THE STORY OF THE HORSE
Ever since men have been familiar with the idea of evolution there has
been a temptation on the part of the zooelogist to draw up pedigrees
expressing the relationship between the various groups of the animal
kingdom. The impulse is natural, and, if the resulting tables are not
accepted with too much confidence, the result is not undesirable. The
truth of the matter is that all of these pedigrees are more or less
hypothetical. They simply show what connection seems most likely. In
all of them are spaces filled with doubtful names. Each addition to
our acquaintance with the past history of animals necessitates
revision of our tables. The student of fossils, trying to rebuild in
imagination the world of the past, finds himself often strangely
unable to link these animals together. The result is that the more we
know of fossils, the more distrustful we become of the easy
connections we have been making between groups. Accordingly we are
more than commonly pleased when we find the clear indication of a
genuine pedigree, actually illustrated by real examples, following
each other in time through the geological history. A few of these
lines are gradually becoming plain, and none of them is clearer than
the pedigree of our familiar and much loved horse. The example is a
particularly interesting one, not only because of our affection for
the animal, but because the horse originated in all likelihood in
North America on the land occupied to-day by our Western plains. As
though he loved the country of his ancestors, he returned after having
circled the globe, and once more went wild in the home of his
forefathers. The problem was first worked out in Europe and later
elaborated in this country. Now the history gets its finest expression
in the American Museum of Natural History in New York City. The
collection of fossil horses in that institution surpasses in
completeness and in excellence of mounting and of sympathetic
restoration any similar collection representing the ancestry of any
other animal in the world.
In the table of Geological Times, given in chapter six, the era of
recent life known as the Cenozoic is seen to occupy something like
five million years. This figure, as was previously suggested, is very
uncertain, and may be three or may be six, but is safely represented
in millions. Through most of this time stretches what is known as the
Age of Mammals, the Tertiary Age. Its close, occupying only the last
few hundred thousand years, is known as the Age of Man, the
Quaternary. Through perhaps three or four millions of these years
stretches the known pedigree of the horse.
When we go back to the early Tertiary we find a forest, with trees
that shed their leaves, interspersed with glades, in which already the
grasses were beginning to be developed. This state of affairs had
existed but for a comparatively short time, geologically speaking. It
had come only in the latter part of the preceding era. Lake and swamp,
meadow and forest intermingled to make a rich and varied scene. Slowly
the land toward the western side of North America lifted itself into
plateau and mountain range. Slowly the westerly winds began to be cut
off by the barriers thus raised across their path. As they swept over
the plateau and down into the eastern plain their moisture came to be
diminished. Gradually a very different state of affairs set in. The
ground became harder, the forest became sparser, the plants became
higher and firmer, the grasses tougher and more wiry, and, by the time
the Quaternary arrived, a condition probably even drier than that of
to-day existed over our western highlands. Throughout this long
change, spread over millions of years, a creature which has become our
horse steadily persisted and steadily advanced. Side lines developed
which finally disappeared, but the main line kept on, and when the
Quaternary came the horse arrived with it. Many of the skeletons in
this series were known before it was realized what they were. As time
went on and intermediate forms were found, it became possible to
recognize these as ancestors of the horse and to assign them their
proper position in the family tree.
[Illustration: THE EVOLUTION OF THE HORSE'S FOOT
_After H. F. Osborne and Charles R. Knight. By permission of the
American Museum of Natural History._]
The earliest of the forerunners of the horse with which we are
acquainted would certainly not be recognized as such by any but the
most careful student of animals, if we could see him to-day. He stood
not higher than a fox-terrier dog, though his shape was very
different. But he would probably be more likely to be classed with the
dog than with the horse by the hasty observer, for he walked with four
toes of each foot upon the ground as the dog does to-day. Like the
dog, he had hanging at the inner side of his front foot a little
useless toe. He was long in body, comparatively short of leg, a little
long of head and neck, and distinctly long of tail. His grinding teeth
had points on them not unlike a pig's, and possessed no apparent
resemblance to the wonderful curved and ridged surfaces seen on the
teeth of the modern horse. What his skin and hair were like can only
be conjectured. In the restoration which Mr. Knight has made, at the
suggestion of Professor Osborne, an interesting inference has been
drawn. That he was a creature of the forest is suggested by his
spreading toes, which would keep him from sinking in the soft soil. It
is consequently surmised that he was dappled with spots which allowed
him to rest unnoticed on the sun-flecked floor of the forest. Mane he
had none, and his tail was probably tufted slightly at the end with
hairs, which were increasingly short as they approached the top. He
had no forelock, and the hair along the ridge of his neck was a little
longer than the rest, and stood erect. Browsing about on the soft and
tender herbage of his woodland home, his teeth had as yet no tendency
to become specialized. The molars had mounds upon them, developing,
perhaps, more into the shape of the points of the hog's, but even
still quite generalized teeth. His main enemies, from whom, perhaps,
he could with little difficulty escape, were creatures related to the
hyenas of to-day. Perhaps, like their modern representatives, they
preferred eating their flesh tainted to exerting themselves enough to
capture and kill their prey. By the time we advance a little further
into the Tertiary, though still in its early portion, a remarkable
change has already come about. The fifth toe, which in the earliest
horse hung upon the side of the front foot, has completely
disappeared. The change in the hind foot has gone still further. The
hind leg in many animals evolves more rapidly than the front. The
heavy work of running is always done by the hind feet, while the front
feet serve rather as a prop to keep the animal from falling than as
the actual means of locomotion. Hence the hind feet and the muscles of
the hind quarters are almost always heavier than the front. Possibly
on the front foot the little fifth toe was less of an obstruction, and
persisted after the early horse had lost the corresponding toe on his
hind foot. This process has gone on still further in this second
stage, and the hind foot has but three toes, while the front still
has four. This is not the only advance. Already the middle toe of the
original set of five is becoming emphasized. The weight is thrown more
forcibly upon it, as with the human foot it is upon the inner or big
toe. The middle toe is growing larger and larger, and the nail upon it
is spreading around it and is growing firmer. The creature, too, is
standing more nearly upon his toes; his legs are getting longer; he
stands higher from the ground, and now has come to be the size of a
hound.
We can only surmise why this creature should have undergone such a
change, but the presence of flesh-eating animals having the size of a
fox, and presumably of the fox's swiftness, probably tells the story.
The little bands of early horses, pursued by their carnivorous foes,
were slowly modified into swifter creatures. It is not so much that
running made them fast, as that the slow ones were continually being
caught. If this process of constant elimination of the slow members of
any herd is kept up long enough, the group will necessarily develop
speed. As time goes on, of these early horses those which happened to
have longer legs and stood higher upon their toes won in the race, and
handed on their qualities to their long-legged descendants. As the
animal rose upon his toes, the inner toe, corresponding to our thumb,
was first raised off the ground and rendered useless, while a similar
change came over the corresponding toe on the hind foot. The hard work
of running being done on the latter, this superfluous toe was more
detrimental there than on the front foot, and disappeared,
consequently, more rapidly. In time, however, it also disappeared from
the front foot. Gradually the further elevation of the foot lifted the
toe, which corresponds to our little finger, off the ground, and this
now disappears also.
With increasing toughness of the grasses, as the climate becomes drier
and the region more elevated, the teeth of the horse are given harder
work. The points begin to spread into ridges and to unite with each
other in such way as to form the crescents, which are later to be so
characteristic of the teeth of the modern horse.
By the middle of the Tertiary this ancestral horse has risen in height
until he is taller and heavier than a setter dog. Three toes are found
on each front foot. The middle toe is getting constantly more
developed, though the smaller toes are evidently still of use. The
ridges of the teeth are quite crescentic now on the outer side, and
becoming better adapted to the evidently firmer food which the
creature is obliged to eat.
As we come toward the end of the Tertiary, the development which had
been all pointing in one direction has advanced very much further. The
creature now would be undoubtedly recognized by anyone as a horse. The
legs are longer and straighter; the middle toe has become the only
useful toe, though on each foot a smaller toe, slender and probably
useless, still hangs on either side. Two similar useless toes to-day
hang at the back of the foot of the cow, which is now walking upon her
two toes, which give her the appearance of carrying a cloven hoof.
That is to say, the first toe on the foot of the cow has disappeared,
the second and fifth hang useless and much diminished at the back of
the foot, while the third and fourth are both well developed and
serviceable in walking.
The late Tertiary horse has grown to be the size of a burro of to-day,
though probably it was a little more slender. The teeth are quite
horselike, both in shape of the crescentic ridges on their surface, in
the length of the teeth in the jaw bone, and in the fact that the
crinkled edges of enamel on the upper surface are protected on either
side by dentine or by cement. These surfaces, being softer than the
enamel, wore away somewhat more rapidly and allowed the sharp edges of
enamel to stand up in ridges. This plan increases the grinding power
of the teeth.
With the oncoming of the Era of Man the horse reaches his modern
splendid development. During the early Quaternary the horse was
perhaps in some of his representatives a larger creature than he is
to-day. Each foot now has but a single toe. The nail has spread around
firmly and heavily, until it has become a splendidly developed hoof,
permitting the animal to travel with speed over firm and often stony
ground. The side toes have disappeared completely from the outside of
the horse's leg, although upon removing the skin it is easy to find
the long splints, which are the remnants of toes, which have not yet
quite disappeared. His heel has been lifted in the air until it is
eighteen inches off the ground, and he is standing like an expert
dancer upon the tip of his toe. The body of the horse thus being
lifted far off the ground, a new development becomes necessary. All
through the growth of the creature the neck and head have been obliged
to lengthen correspondingly. Every animal must be able to bring its
head down to the level of its feet in order that it may drink. Various
animals use different methods to accomplish this result. The giraffe,
with his enormously long legs, has a correspondingly long neck, which
lowers his mouth to the ground. Even with this extended neck the
giraffe's legs are so exceedingly long that he is obliged to spread
his front feet when he wishes to reach the ground with his head. The
elephant has pursued exactly the reverse plan. Using his tremendous
head as a battering ram in fighting, and using his enormous tusks both
in battle and in uprooting young trees, a lengthened neck is
absolutely out of the question. Furthermore his front teeth have grown
so prodigiously that they would interfere with his getting his mouth
to water. Accordingly, his nose has lengthened its tip until it
reaches the level of his feet, and this nose becomes to him the main
organ of grasp and of touch. To drink, its end is inserted in the pool
and water is drawn up the nostril. If the animal were to attempt to
draw it all the way back into his throat, it would inevitably strangle
him by getting into his windpipe. Accordingly, when the nose is well
filled with water, the tip of it is inserted in his mouth, and the
water discharged by a quick puff. The horse has taken a method
intermediate between these. It had moderately lengthened both neck and
head in order to get to the ground with its nipping teeth, and thus to
gather the grasses which serve as its principal food.
The mammalian teeth, while of four kinds, really in most animals serve
but two purposes. The front teeth consist of the incisors and canines,
and are used for biting. The hind teeth, consisting of premolars and
molars, are used for grinding. In the horse, the jaw has lengthened
between these two sets, carrying the biting teeth far forward of the
molars. It is this gap in the row of the horse's teeth which makes it
possible for us to insert the bit into his mouth.
Now comes a strange accident into the life of our American horse.
Creatures of the same kin had been evolving in Europe and Africa, but
the developments are more distinctly horselike, it would seem, in our
own country. Then for some reason the horse disappeared completely
from American soil. Doubtless two things happened. First of all, some
of them migrated across a stretch of open country which then connected
America with Asia in the neighborhood of Bering Strait. These
creatures spread first over Asia and then over Africa and Europe,
leaving their skeletons scattered over this enormous stretch of
country. Asses and zebras are still found abundantly and widely
scattered, but the wild horse of to-day is seen only in western Asia.
What happened to those who remained in America we shall possibly never
know. Some surmise that a fly not unlike the tsetse-fly of Africa
killed them out. Perhaps the members of the cat family, which are
steadily growing larger and fiercer, fed on their young if not upon
the older ones, and exterminated them. Perhaps the Glacial period
which followed was too cold for them. But, whatever may have been the
cause these horses died out not only in North but also in South
America, to which country they had spread.
The old world horse was the companion of man. The skeletons of those
found with early man in the caves of Europe look as if the horse had
been a creature to draw man's burdens and to serve him for food,
rather than to bear him upon its back. Its roasted bones are often
found about the old tribal fires. Upon the discovery of the new world
the Spaniards brought with them to Mexico and to the Mississippi
Valley the horses which carried them in their battles against the
Indians. In the course of these frays many riders were killed and
their horses roamed wild. Slowly they made their way to the western
plains; gradually they became tougher and more wiry; their diminished
hoofs learned to catch more carefully in the rocks of their mountain
home; and the mustang and bronco of more recent years are the
descendants of the little dawn horse, whose dainty skeleton is found
in the rocks over which his later descendants, after a long stretch of
perhaps four million years, are now running.