and it is in such conditions that the systematist must apply his knowledge of the general progressive tendencies through the ages of change, to the determination of the particular changes he should expect to find in the special case before him, and so be enabled to recognize the footprints he is in search of. The genius to do this has been given to few, but in their hands the results have often been brilliant.
Back in the very earliest Tertiary deposits, and in all certainty even earlier, a group of comparatively small mammals was extensively spread through America, and apparently less widely in Europe, characterized by a primitive form of foot structure, each of which had five complete digits, the whole sole being placed upon the ground, as in the animals we call plantigrade. The grinding surfaces of their molar teeth were also primitive, bearing none of the complicated, curved crests and ridges possessed by present ruminants, but instead they had conical cusps, usually not more than three to a tooth; this tritubercular style of molar crown being about the earliest known in true mammals.
In the opinion of many palaeontologists, the ancestors of the present hoofed beasts, or ungulates, were contained among these Condylarthra, as they were named by Prof. Cope.
Of course, these early mammals are known to us only by their fossil and mostly fragmentary skeletons, but it may be said that at least in the ungulate line, the successive geological periods show steady structural progression in certain directions. Of great importance are a decrease in the number of functional digits; a gradual elevation of the heel, so that their modern descendants walk on the tips of their toes, instead of on the whole sole; a constant tendency to the development of deeply grooved and interlocked joints in place of shallow bearing surfaces; and to a complex pattern of the molar crowns instead of the simple type mentioned. To this may be added as the most important factor of all in survival, that these changes have progressed together with an increase in the size of the brain and in the convolutions of its outer layer.
The Condylarthra seem to have gone out of existence before the time of the middle Eocene, but before this they had become separated into the two great divisions of odd-toed and even-toed ungulates, into which all truly hoofed beasts now living fall.
The first group (Perissodactyla) has always one or three toes functionally developed, either the third, or third, second and fourth, the two others having entirely disappeared, except for a remnant of the fifth in the forefoot of tapirs. They have retained some at least of the upper incisor teeth, and, except in some rhinoceroses, the canines are also left; the molars and premolars are practically alike in all recent species, and in all of which we know the soft parts, the stomach has but one compartment, and there is an enormous caecum. It is probable that they took rise earlier than their split-footed relations, and their Tertiary remains are far more numerous, but their tendency is toward disappearance, and among existing mammals they are represented only by horses, asses, rhinoceroses, and tapirs.
Contrasted with these, Artiodactyla have always an even number of functional digits, the third and fourth reaching the ground symmetrically, bearing the weight and forming the "split hoof;" the second and fifth remain, in most cases, as mere vestiges, showing externally as the accessory hoofs or dewclaws; in the hippopotamus alone they are fully developed and the animal has a four-toed foot. In deer and bovine animals the incisors and frequently the canines have disappeared from the upper jaw, and the molars are unlike the premolars in having two lobes instead of one. The stomach is always more or less complex; at its extreme reaching the ruminant type with four compartments, in association with which is a caecum reduced in size and simple in form. Nearly all have horns or antlers, at least in one sex.
Most split-hoofed animals are ruminants, but there is a small remnant, probably of early types, which are not. The present ungulates may be summed up in this way:
Odd-toed: (Perissodactyla)— Horse, Ass, Rhinoceros, Tapir.
Even-toed: (Artiodactyla)—
Non-ruminants—
Hippopotamus,
Swine,
Peccaries.
Ruminants—
Camels, Llamas,
Chevrotains,
Giraffe,
Antelopes,
Sheep, Goats,
Musk-ox,
Oxen,
Deer.
The non-ruminant artiodactyls need not detain us long. Hippopotamuses
are little more than large pigs with four toes; they were never
American, though many species, some very small, are found in the
European Tertiary. The two existing species are African.
In the western hemisphere swine are represented by the peccaries, differing from them chiefly in having six less teeth, one less accessory toe on the hind foot, and in a stomach of more complex character. Peccaries also have the metapodial bones supporting the two functional digits fused together at their upper ends, forming an imperfect "cannon bone," which is a characteristic of practically all the ruminants, but of no other hoofed beasts. One species only enters the United States along the Mexican border.
All non-ruminant ungulates have from four to six incisors in the upper jaw; the canines are present, and sometimes, as in the wart hogs, reach an extraordinary size.
Coming now to the ruminants, all digits except the third and fourth have disappeared from camels and llamas, and the nails on these are limited to their upper surface without forming a hoof, the under side being a broad pad, upon which they tread. No camel-like beasts have inhabited North America since the Pliocene age. Chevrotains, or muis deer (Tragulidae), are not deer in any true sense, as they have but three compartments to the stomach; antlers are absent and in their place large and protruding canine teeth are developed in the upper jaw, and the lateral metacarpal bones are complete throughout their length, instead of being represented by a mere remnant. They are the smallest of ungulates, and inhabit only portions of the Indo-Malayan region. Camels also have upper canines, and the outer, upper incisors as well.
The giraffe is separated from all living ungulates by the primitive character of its so-called "horns," which are not horns in the usual sense, but simply bony prominences of the skull covered with hair. Some of the earliest deer-like animals seem to have had simple or slightly branched antlers which were not shed, and which there is reason to believe were also hairy, and in these, as well as in other characters, giraffes and the early deer may not have been far apart. The "okapi," Sir Harry Johnston's late discovery in the Uganda forests, seems to have come from the same ancestral stock, but the giraffe has no other existing relatives.
The true deer, to which we shall return, are readily enough distinguished from the ox tribe and its allies by their solid and more or less branched antlers, usually confined to males, and periodically shed.
So, through this rapid survey, we have dropped out of the hoofed beasts all but the bovines and their near allies, and are thus far advanced toward our definition of a bison, but from this point we shall not find it easy to draw sharp distinctions, for while the Bovidae, as a whole, are well enough distinguished from all other animals, their characteristics are so much mixed among themselves that it is hardly possible to find any one or more striking features peculiar to one group, and for most of them recourse must be had to associations of a number of lesser characters.
Oxen,
