PREFACE Among all the animals of past and present there is none so deserving of our interest and affection as the horse. It is the most useful of the domestic animals. and has played the largest part in the development of civilization. Since the dawn of recorded history the man with the horse has been in the forefront of progress Whether leading the military civilizations of the past or breaking ground for the industrial civilization of the present the pioneers and leaders of progress have always made large use of this noble animal. It is not too much to say that without his help our ancestors in the Old World might have advanced but little further on the road to civilization than did the inhabitants of the ancient semi-civilized states of Peru and Mexico, where the horse was unknown, and travel, agriculture and military success were limited by the capacity of unaided human strength.
From another point of view the horse is of peculiar interest. It is one of the most perfect machines for swift running that exist among living animals and displays throughout its organization the most exact and finished mechanism adapted to this purpose. It is perhaps the finest example of what nature acting through millions of years, has been able to accomplish in the way of adapting a large quadruped to speed over long distances, and likewise of the extent to which man, during the few thousand years that he has controlled its development, has been able to improve upon nature, in the sense of adapting it to serve more exactly his own purpose. The history of this adaptation, continued over millions of years, constitutes the evolution of the horse. Its records consist of the actual skeletons of the successive stages in the development of the race. Buried in ancient river deposits, petrified and preserved to our day in the successive geological strata, they have been discovered and disinterred through the diligent search of scientists during the last fifty years.
This Museum has taken especial interest in the evolution of the horse. The records of this history are most completely preserved in the western United States, and through the liberality of the trustees of the American Museum, and in particular through special funds provided by the late William C. Whitney, by Professor Henry Fairfield Osborn and by other trustees and friends of the Museum, Messrs. James R. Keene, Randolph Huntington, F. K. Sturgis and others, we have obtained, from expeditions, gifts and purchases, many thousand specimens of fossil species, illustrating numerous successive stages in horse ancestry; and a unique series illustrating the diverse breeds of the domesticated animal and the structure and growth of its teeth and skeleton, so marvelously suited to its requirements. Such parts of the collection as might best serve to illustrate the subject have been placed on exhibition in the hall of fossil mammals. These exhibits show 1. THe EvouuTtion oF THE HORSE IN NATURE 2. THe EvoLution oF THE HoRSE UNDER DOMESTICATION The collections have been gathered together and arranged under the direction and supervision of Professor Henry Fairfield Osborn. Scientific descriptions of parts of the material have been published from time to time by Professor Osborn and his assistants and an extensive monograph on the evolution of the horse is now in preparation.
W.D.MaTtrHew AMERICAN Museum or NatTurRAL History, March, 1913
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Sr, Partl. EVOLUTION OF THE HORSE IN NATURE By W. D. Matthew, Ph.D.
Curator, Department of Vertebrate Palwontology INTRODUCTION S a domestic animal the horse is to be found almost everywhere that man can live. He is spread all over the world—from torrid to arctic climates, in all the continents, in remote oceanic islands he is completely cosmopolitan. But as a wild animal the horse is at present limited to the Old World, and is found there only in the open arid or desert plains of Central Asia and Africa. There are two species in Asia, the Asiatic wild ass (Equus hemionus) and the little known Przewalsky horse (Equus przewalskii), while in Africa there are the African wild ass (Hquus asinus) and the several species of zebra (Hquus zebra, Equus burchelli, Equus grevyz). In the Americas and Australia there are no true wild horses, the mustangs and broncos of the Western Plains and South America being feral (domesticated animals run wild) and descended from the horses brought over from Europe by the early white settlers. When the Spaniards first explored the New World they found no horses on either continent. The Indians were quite unfamiliar with them and at first regarded the strange animal which the newcomers rode, with wonder and terror, like that of the ancient Romans when Pyrrhus and his Greeks brought elephants—‘‘the huge earth-shaking beasts” to fight against them.
The horse is distinguished from all other animals now living by the fact that he has but one toe on each foot. Comparison with other animals shows that this toe is the third or middle digit of the foot. The hoof corresponds to the nail of a man or the claw of a dog or cat, and is broadened out to afford a firm, strong support on which the whole weight of the animal rests. Behind the “cannon bone”’ of the foot are two slender little bones, one on each side, called ‘‘splint bones.’’ These represent the second and fourth digits of other animals, but they do not show on the surface and there is nothing like a separate toe. So that the horse may be said to be an animal that walks on its middle finger nail, all the other fingers having disappeared.
The teeth of the horse are almost equally peculiar. The molars are long, square prisms which grow up from the gums as fast as they wear off on the crowns. Their grinding surface exhibits a peculiar and complicated pattern of edges of hard enamel between which are softer spaces composed of dentine and of a material called ‘‘cement,’’ much like the dentine in quality but formed in a different way. The dentine is formed 4
10 AMERICAN on the inside surfaces of the enamel while the tooth is — still within the jawbone; the cement is deposited on the outside surfaces of the enamel after the tooth has broken through the jawbone and before 1t appears above the gums. Various other peculiarities distinguish the horse from most other animals; some of these MUSEUM GUIDE LEAFLETS are shared by other hoofed animals. The two long bones of the forearm (radius and ulna) are separate in the greater number of animals, ~but in the horse and in many other hoofed animals, they are consolidated into a single bone (See Fig. 17, p. 27). The same consolidation is seen in the bones of the lower leg (tibia and fibula). (See Fig. 18, p. 28.) The lengthening of the foot and stepping on the end of the toe raises the heel in the horse, as in many other animals, to a con- FIG. 2. EVOLUTION OF THE FORE FOOT. PRINCIPAL STAGES (Right to Left) 1.
Four-toed horse Eohippus.
2.
Early three-toed horse Mesohippus.
3.
Later three-toed horse Merychippus. 4.
One-toed horse Equus.
Eocene Epoch Oligocene Epoch Miocene Epoch Pleistocene Epoch and Modern All outside views showing the middle and outer digits of the fore foot. In Eohippus the two outer toes (digits IV and V) are complete. In Mesohippus, digit V is reduced to a small splint and digit IV is notably smaller than the middle digit. In Merychippus, digit V is a tiny nodule of bone and digit IV has become very slender in comparison with the middle digit. while of the fifth digit no trace remains In Equus digit IV is reduced to a long splint,
siderable height above the ground, where it forms the hock joint, bending backward. as the knee bends forward. In these a« In various other ways the legs of the horse are especially fitted foy swift running over hard and level ground, just as its teeth are fitted for f grinding the wiry grasses, which grow on the open plain.
The [zebra and the ass have the same peculiar structure of teeth and feet as the domestic \ horse, and differ only in the color of the skin, proportions of various parts of the body, ete.
FIG. 3. EVOLUTION OF THE HIND FOOT. PRINCIPAL STAGES (Right to Left) 1, Hohippus, Eocene Epoch; 2, Mesohippus, Oligocene; 3, Merychippus, Miocene; 4, Equus, Pleistocene and Modern | | Outer views showing middle and outer digits. Note progressive reduction ol side toe (digit IV) from a well developed digit to a splint bone il
FOSSIL HORSES OF THE AGE OF MAN Hk Age of Man, or Quaternary Period, is the last and by far the shortest of the great divisions of geological time. It includes the Great Ice Age or Glacial Epoch (Pleistocene), when heavy continental glaciers covered the northern parts of Europe and North Ameri- ‘a, and the Recent Epoch, of more moderate climate during which civilization has arisen. ) In the early part of the Quaternary Period, wild species of horse were to be found on every continent except Australia. Remains of these true native horses have been found buried in strata of this age in all parts of the United States, in Alaska, in Mexico, in Ecuador, Brazil and Argentina, as well asin Europe, Asia and Africa. All these horses were much like the living species and most of them are included in the genus Equus. Mr. J. W. Gidley, in charge of the American Museum expedition in northern Texas, discovered in 1899 a number of more or less complete skeletons of a species of fossil horse, Equus scott7, in an old river deposit on Rock Creek in Donley County. These are the most complete specimens known of the extinct horses which inhabited this country at the beginning of the Ice Age. A mounted skeleton and several skulls have been placed on exhibition, and their near resemblance to the modern animal appears at a glance. The difference from the domestic horse is chiefly in proportions: the skull is shorter with deeper jaws, the legs rather short and the feet small in proportion to the body. In these characters this fossil horse resembles an overgrown zebra rather than a domestic horse. We know nothing of its coloring. It may have been striped, and in this case would have been very zebra-like; but there are some reasons for believing that it was not prominently striped. The bones are petrified, brittle and heavy, the animal matter of the bone having entirely disappeared and having been partly replaced by mineral matter. They are not much changed in color however, and are so perfectly preserved that they look almost like recent bone.
All the remains of these native horses which have been found in America have been petrified more or less completely. This means that they have been buried for many thousands of years, for true petrifaction is an exceedingly slow process. This condition serves as an easy means of distinguishing them from bones of the domestic horse, found buried in the earth. These cannot in any case have been buried for more than three or four centuries, and have not had time to petrify. Remains of these fossil horses from various parts of the United States are shown in the counter cases. One very rich locality is on the Niobrara River in Nebraska, another is in central Oregon. Many separate teeth and bones have been found in the phosphate mines near 12
EVOLUTION OF THE HORSE IN NATURE l -- Charleston, South Carolina; other specimens have come from central Florida, from southern Texas, Arizona, Kansas, Louisiana and even from Alaska. They are, in fact, so often found in deposits of rivers and lakes of the latest geological epoch (the Pleistocene) that the formation in the western United States has received the name of “ Equus Beds.”’ In South America, in strata of the Pleistocene Epoch, there occurs, besides several extinct species of the genus Hquus, the Hippidium, a peculiar kind of horse characterized by very short legs and feet, and some peculiarities about the muzzle and the grinding teeth. The legs were hardly as long as those of a cow, while the head was as large as that of a race horse or other small breed of the domestic horse. All these horses became extinct, both in North and South America. Why, we do not know. It is very probable that man—the early tribes of prehistoric hunters—played a considerable part in their disappearance, not indeed by killing them all off directly, but by continual hunting and chasing, driving them from the best feeding-grounds and interfering with their habits and opportunities for grazing. This persecution would tend to reduce their numbers and vigor, and be the prelude to their extinction. The competition with the bison, which had recently migrated to America, may also have made it more difficult than formerly for the American horse to get a living. And finally, some epidemic disease or prolonged season of drought may have exterminated the race. Whatever the cause, the horse had disappeared from the New World when the white man invaded it (unless a few individuals still lingered on the remote plains of South America), and in his place the bison had come and spread over the prairies of the North.
Fossil horses are equally common in the Pleistocene formations of the Old World. They have been found in all parts of Europe and Asia, in North and South Africa, but not in Australasia, the East Indies or Madagascar. In Central Asia, two wild races persist to the present day; others were domesticated by man in the earliest times, and their use in Chaldea and Egypt for draft and riding is depicted in the ancient mural paintings. In Africa the larger species became extinct in prehistoric times,asin America, but the smaller zebras still survive in the southern part of the continent (one species, the quagga, abundant fifty years ago, is now extinct), and the African wild ass is found in the fauna of the northern part. The wild horse of prehistoric Europe, a small race, shortlegged and shaggy-haired, was domesticated by man, a fact that is known from the rude drawings scratched on bone or ivory by men of the Neolithic or Polished Stone Age. But the domesticated horses now in use are derived chiefly from the Asiatic and African species! although it 1They are probably derived from three different wild sources which Professor Cossar Ewart has calledtheForestTypeofNorthernEurope,theSteppeTypeofNorthernAsia,resemblingthePrzewalsky horse, and the Plateau or Desert Type of Northern Africa, resembling the Arabian—Henry Fairfield Osborn.
14 AMERICAN MUSEUM GUIDE LEAFLETS is probable that in some breeds there is a considerable strain of this shaggy, short-legged European race. The domesticated ass is a descendant of the wild ass (Hquus asinus) of North Africa. THE EVOLUTION OF THE HORSE FT HE history of the evolution of the horse through the Tertiary r Period or Age of Mammals affords the best known illustration of the doctrine of evolution by means of natural selection and the adaptation of a race of animals to its environment. The ancestry of this family has been traced back to near the beginning of the Tertiary without a single important break. During this long period of time, estimated at nearly three millions of years, these animals passed through important changes in all parts of the body, but especially in the teeth and feet, adapting them more and more perfectly to their particular environment, namely the open plains of a great plateau region with their scanty stunted herbage, which is the natural habitat of the horse. In the series of ancestors of the horse we can trace every step in the evolution of those marked peculiarities of teeth and feet which distinguish the modern animal from an ancestor which so little suggests a horse that when its remains were first found forty years ago, it was named by the great paleontologist Richard Owen, the Hyracotherium or ‘‘ coney-like beast.’ Its relation to the horse was not at that time suspected by Professor Owen, and was recognized by scientific men only when several of the intermediate stages between it and its modern descendant had been discovered. On the other hand this first ancestor of the horse line is very difficult to distinguish from the contemporary ancestors of tapirs and rhinoceroses, and indicates how all the modern quadrupeds have diverged from a single type, each becoming adapted to the needs of its especial mode of life.
FOUR-TOED HORSES HE earliest known ancestors of the horse were small animals not Ay larger than the domestic cat, with four complete toes on each fore foot and three on each hind foot. There is reason to believe that still more ancient ancestors of this and all other mammals had five toes on each foot. In the fore foot of the earliest known stage there may have been a small, slender rudiment representing the missing first digit or thumb, which no longer appeared on the surface of the foot,' while in the hind foot there is a similar rudiment representing the outer or fifth digit, ‘ItisshowninMarsh'swellknowndiagram;butthereisnoconclusiveevidenceofitspresence on any specimen yet found.