The Cambridge Natural History Part 16

The history of the discovery of the members of this order is very instructive as ill.u.s.trating the dangers of laying too much cla.s.sificatory importance upon detached fragments of animals. So long ago as 1825 terminal phalanges of a new creature were found in the Miocene of Eppelsheim, and sent to Cuvier. Cuvier named them "Pangolin gigantesque," deeming them, on account of their general form and cleft terminations, to pertain to the Edentata. In the same bed some seven years later were found certain teeth clearly of an Ungulate character, to which the generic name of _Chalicotherium_ was applied. It was subsequently discovered that the teeth and the claws belonged to the same animal, and, later, further remains turned up which disclosed a creature having the anomalous composition of an Ungulate with decisively Ungulate teeth, but with the feet to a large extent like those of an unguiculate animal. The same confusion of characters occurs also, it will be remembered, in the distinctly Artiodactyle _Agrioch.o.e.rus_ (see p. 331). Indeed the feet of the latter when first discovered were erroneously, as it now appears, referred to the present order of Ungulates under the name of _Artionyx_. It is probable that the genus _Moropus_ of North America is a member of this group, and that it is probably congeneric with a somewhat different type of Ancylopod known as _Macrotherium_. It is also clear that _Anisodon_, _Schizotherium_, and _Ancylotherium_, if not congeneric with either of the two recognised genera, are at least very close to them.

_Chalicotherium_ has a skull which recalls that of some of the earlier Ungulates; it has, however, no incisors at all, and no canines in the upper jaw; this feature has led to the belief that the animal is related to the Edentata, and that it is in fact a link between them and the Ungulata. The molars, like those of the Perissodactyla, are of the buno-selenodont type.

It also agrees with that group (to which it has been approximated by several writers) in the. tridactyl ma.n.u.s and pes, and in the characters of the tarsus. But although tridactyl, the axis of the limb pa.s.ses through the fourth digit. _Chalicotherium_ is not mesaxonic, as are the Perissodactyles. Moreover, it has no third {212} trochanter, and the unguiculate claws have already been referred to. As to the latter, which are short, it is not the end phalanx but the first which is retracted; thus _Chalicotherium_ differs markedly from both Carnivorous and Edentate types; for in the former it is the last phalanx which is retracted, while in the Edentates the same phalanx is flexed downwards. The limbs of _Chalicotherium_ are nearly of the same size, and the animal seems to have been stout and quadrupedal.[132]

_Macrotherium_, like the last genus, seems to have been common to both New and Old Worlds. It is to be distinguished by a number of characters. It is supposed to have been "semi-arboreal and fossorial"; the fore-limbs are much longer than the hind, the relative proportions of the radius and tibia being 70 to 29. The ulna was distinct from the radius, whereas in _Chalicotherium_ the two are coalesced, or nearly so. Young specimens appear to possess a full set of incisors; whether this is the case or not with _Chalicotherium_ is not known.[133]

_Homalodontotherium_ is sometimes placed in the group.

SUB-ORDER 4. TYPOTHERIA.

It is a little difficult to be confident that the Typotheria are rightly referred to the Ungulata, since they contradict two important Ungulate rules. They have clavicles, which are elsewhere missing, and the thumb looks as if it were opposable.[134] An Ungulate is essentially a running animal, and has no need of a grasping finger. Still Typotheria are placed by most within the Ungulate series, though their undoubted likenesses to other groups, especially to the Rodentia, are admitted, and indeed emphasised. Cope places them definitely with the Toxodonts.

The Typotheria are an extinct group of smallish beasts, confined, like the Toxodontia, to South America, a region which during the Tertiary period, and into the Pleistocene, abounded with strange and varied types of Ungulate animals.

The earlier forms of Typotheria may be exemplified by some {213} account of the genus _Protypotherium_. This animal was of about the size of a _Hyrax_, which indeed it resembles in several points of structure. The teeth have the primitive number of forty-four, and they are close set, leaving no diastema; the molars are rootless and grow persistently; they are simple and Rodent-like in surface pattern. The shape of the lower jaw is like that of _Hyrax_, being rounded in outline posteriorly; there is no projecting angle as in the Rodents, and this remark applies to the Typotheria in general. The aspect of the Rodent lower jaw is characteristically different from that of _Hyrax_ and the forms under consideration.

Some other characters of these early forms of Typotheria can be gathered from an inspection of other genera. In _Icochilus_ both hand and foot were five-toed, and, as in ancient Ungulates generally, the bones of the wrist and of the ankle are serially and not alternately arranged. Moreover, an os centrale is present in the carpus. Both thumb and big toe were opposable.

The skull has a remarkably Rodent-like appearance, but the palate is not so narrowed as in these animals.

In the more recent forms of Typotheria the dent.i.tion has become reduced.

The canines are lost, and as the incisors are reduced also, to one on each side of the upper, and two on each side of the lower jaw, the likeness to a Rodent skull is increased. There is also evidence of a modification from the more primitive forms in the loss of one premolar or even more, in the alternating bones of the carpus, in the disappearance of the centrale, and in the loss of a toe upon the hind-foot. In these more recent forms the fibula articulates with the astragalus instead of with the calcaneum.

Typotheria of these more recent forms may be ill.u.s.trated by the typical genus _Typotherium_. It has the reduced dental formula I 1/2 C 0/0 Pm 2/1 M 3/3; the molars are simple in pattern, and much like those of _Toxodon_.

The upper incisors are powerful and curved, but are surrounded by a layer of enamel, which is not limited to the anterior face, as it practically is in Rodents. The sacrum is composed of a large number of vertebrae--some seven--a state of affairs which recalls the Edentata. The shoulder blade is not Ungulate in form. It has a strong spine, with an acromion and a well-developed metacromion. The terminal phalanges are enlarged and hoof-like.

In the genus _Pachyrucos_ there are three premolars, otherwise {214} the formula is the same as in _Typotherium_. The animal seems to have had nails rather than hoofs. The thumb was opposable. The fibula is fused below with the tibia, whereas in the last genus these two bones are quite separate from each other.

SUB-ORDER 5. TOXODONTIA

The group Toxodontia,[135] like so many others, is exceedingly hard to define. Nor are its limits any easier to mark out than many others of the groups of Ungulates. It will be best perhaps to give an account of _Toxodon_, and of a few types which seem to lie near it in the system, and then to indicate how far they resemble or depart in structure from other Ungulates. _Toxodon_ itself is known from complete skeletons. It lived in Argentina during the "Pampean" period, which seems to be of the Pleistocene age. A large number of species, however, have been described, some of which seem to go farther back in time, and to have existed during the Miocene period further south in Patagonia.

The size of this creature was about that of a large Rhinoceros; it has a bulky body and a large head, which was borne low down, on account of the bending downwards of the anterior vertebrae; in this aspect the figures of the skeletons recall _Glyptodon_ and similar Edentates. The beast was discovered by Darwin, and originally described by Owen. "During his (Mr.

Darwin's) sojourn in Banda Oriental," writes the Rev. H. Hutchinson, "having heard of some 'giants' bones' at a farmhouse on the Sarandis, a small stream entering the Rio Negro, he rode there, and purchased for the sum of eighteenpence the skull which has been described by Sir R. Owen. The people at the farm-house told Mr. Darwin that the remains were exposed by a flood having washed down part of a bank of earth. When found, the head was quite perfect, but the boys knocked the teeth out with stones, and then set up the head as a mark to throw at." The whole of the Pampean area is a valley of dry bones, and the remains of _Toxodon_ are abundant there. The skull of _Toxodon_ is not unlike that of a horse in general aspect; but the orbit is not separated from the temporal fossa. The premaxillae are furnished above with a slight protuberance directed towards {215} the free end of the nasals, which may be related to the presence of a short proboscis. The zygomatic arch is strong and broad: the mandibles are provided with a long symphysis. The dental formula is I 2/3 C (0-1)/1 Pm 4/(3-4) M 3/3. The teeth are prismatic and hypselodont, growing from persistent pulps. The molar teeth are slightly arched in form, whence the name of _Toxodon_, "bow teeth." The strong chisel-shaped incisors suggest the Rodents and _Hyrax_. The cheek teeth, moreover, are by no means unlike those of Rodents in their pattern. They are at any rate not at all like those of existing Ungulates. The small size of the canine and of the first premolar produces a diastema in the tooth series. The sacrum consists of five vertebrae, and the ischium does not articulate with it.

The shoulder blade has a strong spine, but only a rudimentary acromion; nor is the coracoid well developed. The radius crosses the ulna, as in the Elephant; the whole fore-limb is shorter than the hind-limb, which must have exaggerated the hang-dog expression of the creature when alive. The elements of the carpus interlock in the modern fashion. Those of the tarsus, however, are primitive in lying below each other without alternation. The carpus has a centrale. The fibula articulates with the calcaneum. The femur has no third trochanter. There are three toes to all the limbs. It is clear that this a.s.semblage of characters will not allow the placing of _Toxodon_ in any living Ungulate order. If the middle toes appear by their slight pre-eminence to approach the Perissodactyle form, the peculiar surface contour of the molar teeth, letting alone the absence of a third trochanter on the femur, will not permit this cla.s.sification.

Allied to _Toxodon_ is the genus _Nesodon_. It was so named from an "island lobe" on the inner side of the upper molars. This creature, smaller than _Toxodon_, also differs from it in the fact that the dent.i.tion is complete, and in the pattern of the molars, which is rather more complex. There is still the slight projection upon the premaxillary bones, but the nostril is directed more forwards than in _Toxodon_. The zygoma, too, is ma.s.sive. The second pair of incisors in the upper jaw and the outer (third) pair in the lower jaw form biggish tusks in the adult. These and the molar teeth are, however, finally rooted, and do not grow, as in _Toxodon_, from persistent pulps. The genus is from the older Tertiary of Patagonia. Five or six species have been described. Some are as large as a Rhinoceros, others as small as a Sheep. {216}

There is no doubt about the close alliance of the two genera just referred to. It is more doubtful whether _Homalodontotherium_ and its allies should be placed, as they often are, in the neighbourhood of the Toxodonts.

_Homalodontotherium_ owes its name to its even row of teeth without a diastema. It was a creature of equally large size with _Toxodon_, and also came from the Tertiary strata of Patagonia. The teeth are the typical forty-four, and the molars like those of a Rhinoceros; they are, however, brachyodont and not hypselodont as in _Toxodon_. This genus, however, shows an important difference from the Rhinocerotidae and from the other Toxodontia in the fact that it was five-toed, and that the bones of the carpus and tarsus are set in relation to each other in the linear serial fashion.

Undoubtedly a near relative of _Homalodontotherium_ is _Astrapotherium_.

This creature was of equal bulk, and was also Patagonian in range. The teeth are reduced in number, but the animal was provided, like a Wild Boar, with great tusks, which were, however, formed by the incisors. This animal is very imperfectly known; it is the form of the molars and the large size of the incisors which have led to its a.s.sociation with the Toxodontia. As to the resemblance of the teeth of this genus and of _Homalodontotherium_ to those of _Rhinoceros_, it is difficult to regard it as evidence of near affinity. The likeness is probably to be looked upon as a case of parallelism in development. Exactly the same explanation is possibly to be given to the likeness which the teeth of _Toxodon_ and _Nesodon_ show to Rodents, or even to Edentates. As to their affinities Zittel observes:--

"The entirety of their osteological characters argues for the Toxodon a separate position in the neighbourhood of the Perissodactyla, Proboscidea, Typotheria, and Hyracoidea. The relations to the Rodentia rest mainly upon the converging development of the teeth, not upon true relationship."

SUB-ORDER 6. PROBOSCIDEA.

Large vegetable-feeding animals, usually scantily covered with hair, and with the nostrils and upper lip drawn out into a long proboscis. Digits five on both limbs. Femur and humerus not bent upon lower leg and fore-arm in a position of rest. Skull {217} with abundant air cavities in the roofing and other bones. The incisors are developed into long tusks, which exist in the upper jaw alone, in the lower jaw alone, or in both jaws.

There are no canines. The molars are lophodont. The clavicle is absent. The femur has no third trochanter. The bones of the carpus are serially arranged and do not interlock. The stomach is simple. The brain has much convoluted cerebral hemispheres, but the cerebellum is completely uncovered by them. The intestine is provided with a wide caec.u.m. The testes are abdominal. The teats are pectoral in position. The placenta is non-deciduate and zonary. There are two venae cavae superiores.

The position of the limbs in the Elephant tribe is unique among living animals: their straightness that is to say, and the absence or very slight development of angulation at the joints of the limb bones. This same feature has been observed in the extinct Dinocerata and in the t.i.tanotheria. It must not, however, be a.s.sumed from the resemblance to these ancient forms that there is much affinity between them and the Proboscidea, or that the latter have retained an ancient feature of organisation. The oldest Ungulates for the most part, and the Creodonts to which they are undoubtedly related, have much bent limbs. It must be considered, therefore, that the arrangement obtaining in the Elephants is purely secondary. Professor Osborn has put forward the reasonable view[136]

that the vertical limbs of all these colossal creatures are due to "an adaptation designed to transmit the increasing weight" of these animals.

The huge bulk of the body is better borne by vertical pillars than by an angulated limb. Other points, however, such as the exposure of the cerebellum, the two venae cavae, the five digits, and the absence of a third trochanter, argue a low position for the Proboscidea in the Eutherian group.

The group can be readily divided into two families, the Elephantidae and the Dinotheriidae. We will commence with the former.

The Elephants proper, ELEPHANTIDAE, differ from the Dinotheriidae in, and are characterised by, a number of anatomical features. They possess long tusks (incisors) either in both jaws, or, if only in one jaw, in the upper.

The molar teeth are very large--so large that only a few of them are simultaneously in use. There are but three definable genera of Elephantidae, of which {218} _Elephas_ alone survives. This genus also includes many extinct forms, both American and European, as well as Asiatic and African. The entirely extinct genera are _SteG.o.don_ and _Mastodon_. The group is clearly one dwindling towards extermination. From the Middle Miocene downwards these great "pachyderms" have existed; and from the Miocene up to Pleistocene times they were almost world-wide in range and numerous in species.

The genus _Elephas_ comprises usually large, but occasionally (the pygmy Elephant of Malta) quite small forms. The external features of the genus differ slightly in different species, and will therefore be described in relation to those species which we shall notice here. The vertebral formula is C 7, D 19-20, L 3-5, Sa 4-5, Ca 24-30, or even more.

The bodies of the vertebrae are remarkable for their shortness and for the very flattened articular surfaces.

[Ill.u.s.tration]

FIG. 116.--A section of the cranium of a full-grown African Elephant, taken to the left of the middle line, and including the vomer (_Vo_) and the mesethmoid (_ME_); _an_, anterior, and _pn_, posterior narial aperture.

1/12. (From Flower's _Osteology_.)

The skull is large and ma.s.sive. Its large and heavy character is, as has been stated in the definition of the sub-order, due to the immense development of air cavities in the diploe; the diameter of the wall of the skull is actually greater than that of the cranial cavity. These cavities are not obvious in the young animal. They are most conspicuous in the roofing bones of the skull, but are seen elsewhere, and thicken the basis cranii, {219} the maxillae, and so forth. This state of affairs, together with the presence of the huge tusks, has, as it were, pushed back the nasal orifices to near the top of the skull in a very Whale-like fashion. As in the Cetacea, the nasal bones are limited in size, and the premaxillae send up processes to join the frontals and the nasals. There is a straight and somewhat slender zygomatic arch, but the orbit is not separated from the temporal fossa. The malar bone is small, and, as in Rodents, forms the middle part of the zygoma. This is not the case with most Ungulata. The symphysis of the mandibles forms a spout-like rim. The scapula has a narrow prescapular, but a very wide postscapular region. The spine has a strong process projecting backwards from near its middle; this is a point of likeness to certain Rodents. No Elephant has a clavicle. The most remarkable feature about the fore-limb is the separation and crossing of the radius and ulna. The arms of these animals are permanently fixed in the position of p.r.o.nation. The foot is short, and the bones of the carpus are serially arranged. There are, however, traces of a commencing interlocking of these bones in many forms. The hind-feet are somewhat smaller than the fore-feet, and the tibia and fibula are both developed.

As to the teeth, this genus is to be distinguished from allied forms by the presence of tusks in the upper jaw only. These tusks have no bands of enamel such as characterise those of _Mastodon_. They are incisors. There is, however, a trace of the former enamelling in the shape of a patch at the tip, which soon wears away. The molar teeth of _Elephas_ are so large that the jaws cannot accommodate more than at the most two and a part of a third at a time. These are gradually replaced by others to the number of three, the replacement of teeth suggesting that of the Manatee. Each molar is deeply ridged, the interstices between the ridges being filled up with cement. As the tooth wears away, therefore, the surface continues to be flat. Each ridge consists of a core of dentine surrounded by a coat of enamel. The number of these ridges varies greatly from species to species.

The Indian Elephant is one of those which have the greatest number of plates in a single tooth, as many as twenty-seven.[137] Of the six molars which {220} eventually appear, the first three are considered to correspond to premolars. But successional teeth are rare in the genus; that is to say as far as concerns the molars, for the tusks have their milk forerunners.

As to the molars it is apparently only _E. planifrons_ which certainly shows a milk dent.i.tion. In _Mastodon_ and older types a milk dent.i.tion is commoner.

The viscera of the Elephant have been examined by many zoologists. The latest paper, dealing chiefly with the African species, but containing facts about its Indian congener also, is quoted below.[138] The Elephant is remarkable in possessing, in addition to the three usual pairs of salivary glands present in mammals, a fourth, situated in the molar region, and opening on to the cheek by many pores. This gland is especially well developed in Rodents. There is a gland which may be mentioned in this connexion, though it opens externally between the eye and ear, known as the temporal gland; its use does not seem clear. The thoracic cavity of the Elephant, as may be inferred from the large number of ribs, is very large as compared with the abdominal.

The stomach is simple in form, and the epithelium of the oesophagus does not extend into it as is the case with the Horse and Rhinoceros. A gland or a collection of smaller glands occurs in the stomach, and recalls the "cardiac gland" of the Wombat and the Beaver, also that of the Giraffe. The large intestine is long, rather more than half the length of the small intestine. The caec.u.m is well developed in these animals. The liver has a very simple form, being but slightly lobulated. It is actually only bilobed, but it is important to notice that this division does not correspond to the two halves of the liver. As shown by the attachment of the suspensory ligament, one half consists of the left lateral lobe alone, the other half embracing the remaining primary lobes. The simplicity of the liver looks like an archaic character. No Elephant has a gall-bladder. The lungs again are simple in form through their slight lobulation. Each half in fact is without subdivisions, and is of a triangular form. In this the Elephants resemble the Whales, as in the simple liver. In both cases probably the likeness is due to the permanence of primitive features of organisation. The brain[139] of the Elephant {221} has hemispheres which are extremely well convoluted; but they leave the cerebellum entirely uncovered. This suggests a brain which is a great specialisation of a low type. The brain has been particularly compared with that of the Carnivora, with which group the Elephants agree in the characters of the placenta. It is, however, always a matter of the very greatest difficulty to compare the brains of mammals belonging to different orders.

There are but two living species of Elephant, of both of which we shall now proceed to give some account. Only a few of the rather numerous fossil forms can be touched upon here.

The African Elephant, _E. africa.n.u.s_, has been sometimes referred to a distinct genus or sub-genus, _Loxodon_, by reason of the lozenge-shaped areas on the worn grinding-teeth. It lives, as its name denotes, in Africa.

This species has a number of external features which enable it to be distinguished from the Oriental Elephant. The head slopes back more, and has not the two rounded bosses which give so wise a countenance to the Indian species. The ears are very much larger. The tip of the trunk has a slight triangular projection on both the lower and the upper part of the circ.u.mference of the aperture. There are four nails on the fore-feet and three on the hind. As in the Indian form, the toes are all bound together, and do not appear for any part as free digits. A thick pad of fat, etc., makes the animal when alive look as if plantigrade, whereas it is, as a matter of fact, digitigrade. In internal features the most prominent difference from _E. indicus_ is in the molar teeth, which are ridged by much fewer ridges. The outside number for a single tooth in the present species is 10 or 11. In _Elephas indicus_ on the other hand there are as many as 27.

The African Elephant, thinks Sir Samuel Baker, reaches a height of about 12 feet, and it will be remembered that the notorious "Jumbo" was found to be 11 feet high at the shoulder. The tusks are found in both s.e.xes, as in the Indian beast, but are relatively larger in the female in the species now under consideration. It is also a rather more active creature, and is more savage;[140] however it can be tamed, as is shown by several {222} specimens which have been and are in the possession of the Zoological Society, and other proprietors. It was apparently used in the past. Certain Carthaginian coins are stamped with a figure of the African Elephant; but in Africa no attempts are now made to utilise this creature except for food and ivory.

[Ill.u.s.tration]

FIG. 117.--African Elephant. _Elephas africa.n.u.s._ 1/56. (After Sir Samuel Baker.)

The meaning of an Elephant as an emblem. upon a coin appears to be eternity, and there is no question but that the {223} Elephant is a long-lived animal. It is said that it hardly reaches proper maturity before forty, and that 150 years is not beyond probability in the way of longevity. Even longer periods have been a.s.signed to it.

The tusks of the Elephant are by no means necessarily s.e.xual adornments, used for fighting purposes only. The African Elephant is a most "industrious digger," and grubs up innumerable roots as food. It appears to be a fact that during these operations the right tusk is mainly used, and in consequence that tusk is shorter as well as thinner than the other. Two average tusks would weigh respectively 75 and 65 lbs., the latter of course being the weight of the more worn right tusk. These weights, it should be observed, by no means indicate the limits to which finely-developed tusks can attain. The very heaviest tusk known to Sir Samuel Baker[141] weighed 188 lbs. This was sold at an ivory sale in London in the year 1874. The pace of the African Elephant, says the same authority, is at most at the rate of fifteen miles an hour at first, and of course in a furious rush.

This pace cannot be kept up for more than two or three hundred yards, after which ten miles an hour is a better approximation to the rate which can be kept up for long distances.

The Indian Elephant, _Elephas indicus_ (or _Euelephas indicus_, if the genus _Loxodon_ is to be accepted), is better known and has been longer known than the African. It occurs in India and Ceylon, and in some of the Malayan islands, the Elephants of which latter parts of the world have been regarded as a distinct form, an apparently unnecessary procedure.

[Ill.u.s.tration]