and those of its habitation. Arctic animals are white, desert animals
are sand-coloured; dwellers among leaves and grass are green; nocturnal
animals are dusky. These colours are not universal, but are very
general, and are seldom reversed. Going on a little further, we find
birds, reptiles and insects, so tinted and mottled as exactly to match
the rock, or bark, or leaf, or flower they are accustomed to rest
upon--and thereby effectually concealed. Another step in advance, and we
have insects which are formed as well as coloured so as exactly to
resemble particular leaves, or sticks, or mossy twigs, or flowers; and
in these cases very peculiar habits and instincts come into play to aid
in the deception and render the concealment more complete. We now enter
upon a new phase of the phenomena, and come to creatures whose colours
neither conceal them nor make them like vegetable or mineral substances;
on the contrary, they are conspicuous enough, but they completely
resemble some other creature of a quite different group, while they
differ much in outward appearance from those with which all essential
parts of their organization show them to be really closely allied. They
appear like actors or masqueraders dressed up and painted for amusement,
or like swindlers endeavouring to pass themselves off for well-known and
respectable members of society. What is the meaning of this strange
travesty? Does nature descend to imposture or masquerade? We answer, she
does not. Her principles are too severe. There is a use in every detail
of her handiwork. The resemblance of one animal to another is of exactly
the same essential nature as the resemblance to a leaf, or to bark, or
to desert sand, and answers exactly the same purpose. In the one case
the enemy will not attack the leaf or the bark, and so the disguise is a
safeguard; in the other case it is found that for various reasons the
creature resembled is passed over, and not attacked by the usual enemies
of its order, and thus the creature that resembles it has an equally
effectual safeguard. We are plainly shown that the disguise is of the
same nature in the two cases, by the occurrence in the same group of one
species resembling a vegetable substance, while another resembles a
living animal of another group; and we know that the creatures resembled
possess an immunity from attack, by their being always very abundant, by
their being conspicuous and not concealing themselves, and by their
having generally no visible means of escape from their enemies; while,
at the same time, the particular quality that makes them disliked is
often very clear, such as a nasty taste or an indigestible hardness.
Further examination reveals the fact that, in several cases of both
kinds of disguise, it is the female only that is thus disguised; and as
it can be shown that the female needs protection much more than the
male, and that her preservation for a much longer period is absolutely
necessary for the continuance of the race, we have an additional
indication that the resemblance is in all cases subservient to a great
purpose--the preservation of the species.
In endeavouring to explain these phenomena as having been brought about
by variation and natural selection, we start with the fact that white
varieties frequently occur, and when protected from enemies show no
incapacity for continued existence and increase. We know, further, that
varieties of many other tints occasionally occur; and as "the survival
of the fittest" must inevitably weed out those whose colours are
prejudicial and preserve those whose colours are a safeguard, we require
no other mode of accounting for the protective tints of arctic and
desert animals. But this being granted, there is such a perfectly
continuous and graduated series of examples of every kind of protective
imitation, up to the most wonderful cases of what is termed "mimicry,"
that we can find no place at which to draw the line and say,--so far
variation and natural selection will account for the phenomena, but for
all the rest we require a more potent cause. The counter theories that
have been proposed, that of the "special creation" of each imitative
form, that of the action of similar "conditions of existence" for some
of the cases, and of the laws of "hereditary descent and the reversion
to ancestral forms" for others,--have all been shown to be beset with
difficulties, and the two latter to be directly contradicted by some of
the most constant and most remarkable of the facts to be accounted for.
The important part that protective "resemblance" has played in
determining the colours and markings of many groups of animals will
enable us to understand the meaning of one of the most striking facts in
nature, the uniformity in the colours of the vegetable as compared with
the wonderful diversity of the animal world. There appears no good
reason why trees and shrubs should not have been adorned with as many
varied hues and as strikingly designed patterns as birds and
butterflies, since the gay colours of flowers show that there is no
incapacity in vegetable tissues to exhibit them. But even flowers
themselves present us with none of those wonderful designs, those
complicated arrangements of stripes and dots and patches of colour, that
harmonious blending of hues in lines and bands and shaded spots, which
are so general a feature in insects. It is the opinion of Mr. Darwin
that we owe much of the beauty of flowers to the necessity of attracting
insects to aid in their fertilization, and that much of the development
of colour in the animal world is due to "sexual selection," colour being
universally attractive, and thus leading to its propagation and
increase; but while fully admitting this, it will be evident from the
facts and arguments here brought forward, that very much of the
_variety_ both of colour and markings among animals is due to the
supreme importance of concealment, and thus the various tints of
minerals and vegetables have been directly reproduced in the animal
kingdom, and again and again modified as more special protection became
necessary. We shall thus have two causes for the development of colour
in the animal world and shall be better enabled to understand how, by
their combined and separate action, the immense variety we now behold
has been produced. Both causes, however, will come under the general law
of "Utility," the advocacy of which, in its broadest sense, we owe
almost entirely to Mr. Darwin. A more accurate knowledge of the varied
phenomena connected with this subject may not improbably give us some
information both as to the senses and the mental faculties of the lower
animals. For it is evident that if colours which please us also attract
them, and if the various disguises which have been here enumerated are
equally deceptive to them as to ourselves, then both their powers of
vision and their faculties of perception and emotion, must be
essentially of the same nature as our own--a fact of high philosophical
importance in the study of our own nature and our true relations to the
lower animals.[4]
FOOTNOTES:
[4] The author continues this study in Chapter ix of "Darwinism": New
York, Macmillan Co., 1889.--Ed.
THE EVOLUTION OF THE HORSE
THOMAS HENRY HUXLEY
[Professor Huxley as a naturalist, educator, and
controversialist was one of the commanding figures of the
nineteenth century. To physiology and morphology his
researches added much of importance: as an expositor he stood
unapproached. As the bold and witty champion of Darwinism he
gave natural selection an acceptance much more early and wide
than it would otherwise have enjoyed. In 1876 he delivered in
America three lectures on Evolution: the third of the series
is here given. All three are copyrighted and published by D.
Appleton & Co., New York, in a volume which also contains a
lecture on the study of biology. Since 1876 the arguments of
Professor Huxley have been reinforced by the discovery of
many fossils connecting not only the horse, but other
quadrupeds, with species widely different and now extinct.
The most comprehensive collection illustrating the descent of
the horse is to be seen at the American Museum of Natural
History, New York, where also the evolution of tapirs,
camels, llamas, rhinoceroses, dinosaurs, great ground sloths
and other animals are clearly to be traced--in most cases by
remains discovered in America. A capital book on the theme
broached by Professor Huxley is "Animals of the Past," by
Frederic A. Lucas, Curator of the Division of Comparative
Anatomy, United States National Museum, Washington, D. C.,
published by McClure, Phillips & Co., New York.
"The Life and Letters of Professor Huxley," edited by his
son, Leonard Huxley, is a work of rare interest: it is
published by D. Appleton & Co., New York.]
The occurrence of historical facts is said to be demonstrated, when the
evidence that they happened is of such a character as to render the
assumption that they did not happen in the highest degree improbable;
and the question I now have to deal with is, whether evidence in favour
of the evolution of animals of this degree of cogency is, or is not,
obtainable from the record of the succession of living forms which is
presented to us by fossil remains.
Those who have attended to the progress of palaeontology are aware that
evidence of the character which I have defined has been produced in
considerable and continually-increasing quantity during the last few
years. Indeed, the amount and the satisfactory nature of that evidence
are somewhat surprising, when we consider the conditions under which
alone we can hope to obtain it.
It is obviously useless to seek for such evidence, except in localities
in which the physical conditions have been such as to permit of the
deposit of an unbroken, or but rarely interrupted, series of strata
through a long period of time; in which the group of animals to be
investigated has existed in such abundance as to furnish the requisite
supply of remains; and in which, finally, the materials composing the
strata are such as to insure the preservation of these remains in a
tolerably perfect and undisturbed state.
It so happens that the case which, at present, most nearly fulfils all
these conditions is that of the series of extinct animals which
culminates in the horses; by which term I mean to denote not merely the
domestic animals with which we are all so well acquainted, but their
allies, the ass, zebra, quagga, and the like. In short, I use "horses"
as the equivalent of the technical name _Equidae_, which is applied to
the whole group of existing equine animals.
The horse is in many ways a remarkable animal; not least so in the fact
that it presents us with an example of one of the most perfect pieces of
machinery in the living world. In truth, among the works of human
ingenuity it cannot be said that there is any locomotive so perfectly
adapted to its purposes, doing so much work with so small a quantity of
fuel, as this machine of nature's manufacture--the horse. And, as a
necessary consequence of any sort of perfection, of mechanical
perfection as of others, you find that the horse is a beautiful
creature, one of the most beautiful of all land animals. Look at the
perfect balance of its form, and the rhythm and force of its action. The
locomotive machinery is, as you are aware, resident in its slender fore
and hind limbs; they are flexible and elastic levers, capable of being
moved by very powerful muscles; and, in order to supply the engines
which work these levers with the force which they expend, the horse is
provided with a very perfect apparatus for grinding its food and
extracting therefrom the requisite fuel.
Without attempting to take you very far into the region of osteological
detail, I must nevertheless trouble you with some statements respecting
the anatomical structure of the horse; and, more especially, will it be
needful to obtain a general conception of the structure of its fore and
hind limbs, and of its teeth. But I shall only touch upon these points
which are absolutely essential to our inquiry.
Let us turn in the first place to the fore-limb. In most quadrupeds, as
in ourselves, the fore-arms contains distinct bones called the radius
and the ulna. The corresponding region in the horse seem at first to
possess but one bone. Careful observation, however, enables us to
distinguish in this bone a part which clearly answers to the upper end
of the ulna. This is closely united with the chief mass of the bone
which represents the radius, and runs out into a slender shaft which may
be traced for some distance downwards upon the back of the radius, and
then in most cases thins out and vanishes. It takes still more trouble
to make sure of what is nevertheless the fact, that a small part of the
lower end of the bone of the horse's fore-arm, which is only distinct in
a very young foal, is really the lower extremity of the ulna.
What is commonly called the knee of a horse is its wrist. The "cannon
bone" answers to the middle bone of the five metacarpal bones, which
support the palm of the hand in ourselves. The "pastern," "coronary,"
and "coffin" bones of veterinarians answer to the joints of our middle
fingers, while the hoof is simply a greatly enlarged and thickened nail.
But if what lies below the horse's "knee" thus corresponds to the middle
finger in ourselves, what has become of the four other fingers or
digits? We find in the places of the second and fourth digits only two
slender splint-like bones, about two-thirds as long as the cannon bone,
which gradually taper to their lower ends and bear no finger joints, or,
as they are termed, phalanges. Sometimes, small bony or gristly nodules
are to be found at the bases of these two metacarpal splints, and it is
probable that these represent rudiments of the first and fifth toes.
Thus, the part of the horse's skeleton, which corresponds with that of
the human hand, contains one overgrown middle digit, and at least two
imperfect lateral digits; and these answer, respectively, to the third,
the second and the fourth fingers in man.
Corresponding modifications are found in the hind limb. In ourselves,
and in most quadrupeds, the leg contains two distinct bones, a large
bone, the tibia, and a smaller and more slender bone, the fibula. But,
in the horse, the fibula seems, at first, to be reduced to its upper
end; a short slender bone united with the tibia and ending in a point
below, occupying its place. Examination of the lower end of a young
foal's shin-bone, however, shows a distinct portion of osseous matter,
which is the lower end of the fibula; so that the, apparently single,
lower end of the shin-bone is really made up of the coalesced ends of
the tibia and fibula, just as the, apparently single, lower end of the
fore-arm bone is composed of the coalesced radius and ulna.
The heel of the horse is the part commonly known as the hock. The hinder
cannon bone answers to the middle metatarsal bone of the human foot, the
pastern, coronary, and coffin bones, to the middle toe bones; the hind
hoof to the nail; as in the fore-foot. And, as in the fore-foot, there
are merely two splints to represent the second and the fourth toes.
Sometimes a rudiment of a fifth toe appears to be traceable.
The teeth of a horse are not less peculiar than its limbs. The living
engine, like all others, must be well stoked if it is to do its work;
and the horse, if it is to make good its wear and tear, and to exert the
enormous amount of force required for its propulsion, must be well and
rapidly fed. To this end good cutting instruments and powerful and
lasting crushers are needful. Accordingly, the twelve cutting teeth of a
horse are close-set and concentrated in the fore-part of its mouth, like
so many adzes or chisels. The grinders or molars are large, and have an
extremely complicated structure, being composed of a number of different
substances of unequal hardness. The consequence of this is that they
wear away at different rates; and, hence, the surface of each grinder
is always as uneven as that of a good millstone.
I have said that the structure of the grinding teeth is very
complicated, the harder and the softer parts being, as it were,
interlaced with one another. The result of this is that, as the tooth
wears, the crown presents a peculiar pattern, the nature of which is not
very easily deciphered at first, but which it is important we should
understand clearly. Each grinding tooth of the upper jaw has an _outer
wall_ so shaped that, on the worn crown, it exhibits the form of two
crescents, one in front and one behind, with their concave sides turned
outwards. From the inner side of the front crescent, a crescentic _front
ridge_ passes inwards and backwards, and its inner face enlarges into a
strong longitudinal fold or _pillar_. From the front part of the hinder
crescent, a _back ridge_ takes a like direction, and also has its
_pillar_.
The deep interspaces or _valleys_ between these ridges and the outer
wall are filled by bony substance, which is called _cement_, and coats
the whole tooth.
The pattern of the worn face of each grinding tooth of the lower jaw is
quite different. It appears to be formed of two crescent-shaped ridges,
the convexities of which are turned outwards. The free extremity of each
crescent has a _pillar_, and there is a large double _pillar_ where the
two crescents meet. The whole structure is, as it were, imbedded in
cement, which fills up the valleys, as in the upper grinders.
If the grinding faces of an upper and of a lower molar of the same side
are applied together, it will be seen that the opposed ridges are
nowhere parallel, but that they frequently cross; and that thus, in the
act of mastication, a hard surface in the one is constantly applied to a
soft surface in the other, and _vice versa_. They thus constitute a
grinding apparatus of great efficiency, and one which is repaired as
fast as it wears, owing to the long-continued growth of the teeth.
Some other peculiarities of the dentition of the horse must be noticed,
as they bear upon what I shall have to say by and by. Thus the crowns of
the cutting teeth have a peculiar deep pit, which gives rise to the
well-known "mark" of the horse. There is a large space between the outer
incisors and the front grinders. In this space the adult male horse
presents, near the incisors on each side, above and below, a canine or
"tush," which is commonly absent in mares. In a young horse, moreover,
there is not unfrequently to be seen, in front of the first grinder, a
very small tooth, which soon falls out. If this small tooth be counted
as one, it will be found that there are seven teeth behind the canine on
each side; namely, the small tooth in question, and the six great
grinders, among which, by an unusual peculiarity, the foremost tooth is
rather larger than those which follow it.
I have now enumerated those characteristic structures of the horse which
are of most importance for the purpose we have in view.
To any one who is acquainted with the morphology [comparative forms] of
vertebrated animals, they show that the horse deviates widely from the
general structure of mammals; and that the horse type is, in many
respects, an extreme modification of the general mammalian plan. The
least modified mammals, in fact, have the radius and ulna, the tibia and
fibula, distinct and separate. They have five distinct and complete
digits on each foot, and no one of these digits is very much larger than
the rest. Moreover, in the least modified mammals the total number of
the teeth is very generally forty-four, while in horses the usual number
is forty, and in the absence of the canines it may be reduced to
thirty-six; the incisor teeth are devoid of the fold seen in those of
the horse: the grinders regularly diminish in size from the middle of
the series to its front end; while their crowns are short, early attain
their full length, and exhibit simple ridges or tubercles, in place of
the complex foldings of the horse's grinders.
Hence the general principles of the hypothesis of evolution lead to the
conclusion that the horse must have been derived from some quadruped
which possessed five complete digits on each foot; which had the bones
of the fore-arm and of the leg complete and separate; and which
possessed forty-four teeth, among which the crowns of the incisors and
grinders had a simple structure; while the latter gradually increased in
size from before backwards, at any rate in the anterior part of the
series, and had short crowns.
And if the horse has been thus evolved, and the remains of the different
stages of its evolution have been preserved, they ought to present us
with a series of forms in which the number of the digits becomes
reduced; the bones of the fore-arm and leg gradually take on the equine
condition; and the form and arrangement of the teeth successively
approximate to those which obtain in existing horses.
Let us turn to the facts, and see how far they fulfil these requirements
of the doctrine of evolution.
In Europe abundant remains of horses are found in the Quaternary and
later Tertiary strata as far as the Pliocene formation. But these
horses, which are so common in the cave-deposits and in the gravels of
Europe, are in all essential respects like existing horses. And that is
true of all the horses of the latter part of the Pliocene epoch. But in
deposits which belong to the earlier Pliocene and later Miocene epochs,
and which occur in Britain, in France, in Germany, in Greece, in India,
we find animals which are extremely like horses--which, in fact, are so
similar to horses that you may follow descriptions given in works upon
the anatomy of the horse upon the skeletons of these animals--but which
differ in some important particulars. For example, the structure of
their fore and hind limbs is somewhat different. The bones which, in the
horse, are represented by two splints, imperfect below, are as long as
the middle metacarpal and metatarsal bones; and attached to the
extremity of each is a digit with three joints of the same general
character as those of the middle digit, only very much smaller. These
small digits are so disposed that they could have had but very little
functional importance, and they must have been rather of the nature of
the dew-claws, such as are to be found in many ruminant animals. The
_Hipparion_, as the extinct European three-toed horse is called, in
fact, presents a foot similar to that of the American _Protohippus_
(Fig. 9), except that in the _Hipparion_ the smaller digits are situated
farther back and are of smaller proportional size than in the
_Protohippus_.
The ulna is slightly more distinct than in the horse; and the whole
length of it, as a very slender shaft intimately united with the radius,
is completely traceable. The fibula appears to be in the same condition
as in the horse. The teeth of the _Hipparion_ are essentially similar to
those of the horse, but the pattern of the grinders is in some respects
a little more complex, and there is a depression on the face of the
skull in front of the orbit, which is not seen in existing horses.
In the earlier Miocene, and perhaps the later Eocene deposits of some
parts of Europe, another extinct animal has been discovered, which
Cuvier, who first described some fragments of it, considered to be a
_Palaeotherium_. But as further discoveries threw new light on its
structure, it was recognized as a distinct genus under the name of
_Anchitherium_.
In its general characters, the skeleton of _Anchitherium_ is very
similar to that of the horse. In fact, Lartet and De Blainville called
it _Palaeotherium equinum_ or _hippoides_; and De Christol, in 1847, said
that it differed from _Hipparion_ in little more than the characters of
its teeth, and gave it the name of _Hipparitherium_. Each foot possesses
three complete toes; while the lateral toes are much larger in
proportion to the middle toe than in _Hipparion_, and doubtless rested
on the ground in ordinary locomotion.
The ulna is complete and quite distinct from that radius, though firmly
united with the latter. The fibula seems also to have been complete. Its
lower end, though intimately united with that of the tibia, is clearly
marked off from the latter bone.
There are forty-four teeth. The incisors have no strong pit. The canines
seem to have been well developed in both sexes. The first of the seven
grinders, which, as I have said, is frequently absent, and when it does
exist, is small in the horse, is a good-sized and permanent tooth,
while the grinder which follows it is but little larger than the hinder
ones. The crowns of the grinders are short, and though the fundamental
pattern of the horse-tooth is discernible, the front and back ridges are
less curved, the accessory pillars, are wanting, and the valleys, much
shallower, are not filled up with cement.
Seven years ago, when I happened to be looking critically into the
bearing of palaeontological facts upon the doctrine of evolution, it
appeared to me that the _Anchitherium_, the _Hipparion_, and the modern
horses, constitute a series in which the modifications of structure
coincide with the order of chronological occurrence, in the manner in
which they must coincide, if the modern horses really are the result of
the gradual metamorphosis, in the course of the Tertiary epoch, of a
less specialized ancestral form. And I found by correspondence with the
late eminent French anatomist and palaeontologist, M. Lartet, that he had
arrived at the same conclusion from the same data.
That the _Anchitherium_ type had become metamorphosed into the
_Hipparion_ type, and the latter into the _Equine_ type,[5] in the
course of that period of time which is represented by the latter half
of the Tertiary deposits, seemed to me to be the only explanation of the
facts for which there was even a shadow of probability.
And, hence, I have ever since held that these facts afford evidence of
the occurrence of evolution, which, in the sense already defined, may be
termed demonstrative.