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.