but not in the direct line of descent.

In the Oligocene is found Mesohippus, of which several complete skeletons are known. The various species range in size from that of a red fox to a mastiff. There are but three toes in each foot, the outer digit of the forefoot being now reduced to a splint bone and no longer appearing as a separate toe. The central toe in both fore and hind feet is much larger, and the side toes, although they still reach the ground, are quite slender and can support but a small part of the weight of the animal.

The teeth are of the crested or lophodont type, the crests higher and sharper than in the preceding genera. This constitutes the necessary intermediate stage in the conversion of the low, round-knobbed, or bunodont crown into the high, sharply crested crown with cement bracing which characterizes the later horses. See TOOTH.

In the Miocene is found Protohippus, in which the side toes, although still complete, are extremely small and slender and do not reach the ground. They can therefore no longer assist in supporting the weight of the animal and are merely useless rudiments. Various species range from the size of a mastiff to that of a Shetland pony. The teeth in this animal are much more like those of the modern horse; the crown is greatly lengthened, the crests or ridges being higher and more complicated, and the valleys between the ridges are filled up with a material (cement) approximating the dentine in texture and hardness. A new and very effective method of grinding is thus begun, for when the sharp enamel crests wear down, they form a double ridge of enamel supported within by dentine and without by cement; the two latter are softer than the enamel and wear away more rapidly, leaving it as a sharp projecting ridge, continually renewed with the wear of the tooth. The tooth is pushed up from the jaw as fast as it wears off on the grinding surface, so that it becomes an efficient grinder for those hard, siliceous grasses which would rapidly wear down a tooth of the old pattern to a useless stump.

In the Pliocene lived the Pliohippus, of which very little is known, except that it was either one-toed or had the side toes reduced to extremely small rudiments, and the teeth were much like those of Protohippus, which it a little exceeded in size.

In the Pleistocene is found the modern genus Equus, of larger size, with but one toe on each foot, the lateral digits represented by splint bones, and with teeth longer-crowned than those of Protohippus, enabling the animal to grind hard grasses still more efficiently. Extinct species have been found in Europe, Asia, Africa, North and South America, as we have seen above.

Meaning of the Changes in Feet and Teeth. Along with the disappearance of the side toes in the evolution of the horse there is a considerable increase in the proportionate length of the limbs and especially of the lower part of the leg and foot. The surfaces of the joints, at first more or less of the ball-andsocket kind, permitting of free motion of the limb in all directions, become keeled and grooved

like a pulley, thus permitting free motion forward and backward, but limiting the motion in all other directions and increasing considerably the strength of the joint. By this means the foot is made more efficient for locomotion over a smooth, regular surface, but less so over very rough ground, of little use for striking or grasping, or the varied purposes for which the feet of many-toed animals are used.

The increased length in the lower leg and foot increases the length of the stride without decreasing its quickness, for the heavy muscles of the leg are chiefly in the upper part, and to increase the length of the lower part changes the centre of gravity of the limb very little, and it consequently swings to and fro from the socket nearly as fast; for in an ordinary step the leg swings like a pendulum, and the speed of the swing is regulated by the distance of the centre of gravity from the attachment, as that of a pendulum is by the height of the bob.

To increase the length of lower leg and foot will therefore give the animal greater speed; but it puts an increased strain on the ankle and toe joints, and these must be strengthened correspondingly, by converting them from ball-andsocket joints to ginglymoid or pulley joints. Additional strength, likewise at the expense of flexibility, is obtained also by the consolidation of the two bones of the forearm (ulna and radius) and leg (tibia and fibula) into one, the shaft of the lesser bone practically disappearing while its ends become fused solidly to its larger neighbor.

Corresponding with the increase in length of limb, it is necessary for a grazing animal that the head and neck should increase in length in order to enable the mouth to reach the ground. So in the modern horse we find the neck and head much elongated when compared with the little Hyracotherium, and this elongation has taken place at equal pace with the elongation of the legs. The reduction and disappearance of the side toes, and the concentration of the step on the single central toe, serve likewise to increase the speed over smooth ground.

The

soft yielding surface of the polydactyl foot is able to accommodate itself to a rough, irregular surface, but on smooth ground the yielding step entails a certain loss of speed. An illustration is afforded by the pneumatic tire of a bicycle; a soft tire accommodates itself to a rough road and makes easier riding, but a hard tire is faster, especially on a smooth road. Similarly the hard, firm step from the single toe allows of more speed over a smooth surface, although compelling the animal to pick its way slowly and with care on rough, irregular ground.

The change in the character of the teeth from brachydont, or short-crowned, to hypsodont, or long-crowned, enables the animal to subsist on the hard innutritious grasses of the dry plains, which require much more thorough mastication before they can be of any use as food than do the softer green foods of the swamps and forests.

All these changes in the evolution of the horse are adaptations to a life in a region of level, smooth, and open grassy plains, which are the natural habitat of the horse. The race, better fitted at first for a forest life, has become more and more completely adapted to live and compete with its enemies or rivals under the conditions which prevail in the high, dry plains of the interior of the great continents. The great increase in size, which has occurred in