The Cambridge Natural History Part 18

[Ill.u.s.tration]

FIG. 123.--Anterior aspect of right femur of Rhinoceros (_Rhinoceros indicus_). . _h_, Head; _t_, great trochanter; _t_', third trochanter.

(From Flower's _Osteology_.)

The living Perissodactyles belong to three types only, indeed to three genera only (in the estimation of most), which are the Horses, Tapirs, and Rhinoceroses. But taking into account the extinct forms, they may be divided primarily (according to Professor Osborn) into the four following groups:--(1) t.i.tanotherioidea, including but one family, t.i.tanotheriidae; (2) Hippoidea, including the families Equidae and Palaeotheriidae; (3) Tapiroidea, with two families, Tapiridae and Lophiodontidae; and (4) Rhinocerotoidea with families Hyracodontidae, Amynodontidae, and Rhinocerotidae. It is conceivable, according to the same writer, that the Chalicotheres (here treated of as a separate sub-order, Ancylopoda) should be added to the Perissodactyle series.

[Ill.u.s.tration]

FIG. 124.--Side view of skull of Horse with the bone removed so as to expose the whole of the teeth. _c_, Canine; _Fr_, frontal; _i_^1, _i_^2, _i_^3, incisors; _L_, lachrymal; _m_^1, _m_^2, _m_^3, molars; _Ma_, malar or jugal; _Mx_, maxilla; _Na_, nasal; _oc_, occipital condyle; _Pa_, parietal; _pm_^1, situation of the vestigial first premolar, which has been lost in the lower, but is present in the upper jaw; _pm_^2, _pm_^3, _pm_^4, remaining premolars; _PMx_, premaxilla; _pp_, paroccipital process; _Sq_, squamosal. (After Flower and Lydekker.)

FAM. 1. EQUIDAE.--This family, which includes the living Horse, Zebras, and a.s.ses, as well as a number of extinct genera agreeing with those types in structure, may be defined by the possession of but one functional toe, the two lateral ones being mere splints, or but little more. The molar teeth are hypselodont, and {238} the premolars, with the exception of the first, resemble the molars in their pattern. The orbit is completely surrounded by bone. The incisors are chisel-shaped, with a pit on the free surface. The canines are rudimentary if present. The radius and ulna are fused, as are the tibia and fibula. Although for the sake of uniformity a family, Equidae, is here separated from its allies, it is quite impossible owing to the full state of our knowledge of this group to draw a really hard-and-fast line between this family and the Palaeotheriidae. We shall deal presently with the conjectured pedigree of the Horse, which naturally involves that family, and which presents an unbroken series from four-toed Perissodactyles to the present one-toed Horse, the various bones and teeth becoming modified in the course of the descent "with the regularity of clockwork." We are compelled to draw the line at functional second and third toes; directly these are no longer used the animal is a Horse in the strict sense! This is irrational and regrettable, but necessary for practical purposes, if {239} we are to continue the plan of defining the various families of Mammalia.

The genus _Equus_[150] contains not only the Horse, but the a.s.ses and Zebras. The genus is to be distinguished as regards external characters by the following features:--The body is thickly clothed with hair; there is a more or less bushy tail and mane; the colours are apt to be disposed in stripes of black or blackish upon a yellowish brown ground; this is of course best seen in the Zebras, but the wild a.s.ses also have some traces of it, if only in the single cross-bar of the African Wild a.s.s, and it is even "reversionary" in the domestic Horse at times. There are no horns upon the forehead or elsewhere; the fore-limbs or both pairs have a callous pad upon the inside, which is possibly to be looked upon as an aborted gland, possibly originally of use as secreting some odorous substance calculated to enable strayed members of the herd to regain their companions. The terminal phalanx of each of the (functionally) single digits is enclosed in a large h.o.r.n.y hoof.

The main internal features of structure which divide this genus of Perissodactyles from the Rhinoceros or the Tapir, or from both, are: the existence of strong incisors, three on each side of each jaw; there are canines, but these are small and do not always persist in the full-grown mare. They are popularly known as "tusks" or "tushes." The first of the four premolars (the "wolf tooth") is small and quite rudimentary; it is often absent. As there are three molars, the present genus has the "typical" number of the Eutherian dent.i.tion, _i.e._ forty-four. In the skull the orbit is--as it is not in Tapirs and Rhinoceroses--completely encircled by bone. There is but one functional finger and toe on each hand (Fig. 121 C) and foot; the second and third digits are represented by mere splints, one of which may as an abnormality be enlarged, and reach nearly as far as the well-developed digit. There are even occasionally traces of digit number two.

The Horse, _E. caballus_, is to be distinguished from its congeners by the small callosities on the hind-limbs which it possesses in addition to the larger ones on the fore-limbs. The hairy covering of the tail is more abundant, as is also the mane. The head too is proportionately smaller, and the general contour {240} more graceful. Though Zebra markings are not usual upon _E. caballus_, there are plenty of examples of--what we may perhaps in this case term--a "reversion" to a striped state. The celebrated "Lord Morton's mare,"[151] whose portrait hangs in the Royal College of Surgeons, is an interesting case of this. It was as a matter of fact thought to be an example of that rather doubtfully-occurring phenomenon, "telegony." Its history is briefly this. The animal was the offspring of a mare that had previously produced to a male Quagga a hybrid foal.

Afterwards a second foal was produced by the same mare to an Arab sire.

This foal, the one in question, was striped, and hence was thought to be an example of male prepotency. But instances are known of unquestioned Horses which show the same stripes, such as a Norway pony which had not even _seen_ a Zebra!

A last remnant of the naked palm of the hand and sole of the foot is left in the shape of a small bare area, smaller in the Horse than in the a.s.ses, known technically as the "ergot," the term being that of the French veterinarians. As already mentioned, the Horse differs from the a.s.ses and Zebras in the fact that the hind-limbs have callosities on the inner side.

They are known as "chestnuts," and their nature has been much disputed. It has been suggested that they are the last rudiment of a vanished toe; but in all probability they are, as already suggested, traces of glandular structures, which are common, upon the limbs in many animals (see above, p.

12).

It is a singular fact that there are apparently no wild Horses of this species. The case is curiously a.n.a.logous to that of the Camel, which also is only known as feral or domesticated. Why the Horse should have become extinct as a wild animal, considering that when it does run wild it can thrive abundantly, is impossible to understand. Sir W. Flower thinks[152]

that "the nearest approach to truly wild horses existing at present are the so-called Tarpans, which occur in the Steppe country north of the sea of Azov between the river Dnieper and the Caspian. They are described as being of small size, dun colour, with short mane and rounded obtuse nose." But he adds that there is no evidence to prove whether they are really wild. In favour, however, of their possibly being wild and indigenous European Horses, may be {241} mentioned the fact that their general build and appearance is highly suggestive of the wild Horses sketched by primitive man upon ivory.

A really wild Horse, and possibly the ancestor of the European domestic Horse, is _E. przewalskii_ of the sandy deserts of Central Asia. This animal has been believed to be a mule between the Wild a.s.s and a feral Horse; but if a distinct form, and probability seems to urge that view, it is interesting as breaking down the distinctions between Horses and a.s.ses.

The species possesses the four callosities of the Horse, but has a poorer mane and an asinine tail.

There is no question that the Horse has been a domestic animal for very many centuries. Hieroglyphics appear to show that the Egyptians had not originally domesticated the Horse; it seems to have been first introduced among them by the Hyksos or Shepherd Kings.[153] Whatever the date may be, it is certain that considerably anterior to the Egyptians the a.s.syrians and Phoenicians possessed Horses. In Western Europe the date of the introduction of the Horse seems to have been during the bronze epoch. Lord Avebury[154] has pointed out that out of eighteen cases of graves in which the remains of Horse were found, twelve contained metal implements, _i.e._ 66 per cent. This does not of course prove that the Horse was domesticated at that period, but it throws doubt upon the earlier occurrence of the Horse in abundance. The Horse, however, does occur on the Continent a.s.sociated with the remains of man during the Quaternary period.[155]

Messrs. Cuyer and Alix enumerate between fifty and sixty domesticated races of Horse, not counting the supposed wild varieties which have been already referred to. These may be further subdivided; for instance, under the race "pony" we may distinguish the Irish, Scotch, and Shetland varieties, all of which, however, according to Sanson, have originated in Ireland. They are used, remark the authors above quoted, "par les jeunes filles des lords pour leurs promenades." The Arab, the Barb, the Suffolk Punch, etc., are among the numerous races of domestic Horses, into which to enter properly would require another volume, and that of large size. {242}

The a.s.ses and Zebras differ from the Horse in the characters mentioned under the description of _Equus caballus_. In addition to these may be pointed out a feature to which attention has been directed by Mr.

Tegetmeier.[156] According to him the period of gestation in the Horse is only eleven months; in the others more than twelve.

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FIG. 125.--Asiatic Wild a.s.s. _Equus onager._ 1/20.

Opinions as to the number of species of a.s.ses differ. On the most liberal estimate there are three Asiatic and two African species. The best known of the Asiatic Wild a.s.ses is the Onager, _E. onager_. It is of a uniform yellowish, "desert" colour, with a dark stripe along the middle of the back, and is found in Persia, the Punjab, and the country of Cutch. The creature is of great swiftness; it has been stated to be untameable, but Mr. Tegetmeier makes the absolutely opposite statement that the a.s.s occasionally "becomes so tame as to be troublesome"! The Syrian Wild a.s.s, _E. hemippus_, hardly, if at all, differs from this.

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FIG. 126.--Nubian Wild a.s.s. _Equus africa.n.u.s._ 1/20.

The Kiang, _E. hemionus_, seems to have more claims to distinctness. In the first place it has a more limited and a {243} different distribution; it is confined to the high tablelands of Thibet at an elevation of 15,000 feet and upwards. In correlation with this habitat it has a thicker and more "furry" coat, which is, moreover, of a darker shade than that of the Onager. This coat is shed in the summer, and replaced by one which is not so dark in hue. It is an interesting fact that the African Wild a.s.ses approach to the zebra type in having at least traces of stripings. There are apparently two species. The best known, the Nubian a.s.s, _E. africa.n.u.s_, is probably the parent of the domestic donkey. It has a dorsal longitudinal stripe, and another across the shoulder--in legend the marks of the Saviour. The matter of the name of this a.s.s seems difficult to decide. It has been called also _E. asinus_ and _E. taeniopus_. It has been observed that this animal has a great aversion to water, and a delight in rolling in the dust--both of which characteristics argue a desert existence. But on the other hand the Kiang will plunge boldly into streams, yet it would seem to be the descendant of a purely desert form. The a.s.s is a longer-lived {244} animal than the Horse. Mr. Tegetmeier calls attention to a donkey living in 1893 which had been ridden fifty-five years previously. The Horse, on the other hand, lives not much more than twenty-five years.

A second species of African Wild a.s.s, _E. somalicus_,[157] is distinguished by its greyer colour, by the absence of the shoulder stripe, by the very faint development of the dorsal stripe, and by the presence of numerous cross stripes upon the legs. It has, too, smaller ears, and a longer and more flowing mane. Mr. Lort Phillips, an experienced naturalist and traveller, saw a herd of these Wild a.s.ses in Somaliland, which he regarded as being of quite a new species. A living example in the Zoological Society's Gardens led Mr. Sclater to an identical conclusion, which was supported, as he pointed out, by the fact that this a.s.s has a different range to the African or Nubian Wild a.s.s.

Of the Zebras three species are usually allowed; these are _E. zebra_, the "Mountain" or "Common" Zebra, _E. burch.e.l.li_, _E. grevyi_, as well as _E.

quagga_. Professor Ewart thinks that the Common Zebra, Burch.e.l.l's, and the Quagga are not very distinctly marked off from each other. No one, however, has any doubt of the distinctness of _E. grevyi_. This latter differs from the rest in its larger size, in the large head and ears, and in the marked hairiness of the ears. It would seem to be a primitive type of Zebra, if the fact that the occasional reversion of hybrids to a parent form be allowed; for Professor Ewart found a cross-bred Zebra to present several characteristics in the face-marking of this, the finest of the Zebra tribe.

Only four specimens of _E. grevyi_ have been exhibited alive in Europe--two in Paris, and two in the Zoological Society's Gardens in London. The latter were presented to Queen Victoria by King Menelek of Abyssinia. The species was named by Professor A. Milne-Edwards in honour of a late President of the French Republic, from an example also sent by King Menelek.

The Common Zebra has closer and darker stripes than Burch.e.l.l's, but not quite so close as in _E. grevyi_. It has also a very characteristic arrangement of stripes on the withers in the form of a gridiron. This latter is wanting in both the other species. In _E. grevyi_, in fact, this part of the back is white. _E. zebra_ has also a dewlap in front. _E.

burch.e.l.li_ has fewer and broader {245} stripes, and between them lie in many cases shadow-stripes of a faint brown.

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FIG. 127.--Burch.e.l.l's Zebra. _Equus burch.e.l.li._ 1/20.

All these animals, and the Quagga too, are absolutely confined to Africa.

Mr. R Crawshay,[158] in describing what he considered to be a new variety, remarked upon the curiosity of _E. burch.e.l.li_. "They remain out in the sun on the plains all day long, not retiring into covert at all. They are then an intolerable nuisance to any one in pursuit of other game; indeed this may be said of them at all times. If once they notice you, they draw in and mob you in their curiosity--only, however, when one takes no interest in them, for when they fancy they are the object of the intruder's attention, no animals are more watchful and cunning in safeguarding themselves. If only their curiosity were manifested in silence it would not so much matter, but it vents itself in snorts and thundering stampedes, which puts every beast within earshot on the _qui vive_."

Whether Burch.e.l.l's Zebra[159] can be further subdivided into species or sub-species appears to be doubtful. Dr. Matschie considers that _Equus boehmi_ may be regarded as a valid form, and in addition to this two sub-species, _E. burch.e.l.li granti_ and {246} _E. burch.e.l.li selousi_, have been proposed for what are at most local races. But it is at present far from certain whether their distribution favours this subdivision.

The Quagga was more striped than is sometimes represented in ill.u.s.trations.

According to Dr. Noack, from whose paper[160] upon the animal I quote here, the transverse stripes reached back as far as the b.u.t.tocks; they were, however, completely absent from the legs. The animal is, as every one knows, probably completely extinct. In the year 1836 it was still abundant; in 1864 the last specimen ever exhibited was received by the Zoological Society. Mr. W. L. Sclater thinks that it may have survived in the Orange River Colony as late as 1878, but admits that any certainty is difficult, as it was frequently confounded by the Boers with Burch.e.l.l's Zebra. Its rarity is emphasised by the fact that it is not mentioned in the recent work of that most skilful of hunters, Mr. F. Selous. Gaudry places the Quagga nearest of all living Equidae to the _Hipparion gracile_ of Pikermi.

FOSSIL EQUIDAE.--The existing Equidae all belong to the genus _Equus_, though there are some who would (quite unnecessarily) divide off the Zebras as a genus _Hippotigris_. The genus _Equus_ itself goes back in time to the Pliocene, during which epoch there lived in India _E. sivalensis_, the same species according to some with the _E. stenonis_ of Europe. None of these species, Old World or New, are easily to be separated from _E. caballus_.

But many names have been given to them. It is of course perfectly conceivable that they may have differed among themselves as much as do the existing Zebras and a.s.ses, the separation of which would be hardly possible did we know their bones only. There are, however, extinct genera, undoubtedly related so closely to _Equus_ as to be placed in the same family, though clearly separable as genera. _Hipparion_ is one of these genera; its remains are known from Europe, Asia, and North Africa, from beds of Miocene and Pliocene times. A large number of different species have been described. It was a beast of about the size of a Zebra. The princ.i.p.al characters are that each foot has three toes, of which, however, the two side ones are smaller than the central toe. There is a marked round fossa on the maxillary bone, a feature shared by the South American _Onohippidium_.[161] The pattern of {247} the molar teeth is, too, a little different from that of _Equus_. _Protohippus_ of the North American Pliocene is also three-toed, but the two additionally-developed toes are smaller than in _Hipparion_. Other forms are dealt with below in connexion with the ancestry of Perissodactyles. It is a curious fact about _Hipparion_, which is not now regarded as on the direct line of equine descent, that the edges of the enamel plates of the molars may show a complicated folding very like that presented by that clearly terminal form of Perissodactyle life, the gigantic _Elasmotherium_. This is indicative of high specialisation, which ended in extinction.

ANCESTRY OF THE HORSES.--The LOPHIODONTIDAE and the PALAEOTHERIIDAE are two of the most interesting extinct families of Perissodactyles; for among them we find what would appear to be the ancestral forms of both the existing Tapirs and Horses. The Rhinoceroses also would seem to be derivable from the Palaeotheriidae. The very vagueness of the characters of these creatures, considered from a cla.s.sificatory point of view, has led to much diversity in their placing. This though gratifying to the evolutionist is tiresome to the writer who wishes to give a methodical account of their various characters. It will be best perhaps not to attempt an accurate placing or to reconcile conflicting opinion, but to give some salient features of osteology which lead to the belief in their relations.h.i.+p to existing groups of Perissodactyles. A book upon the history of mammals would be incomplete without some account of that well-ascertained series of forms which seem to connect these primitive Perissodactyles with the modern Horse. _Equus_, in fact, is not only the "show horse" of the doctrine of evolution, but also the "stalking horse."

In the Eocene of both Europe and America are met with a number of forms from which we may start. _Hyracotherium_, regarded on the one hand as the type of a sub-family of the Equidae themselves, and on the other as a member of the family Lophiodontidae, was a small-sized animal, three feet or so in length; it possesses the complete Eutherian dent.i.tion with a slight diastema. The orbits are not separated from the temporal fossa; the fore-limbs were four-toed, the hind three-toed, with moderately long metapodia, especially on the hind-feet. The shoulder blade {248} has a well-marked coracoid process. The radius and ulna are separate; so too are the tibia and fibula. _Eohippus_, belonging to the same sub-family, is slightly more primitive; for the hind-feet have a rudiment of digit I.

_Orohippus_ is a little nearer to the Horses in that the molar teeth have acquired a little further advance towards the equine type. Instead of the tubercles of the teeth remaining for the most part separate, they have fused into a set of ridges, of which, however, the pattern is less complex than in the modern Horses. In other respects _Orohippus_ is much like _Hyracotherium_. _Pachynolophus_ seems to be but a synonym.

The next stage is shown by _Mesohippus_, a Lower Miocene form, usually referred to the neighbourhood of _Palaeotherium_. It has nearly lost one of the toes of the fore-foot, a rudiment only remaining; the metapodials, at any rate of the fore-feet, seem to be slightly increased in length. The orbit is not encircled by bone, but there is a strong process from the frontal, which nearly meets the zygomatic arch.

_Anchitherium_, from the Upper Miocene, is not far removed in structure from the last-mentioned form; it is a trifle nearer the existing Horse in several points. The ulna is further reduced and fused with the radius below: the rudiment of digit V is still more rudimentary; the two lateral digits are smaller in proportion to the central one than they are in _Mesohippus_; the fibula is fused below with the tibia. From this form to _Equus_ is a small series of steps, characterised by the still further reduction of all the digits except III, by the still further reduction of the already rudimentary ulna and fibula, and by the increasing depth of the molar teeth, which are of course, in _Equus_, hypselodont.

Another interesting conclusion may seem to follow when we consider the geographical range of the ancestral Horses. _Hyracotherium_ and _Pachynolophus_ occurred both in the Old and New World. From them may have arisen the Horses of both hemispheres. After that point there is a division. _Mesohippus_ is American, and we get at _Equus_ in that continent through _Desmatippus_ and _Protohippus_. On the other hand there are no remains known of _Mesohippus_ in Europe; and unless subsequent researches prove the existence of _Mesohippus_, we have to rely upon forms which are placed with _Anchitherium_ and _Hipparion_.

It seems that in America the next genus in the direct line of equine descent to _Mesohippus_ is _Miohippus_. It is smaller in {249} size than _Anchitherium_, to be considered immediately. The odontoid process of the axis is just beginning to a.s.sume the characteristic spout-like shape of that of the existing Horse and many modern Ungulates. The median digit of both fore- and hind-limbs has become greatly enlarged as compared with the corresponding digit of earlier forms.

It is held, however, that _Anchitherium_ is not on the direct line of descent either in America or in Europe, in both of which it occurs. Its teeth are in some respects less Horse-like than in some of the more ancient genera, to which the converse would be expected on the descent theory. Its hoofs are much elongated and flattened, a mark of specialisation and not appropriate to a creature holding an intermediate position in the equine series. Both the American (_A. equinum_) and the European species (_A.

aureliense_) are of very large size, larger than its successors, and such "alternations in bulk are unlikely."

The genus _Desmatippus_ of Professor Scott[162] fills in the gap between _Miohippus_ and _Protohippus_. The molars and premolars are brachyodont, but there is a thin deposit of cement in the tooth valleys, leading towards the more complete filling of these valleys with cement, which is found in _Protohippus_. This genus of Horses, of which there is at present but one species, _D. crenidens_, was three-toed, and "the lateral digits, so far as can be judged by fragmentary remains, were still fairly developed, and though much more reduced than in _Miohippus_, appear to be somewhat less so than in _Protohippus_."

To recapitulate, the following is the probable series of equines in America--_Mesohippus_, _Miohippus_, _Desmatippus_, _Protohippus_.

The development of the limbs of the Horse shows a most interesting series, of stages, which correspond in part to the ancestral forms which palaeontology seems to prove to be the line of the descent of our existing Equidae. This matter has recently been elucidated by Professor Ewart, who details the following facts and comparisons:--

In the youngest embryo (about 20 mm. in length) the humerus is somewhat curved, and considerably longer than the radius and carpus taken together.

The first-named bone is shorter in the adult, and the proportions of that bone in the young as well as its curvature are suggestive of that ancient {250} Ungulate _Phenacodus_ (see p. 202). In the next stage (an embryo of 25 mm.) the humerus has slightly decreased in proportionate length, and has come to be more like that of _Hipparion_. In both of these embryos it should be noted that the ulna is complete and separate from the radius. In the second of the two it has more distinctly acquired the form which it will possess in the adult. The second metacarpal--one of the splint bones of the adult--is tipped with a small nodule of cartilage, which is clearly the representative of one or more of the phalanges belonging to that digit.