CHAPTER II

METHODS OF INVESTIGATION—PALÆONTOLOGICAL

Palæontology is the science of ancient life, animal and vegetable, the Zoölogy and Botany of the past, and deals with fossils. Fossils are the recognizable remains or traces of animals or plants, which were buried in the rocks at the time of the formation of those rocks. In a geological sense, the term _rock_ includes loose and uncompacted materials, such as sand and gravel, as well as solid stone. Granting the possibility of so determining the relative dates of formation of the rocks, that the order of succession of the fossils in time may be ascertained in general terms, the question remains: What use, other than geological, can be made of the fossils? In dealing with this question, attention will be directed almost exclusively to the mammals, the group with which this book is concerned.

As a preliminary to the discussion, something should be said of the ways in which mammals became entombed in the rocks in which we find them. In this connection it should be remembered that, however firm and solid those rocks may be now, they were originally layers of loose and uncompacted material, deposited by wind or water, and that each layer _formed in its turn the surface of the earth, until buried by fresh accumulations upon it_, it may be to enormous depths.

One method of the entombing of land-mammals, which has frequently been of great importance, is burial in volcanic dust and so-called ash, which has been compacted into firm rock. During a great volcanic eruption enormous quantities of such finely divided material are ejected from the crater and are spread out over the surrounding country, it may be for distances of hundreds of miles. Thus will be buried the scattered bones, skeletons, carcasses, that happen to be lying on the surface; and if the fine fragments are falling rapidly, many animals will be buried alive and their skeletons preserved intact. A modern instance of this is given by the numerous skeletons of men and domestic animals buried in the volcanic ash which overwhelmed Pompeii in 79 A.D. Pliny the Younger, who witnessed that first recorded eruption of Vesuvius, tells us in a letter written to Tacitus, that far away at Misenum, west of Naples, it was often necessary to rise and shake off the falling ashes, for fear of being buried in them. In the Santa Cruz formation of Patagonia (see p. 124), which has yielded such a wonderful number and variety of well-preserved fossils, the bones are all found in volcanic dust and ash compacted into a rock, which is usually quite soft, but may become locally very hard. The Bridger formation of Wyoming (p. 110) and the John Day of eastern Oregon (p. 116) are principally made up of volcanic deposits; and no doubt there are several others among the Tertiary stages which were formed in the same way, but have not yet received the microscopic study necessary to determine this.

Much information concerning the mammalian life of the Pleistocene, more especially in Europe and in Brazil (p. 211), has been derived from the exploration of caverns. Some of these caves were the dens of carnivorous beasts and contain multitudes of the bones of their victims, as well as those of the destroyers themselves. Others, such as the Port Kennedy Cave, on the Schuylkill River above Philadelphia, the Frankstown Cave in central Pennsylvania, the Conard Fissure in Arkansas, are hardly caverns in the ordinary sense of the word, but rather narrow fissures, into which bones and carcasses were washed by floods, or living animals fell from above and died without being able to escape. The bones are mostly buried in the earth which partially or completely fills many caverns and may be covered by a layer of stalagmite, derived from the solution and re-deposition of the limestone of the cavern-walls, by the agency of percolating waters.

A mode of preservation which is unfortunately rare is exemplified by the asphaltic deposits near Los Angeles, at Rancho La Brea, which have been very fully described by Professor J. C. Merriam of the University of California. The asphalt has been formed by the oxidation and solidification of petroleum, which has risen up through the Pleistocene rocks from the oil-bearing shales below. At one stage in the conversion of petroleum into asphalt, tar-pools of extremely viscid and adhesive character were, and still are, formed on the surface of the ground; and these pools were veritable traps for mammals and birds and for the beasts and birds of prey which came to devour the struggling victims.

“The manner in which tar or asphalt pools may catch unsuspecting animals of all kinds is abundantly illustrated at the present time in many places in California, but nowhere more strikingly than at Rancho La Brea itself, where animals of many kinds have frequently been so firmly entrapped that they died before being discovered, or if found alive were extricated only with the greatest difficulty. As seen at this locality, the tar issuing from springs or seepages is an exceedingly sticky, tenacious substance which is removed only with the greatest difficulty from the body of any animal with which it may come in contact. Small mammals, birds, or insects running into the soft tar are very quickly rendered helpless by the gummy mass, which binds their feet, and in their struggles soon reaches every part of the body. Around the borders of the pools the tar slowly hardens by the evaporation of the lighter constituents until it becomes as solid as an asphalt pavement. Between the hard and soft portions of the mass there is a very indefinite boundary, the location of which can often be determined only by experiment, and large mammals in many cases run into very tenacious material in this intermediate zone, from which they are unable to extricate themselves.”

The foregoing account refers to what may actually be observed at the present time; in regard to the Pleistocene, Professor Merriam says: “In the natural accumulation of remains at the tar pools through accidental entangling of animals of all kinds, it is to be presumed that a relatively large percentage of the individuals entombed would consist of young animals with insufficient experience to keep them away from the most dangerous places, or with insufficient strength to extricate themselves. There would also be a relatively large percentage of old, diseased, or maimed individuals that lacked strength to escape when once entangled. In the census of remains that have been obtained up to the present time the percentages of quite young, diseased, maimed, and very old individuals are certainly exceptionally large.... In addition to the natural accumulation of animal remains through the entangling of creatures of all kinds by accidental encountering of the tar, it is apparent from a study of the collections obtained that some extraordinary influence must have brought carnivorous animals of all kinds into contact with the asphalt with relatively greater frequency than other kinds of animals. In all the collections that have been examined the number of carnivorous mammals and birds represented is much greater than that of the other groups.... Whenever an animal of any kind is caught in the tar, its struggles and cries naturally attract the attention of carnivorous mammals and birds in the immediate vicinity, and the trapped creature acts as a most efficient lure to bring these predaceous animals into the soft tar with it. It is not improbable that a single small bird or mammal struggling in the tar might be the means of entrapping several carnivores, which in turn would naturally serve to attract still others.... In the first excavations carried on by the University of California a bed of bones was encountered in which the number of saber-tooth and wolf skulls together averaged twenty per cubic yard.”[1]

As the animals were thus entombed alive, it would be expected that a large number of complete skeletons would be preserved, but this is not the case: “connected skeletons are not common.” This scattering and mingling of the bones were due partly to the trampling of the heavier animals in their struggles to escape, but, in more important degree, to the movements within the tar and asphalt.

In arid and semi-arid regions great quantities of sand and dust are transported by the wind and deposited where the winds fail, or where vegetation entangles and holds the dust. Any bones, skeletons or carcasses which are lying on the surface will thus be buried, and even living animals may be suffocated and buried by the clouds of dust. An example of such wind-made accumulations is the Sheridan formation (Equus Beds, see p. 131), which covers vast areas of the Great Plains from Nebraska to Mexico and contains innumerable bones, especially of horses. In this formation in northwestern Kansas, Professor Williston found nine skeletons of the large peccary (_†Platygonus leptorhinus_), lying huddled together, with their heads all pointing in the same direction, and in the upper Miocene (p. 121) of South Dakota Mr. Gidley discovered six skeletons of three-toed horses (_†Neohipparion whitneyi_) crowded together, killed and buried probably by a sandstorm. Similar illustrations might be gathered from many other parts of the world.

Swamps and bogs may, especially under certain conditions, become the burial places of great numbers of animals, which venture into them, become buried and are unable to extricate themselves. Especially is this true in times of great drought, when animals are not only crazed with thirst, but very much weakened as well, and so unable to climb out of the clinging mud. In an oft-quoted passage, Darwin gives a vivid description of the effects of a long drought in Argentina between the years 1827 and 1830. “During this time so little rain fell, that the vegetation, even to the thistles, failed; the brooks were dried up, and the whole country assumed the appearance of a dusty high road.” “I was informed by an eyewitness that the cattle in herds of thousands rushed into the Paraná, and being exhausted by hunger they were unable to crawl up the muddy banks, and thus were drowned. The arm of the river which runs by San Pedro was so full of putrid carcasses, that the master of a vessel told me that the smell rendered it quite impassable. Without doubt several hundred thousand animals thus perished in the river; their bodies when putrid were seen floating down the stream; and many in all probability were deposited in the estuary of the Plata. All the small rivers became highly saline, and this caused the death of vast numbers in particular spots; for when an animal drinks of such water it does not recover. Azara describes the fury of the wild horses on a similar occasion, rushing into the marshes, those which arrived first being overwhelmed and crushed by those which followed. He adds that more than once he has seen the carcasses of upwards of a thousand wild horses thus destroyed.... Subsequently to the drought of 1827 to 1832, a very rainy season followed, which caused great floods. Hence it is almost certain that some thousands of the skeletons were buried by the deposits of the very next year.”[2]

In the arid and desolate regions of the interior of South Australia is a series of immense dry lakes, which only occasionally contain water and ordinarily “are shallow, mud-bottomed or salt-encrusted claypans only.” One of these, Lake Callabonna, is of great interest as having preserved in its soft mud many remains of ancient life, of creatures which were mired in the clay and destroyed, as has been described by Dr. E. C. Stirling. “There is, however, compensation for the unpromising physical features of Lake Callabonna in the fact that its bed proves to be a veritable necropolis of gigantic extinct marsupials and birds which have apparently died where they lie, literally, in hundreds. The facts that the bones of individuals are often unbroken, close together, and, frequently, in their proper relative positions, the attitude of many of the bodies and the character of the matrix in which they are embedded, negative any theory that they have been carried thither by floods. The probability is, rather, that they met their deaths by being entombed in the effort to reach food or water, just as even now happens in dry seasons, to hundreds of cattle which, exhausted by thirst and starvation, are unable to extricate themselves from the boggy places that they have entered in pursuit either of water or of the little green herbage due to its presence. The accumulation of so many bodies in one locality points to the fact of their assemblage around one of the last remaining oases in the region of desiccation which succeeded an antecedent condition of plenteous rains and abundant waters.”

It is a very general experience in collecting fossil mammals to find that they are not evenly or uniformly distributed through the beds, but rather occur in “pockets,” where great numbers of individuals are crowded together, while between the “pockets” are long stretches of barren ground. It is equally common to find the bones thickly distributed in certain layers, or beds, and the layers above and below entirely wanting in fossils. The reasons for this mode of occurrence have been partially explained in the foregoing paragraphs, but the reason differs for each particular mode of entombment. The important part played by drought in causing such accumulation of closely crowded bodies in swamps and mud-holes is indicated in the quotations from Darwin and Stirling; but similar accumulations may take place on hard ground, as was observed in central Africa by Gregory. “Here and there around a water hole we found acres of ground white with the bones of rhinoceroses and zebra, gazelle and antelope, jackal and hyena.... These animals had crowded around the dwindling pools and fought for the last drops of water.”[3] Even in normal seasons springs and water holes and the drinking places in streams are the lurking places of beasts of prey and crocodiles, so that great accumulations of bones are made around these spots. A succession of unusually severe winters frequently leads to great mortality among mammals, as happened in Patagonia in the winter of 1899, when enormous numbers of Guanaco perished of starvation on the shore of Lake Argentine, where they came to drink.

Bones which are exposed on the surface of the ground decay and crumble to pieces in the course of a very few years; and if they are to be preserved as fossils, it is necessary that they should be buried under sedimentary or volcanic deposits. Several such modes of burial have been described in the foregoing paragraphs, but there are other and equally important methods, which remain to be considered.

The deposits made by rivers are often extremely rich in fossils, and most of the Tertiary formations of the Great Plains are now ascribed to the agency of rivers. The flood-plain of a stream, or that part of its basin which is periodically overflowed, is gradually built up by the layers of clay and silt thrown down by the relatively still waters of the flooded area, and scattered bones, skeletons or carcasses that may have been lying on the ground before the freshet are buried in the deposits. Bones covered up in this manner frequently show the marks of teeth of rodents or carnivores which have gnawed them when lying exposed. Deposits made in the stream-channels, where the current was swiftest, are of coarser materials such as gravel and sand, and these often contain the skeletons of animals which were drowned and swept downward by the flooded stream. When the Bison (the mistakenly so-called Buffalo) still roamed in countless herds over the western plains, immense numbers of them were drowned in the upper Missouri River by breaking through the ice, when they attempted to cross at times when the ice had not attained its winter thickness, or was weakened by melting in the spring. No doubt, the bed of that river contains innumerable bones of the Bison. Frequently, too, animals are caught in quicksands and, unable to escape, are buried in the soft mass; fossil skeletons which are preserved in sandstones in an erect or standing position are usually to be interpreted in this manner.

The sedimentary accumulations formed in lakes and ponds sometimes yield fossil bones or skeletons in considerable numbers, which have, for the most part, been derived from the carcasses of animals carried into the lake by streams. A newly drowned mammal sinks to the bottom and, if sufficient sediment be quickly deposited upon it, it may be anchored there and fossilized as a complete skeleton. Otherwise, when distended by the gases of putrefaction, the body will rise and float on the surface, where it will be attacked and pulled about by crocodiles, fishes and other predaceous creatures. As the bones are loosened in the course of decomposition, they will drop to the bottom and be scattered, now here, now there, over a wide area.

Land mammals are rarely found in marine rocks, or such deposits as were made on the sea-bottom; but the remains of marine mammals, whales, porpoises, dolphins, seals, etc. are often found in large numbers. In principle, the method of entombment is the same as in the case of lakes, but currents may drift to some bay or cove multitudes of carcasses of these marine mammals. At Antwerp, in Belgium, incredible quantities of such remains have been exposed in excavations and in all probability were drifted by currents into a quiet and shallow bay, which was subsequently converted into land.

While the foregoing account by no means exhausts the various methods of accumulation and burial of the skeletons and scattered bones of mammals, it covers the more important of these methods sufficiently for a general understanding of the different processes. In whatever manner the preservation may have been effected, there is great difference in the relative abundance and completeness among the fossils of the various kinds of mammals which were living at the same time and in the same area. It need hardly be said, that the more abundant any species was, the better was the chance of its being represented among the fossils; hence, gregarious species, living in large herds, were more likely to be preserved than those which led a solitary existence, or were individually rare. Most of the hoofed mammals are and apparently always have been gregarious, and are therefore much better represented among the fossils, and are, in consequence, better known than the beasts of prey, which, of necessity, were individually less numerous and generally solitary in habits. Not only this, but large and medium-sized mammals, with strong and heavy bones, were better fitted to withstand the accidents of entombment and subsequent preservation than small creatures with delicate and fragile skeletons. The mere dead weight of over-lying sediments often crushes and distorts the bones, and the movements of uplift, compression, folding and fracture, to which so many strata have been subjected, did still further damage to the fossils. The percolating waters, which for ages have traversed the porous rocks, often attack and dissolve the bones, completely destroying the minute ones and greatly injuring those which are massive and strong. In consequence of all those accidents it frequently happens that only the teeth, the hardest and most resistant of animal structures, and it may be the dense and solid jaw-bones, are all that remain to testify of the former existence of some creature that long ago vanished from the earth. Very many fossil mammals are known exclusively from the teeth, and it is this fact which makes the exact study of teeth so peculiarly important to the palæontologist.

In view of all these facts, it is not surprising that concerning the history of many mammalian groups we have but scanty information, or none at all, while in the case of others the story is wonderfully full and detailed. The latter are, very generally, the groups which were not only numerically abundant at all stages of their history, but also had skeletons that were strong enough to resist destruction; while the groups as to which there is little or no information are chiefly of small and fragile animals, or such as were always rare. For example, a great deal has been learned regarding the development of horses and rhinoceroses in North America, but the history of the tapirs is very unsatisfactorily known, because, while horses and rhinoceroses were common, tapirs were solitary and rare. In Europe bats have been found in the Eocene, Oligocene and Miocene, and there is no reason to suppose that they were not equally ancient and equally abundant in America; but none have been found in the western hemisphere in any formation older than the Pleistocene. All things considered, the extraordinary fact is, not that so many forms have irretrievably perished, but that so much has been preserved, escaping all the chances of destruction.

As to the degree of preservation in fossil mammals, we have to do almost entirely with bones and teeth. With very rare exceptions, and those all of late geological date, the viscera, muscles, skin, hair, horns, hoofs and claws have been completely destroyed and have vanished without leaving a trace. In northern Siberia the gravel soil is permanently frozen to a depth of several hundred feet and contains the intact carcasses of elephants and rhinoceroses of Pleistocene date and notably different from any species of these animals now in existence. Sometimes such a carcass is disinterred from a bluff by the cutting action of a stream and is in a state of nearly complete preservation, with hide, hair and flesh almost as in an animal freshly killed. From these remains it has been learned that the †Mammoth was an elephant densely covered with hair and wool, just as he was depicted in the carvings and cave-paintings of Pleistocene Man in Europe, where †Mammoth bones have been abundantly found, and also that there were Siberian rhinoceroses similarly protected against the cold. †Mammoth remains with hide and flesh, but much less complete, have likewise been found in Alaska.

In a cavern in southern Patagonia an expedition from the La Plata Museum discovered, with the remains of a gigantic, extinct †ground-sloth, large pieces of the skin still covered with hair and affording most welcome information as to the colouration of these most curious animals. The skin had been preserved from decay by deep burial in dry dust. Mummies of Pleistocene rodents have been found in the dry caves of Portugal, whereas in the ordinary caves which are damp or wet, only bones are preserved. Unfortunately, as has been said, such instances of complete preservation are very rare, and none are known of mammals more ancient than those of the Pleistocene epoch.

In general, it may be said that the higher the geological antiquity of a skeleton is, the greater is the chemical alteration which it has undergone. Bones of Pleistocene or later date have, as a rule, suffered little change beyond the loss of more or less of their animal matter, the amount of such loss depending chiefly upon exposure to the air. Bones which, for thousands or tens of thousands of years, have been buried in dense cave-earth, in an antiseptic peat-bog, or in asphalt, are often perfectly sound and fresh when taken up. Skeletons of the antecedent (Tertiary) period are, on the other hand, very frequently _petrified_; that is to say, the original substance of the bones has been completely removed and replaced by some stony material, most commonly lime or flint. This substitution took place very gradually, molecule by molecule, so that not only is the form of the bone or tooth most accurately reproduced, but the internal, microscopic structure is perfectly retained and may be studied to as great advantage as in the case of modern animals.

While, save in the rarest instances, only the hard parts of fossil mammals remain to testify of their structure, very important information as to the size, form and external character of the brain may be secured from “brain-casts,” which may be natural or artificial. The pressure of the mud, sand or other material, in which the fossil was embedded, filled up all openings in the skeleton and, as the brain decayed and disappeared, its place was taken by this material, which subsequently hardened and solidified and quite accurately reproduces the external form and character of the brain. When a fossil skull is exposed and shattered by weathering, the natural brain-cast often remains intact, and a great many such specimens are in the collections. An artificial cast is made by sawing open the cranial cavity, cleaning out the stony matrix which fills it and then pouring liquid gelatine or plaster of Paris into the cavity. These artificial casts are often quite as satisfactory as the natural ones.

As has been shown above, the history of the mammals is recorded, save in a very few instances, in terms of bones and teeth and, to the uninitiated, it might well seem that little could be accomplished with such materials. However, it is the task, and the perfectly feasible task, of palæontology to make these dry bones live. It is a current and exceedingly mischievous notion that the palæontologist can reconstruct a vanished animal from a single bone or tooth and, in spite of repeated slayings, this delusion still flourishes and meets one in modern literature at every turn. No doubt, much of the scepticism with which attempts to restore extinct animals are met by many intelligent people is traceable to the widespread belief that such off-hand and easy-going methods are used in the work. So far from being able to make a trustworthy reconstruction from a few scattered bones, competent palæontologists have been sometimes led completely astray in associating the separated parts of the same skeleton. More than once it has happened that the dissociated skull and feet of one and the same animal have been assigned to entirely different groups, just because no one could have ventured, in advance of experience, to suppose that such a skull and teeth could belong to a creature with such feet. In all these cases (and they are few) the error has been finally corrected by the discovery of the skeleton with all its essential parts in their natural connection.

While the number of complete skeletons of Tertiary mammals as yet collected is comparatively small, it is often possible to construct a nearly complete specimen from several imperfect ones, all of which can be positively shown to belong to the same species. Such composite skeletons are almost as useful as those in which all the parts pertain to a single individual, though in making the drawings it is not easy to avoid slight errors of proportion. It must not be supposed that no successful restoration of missing bones is practicable; on the contrary, this can often be done very easily, but only when all the essential parts of the skeleton are known.

Even if an unlimited number of perfect skeletons were available, of what use would they be? A skeleton is a very different looking object from a living animal, and how is it possible to infer the latter from the former? Do the many restorations of extinct mammals which this book owes to the skill of Mr. Horsfall and Mr. Knight deserve any other consideration than that due to pleasing, graceful or grotesque fancies, with no foundation of solid fact? To answer these questions, it is necessary first to consider the relations of the bony structure to the entire organism and then to discuss the principles in accordance with which the restorations have been made.

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.

[Illustration: 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.

[Illustration: 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.

[Illustration: FIG. 6.—_Tapirus terrestris_, 3 days old. Compare with Fig. 137, p. 320. (By permission of W. S. Berridge, London.)]

It would, however, be misleading to claim a greater authority for these attempts at restoring a long-vanished life than can fairly be ascribed to them. The general form and proportions of the head, neck, body, tail, limbs and feet may be deduced with a high degree of accuracy from the skeleton, while the external characters of skin, hair and colouration are largely conjectural, but not altogether imaginary. It cannot be doubted that among the extinct mammals were many which, owing to some uncommon growth of subcutaneous fat, or some unusual local development of hair, were much more curious and bizarre in appearance than we can venture to represent them. If, for example, the Camel, the Horse, the Lion and the Right Whale were extinct and known only from their skeletons, such restorations as we could make of them would assuredly go astray in some particulars. The Camel would be pictured without his hump, for there is nothing in the skeleton to suggest it; the forelock, mane and characteristic tail of the Horse and the Lion’s mane would certainly not be recognized; while the immense development of blubber in the head of the Whale gives to it a very different appearance from that which the skull would seem to indicate. Such cases are, however, exceptional and restorations made by competent hands from complete skeletons probably give a fair notion of the appearance of those animals when alive.

It will thus be sufficiently plain that the work of restoration is beset with difficulties, but that there is no good ground for the uncritical scepticism which summarily rejects the results as being purely fanciful, or for the equally uncritical credulity which unhesitatingly accepts them as fully and incontestably accurate. It is altogether likely that one of the main sources of error consists in making the extinct animal too closely resemble some existing species which is selected as a model.

Too much space has perhaps been devoted to the problem of restoring the external form of these extinct mammals, a problem which, after all, is of distinctly subordinate importance. The most valuable results which may be gained from a study of these fossil mammals are the answers which they afford to the great questions of relationship, classification and genetic descent, and the light which they throw upon the processes of evolution and the course of geographical arrangement. The bones and teeth afford admirable means of tracing the gradual steps of modification by which the modern mammals have arisen from very different ancestors and of following their wanderings from region to region and continent to continent. It is to these questions that most of the subsequent chapters are devoted.