A history of land mammals in the western hemisphere

by William Berryman Scott

The skeleton is far from being merely the mechanical frame-work of the animal. Such a frame-work it is, of course, but it is much more than that; it is the living and growing expression of the entire organism and is modified, not only by age, but by the conditions of the environment and accidental circumstances as well. The bones of the same individual differ very materially in early youth, maturity and old age; so long as the animal lives, its bones are perpetually changing, slowly it is true, but with ready response to needs. Not only that, but dislocated bones may and frequently do develop entirely new joints, and their internal structure is remodelled to meet the requirements of stresses differing in character or direction from those of normal, uninjured bones. The general form and proportions of any mammal are determined chiefly by its muscular system and this may be directly and confidently inferred from its skeleton, for the muscles which are of importance in this connection are attached to the bones and leave their indelible and unmistakable mark upon them. In any good text-book of anatomy this extremely intimate relation of bone and muscle is made clear; and it is shown how each attachment of muscle, tendon and ligament is plainly indicated by rough lines, ridges, projections or depressions, which speak a language intelligible enough to those who have learned to interpret it. Given the skeleton, it is no very difficult task to reconstruct the muscular system in sufficient detail. Further, the teeth afford valuable information as to the food, habits and appearance of the animal, for the bulk of the viscera, a significant element in the general form, is principally conditioned by the character of the diet.

Beasts of prey, which live by catching and devouring other animals, have a certain likeness to one another, even though they are in no wise related, except as all mammals are. The Thylacine, or so-called “Tasmanian Wolf” (Thylacynus cynocephalus), a marsupial and related to the opossums, is deceptively like the true wolves in appearance, although belonging to an order (Marsupialia) almost as widely separated from that to which the wolves belong (Carnivora) as two mammalian groups well can be. This resemblance is as clearly indicated by the skeletons as by the living animals themselves, though the fundamental differences of structure which distinguish the marsupial from the carnivore are no less clearly displayed. Large herbivorous mammals too, though referable to very different orders, bear a strong resemblance to one another, the characteristic differences, so far as the living animal is concerned, appearing chiefly in the head. It was this general likeness that induced Cuvier to form his order, “Pachydermata,” which comprised elephants, rhinoceroses, hippopotamuses, tapirs, etc., animals that are now distributed into no less than three separate orders; aside from the head, all of these forms are quite distinctly similar in appearance.

Of course, the external features, such as ears, tail, skin and hair, are most important factors in the general make-up of any mammal; and, as to these matters, the fossils leave us largely in the lurch, save in the all too rare cases, like the Siberian †Mammoth, in which these external features are actually preserved. Two artists may so restore the same animal as to result in two very different pictures, and no one can positively decide between them; just as two modern mammals, which are closely related and have very similar skeletons, may yet differ markedly in outward appearance, because of the different character of the skin, as do, for example, the Bornean and Indian rhinoceroses. Yet even in dealing with purely external features, we are not left altogether to conjecture. Ears of unusual size or form frequently leave some indication of this on the skull, and the presence or absence of a proboscis can nearly always be inferred with confidence from the character of the bones of the nose and muzzle. The length and thickness of the tail may be generally directly deduced from the caudal vertebræ, but whether it was close-haired and cylindrical, or bushy, or tufted at the end, or flat and trowel-shaped, as in the Beaver, is not determinable from the bones alone.

Fig. 4.—Wild sow and pigs, showing the uniform colour of the adult and stripes of the young.

Most uncertain of all the characters which determine outward appearance are the hair and the pattern of colouration; the Horse and Zebra differ much more decidedly in the living form than their skeletons would lead one to expect, as do also the Lion, the Tiger and the Leopard. The curious and exceptional colour-pattern of the Okapi, that remarkable giraffe-like animal but lately discovered in the equatorial forests of western Africa, could never have been inferred from a study of the skeleton alone. However, even in the problem of colour-patterns there is more to go upon than sheer guess-work, for certain definite principles of animal colouration have been ascertained; the great difficulty lies in the application of these principles to a particular case. It is quite certain that the naked, hairless skin is never primitive, but always a comparatively late acquisition and, in many mammalian orders, is not found at all. Aside from a few domesticated animals, this type of skin occurs only in very large herbivorous mammals living in warm climates, such as elephants, rhinoceroses and hippopotamuses, in a few burrowers, and in marine mammals, like the walruses, whales, porpoises, etc. Useful hints as to the colouring of ancient and extinct forms may be gathered from a study of series of living animals, such as lizards and butterflies, in which the development of a definite scheme of colouration may be followed step by step. Young animals very frequently retain more or less distinct traces of the ancestral colouration, which disappear in the adult, for the development of the individual is, in some respects at least, an abbreviated and condensed recapitulation of the history of the species. In many mammals which, in the adult condition, have a solid body-colour, the young are striped or spotted, a strong indication that these mammals were derived from striped or spotted ancestors. Thus, the Wild Boar has a uniform body-colour in the full-grown stage, but the pigs are longitudinally striped; many deer are spotted throughout life, as in the Fallow Deer, the Axis Deer of India and others, but the great majority of the species, including all the American forms, have uniform colouration, while the fawns are always spotted. Lion cubs are also spotted and the adults have a uniform tawny colour, and many such examples might be given.

Fig. 5.—Fawns of the Mule Deer (Odocoileus hemionus). Compare with [Fig. 83, p. 167]. (By permission of the N. Y. Zoölog. Society.)

The study of colouration among existing animals has led to the conclusion that in mammals the primitive colour-pattern was that of stripes, either longitudinal or transverse and more probably the former. In the second stage these bands break up into spots, which still show the longitudinal arrangement and may be either light on a dark ground, or dark on a light ground. In a third stage the spots may again coalesce into stripes, the course of which is at right angles to that of the original stripes, or the spots may disappear, leaving a uniform body-colour, lighter or white on the belly. These changes of colour-pattern have not proceeded at a uniform rate in the various mammalian groups, or even within the same group, for an all-important factor is the mode of life of the particular animal. In general, it may be said that the scheme of colour is such as to render its possessor inconspicuous, or even invisible, and many a creature that seems to be very conspicuous and striking in a museum case can hardly be seen at all when in its natural surroundings. Thus, Arctic mammals and birds, in their winter dress, are white; desert animals are tawny or sandy-brown; forest animals are frequently striped or spotted; while those that live on open plains are more commonly of uniform colouration. There are exceptions to these rules, but they hold good for the most part. From careful comparative study of the teeth and skeletons a clew may be gained as to the habits of animals and from the habits something may be inferred as to the colouration.