common with the horse, on the one hand, and with the rhinoceros on the
other, that it is the intermediate form through which rhinoceroses have
passed to become horses, or _vice versa_; on the contrary, any such
supposition would certainly be erroneous. Nor do I think it likely that
the transition from the reptile to the bird has been effected by such a
form as _Archæopteryx_. And it is convenient to distinguish these
intermediate forms between two groups, which do not represent the actual
passage from the one group to the other, as _intercalary_ types, from
those _linear_ types which, more or less approximately, indicate the
nature of the steps by which the transition from one group to the other
was effected.
I conceive that such linear forms, constituting a series of natural
gradations between the reptile and the bird, and enabling us to
understand the manner in which the reptilian has been metamorphosed into
the bird type, are really to be found among a group of ancient and
extinct terrestrial reptiles known as the _Ornithoscelida_. The remains
of these animals occur throughout the series of Mesozoic formations,
from the Trias to the Chalk, and there are indications of their
existence even in the later Palæozoic strata.
Most of these reptiles, at present known, are of great size, some having
attained a length of forty feet or perhaps more. The majority resembled
lizards and crocodiles in their general form, and many of them were,
like crocodiles, protected by an armour of heavy bony plates. But, in
others, the hind-limbs elongate and the fore-limbs shorten, until their
relative proportions approach those which are observed in the
short-winged, flightless, ostrich tribe among birds.
The skull is relatively light, and in some cases the jaws, though
bearing teeth, are beak-like at their extremities and appear to have
been enveloped in a horny sheath. In the part of the vertebral column
which lies between the haunch bones and is called the sacrum, a number
of vertebræ may unite together into one whole, and in this respect, as
in some details of its structure, the sacrum of these reptiles
approaches that of birds.
But it is in the structure of the pelvis and of the hind limb that some
of these ancient reptiles present the most remarkable approximation to
birds, and clearly indicate the way by which the most specialised and
characteristic features of the bird may have been evolved from the
corresponding parts in the reptile.
In Fig. 6, the pelvis and hind-limbs of a crocodile, a three-toed bird,
and an ornithoscelidan are represented side by side; and, for facility
of comparison, in corresponding positions; but it must be recollected
that, while the position of the bird's limb is natural, that of the
crocodile is not so. In the bird, the thigh-bone lies close to the body,
and the metatarsal bones of the foot (ii., iii., iv., Fig. 6) are,
ordinarily, raised into a more or less vertical position; in the
crocodile, the thigh-bone stands out at an angle from the body, and the
metatarsal bones (i., ii., iii., iv., Fig. 6) lie flat on the ground.
Hence, in the crocodile, the body usually lies squat between the legs,
while, in the bird, it is raised upon the hind legs, as upon pillars.
In the crocodile, the pelvis is obviously composed of three bones on
each side: the ilium (_Il._), the pubis (_Pb._), and the ischium
(_Is._). In the adult bird there appears to be but one bone on each
side. The examination of the pelvis of a chick, however, shows that
each half is made up of three bones, which answer to those which remain
distinct throughout life in the crocodile. There is, therefore, a
fundamental identity of plan in the construction of the pelvis of both
bird and reptile; though the difference in form, relative size, and
direction of the corresponding bones in the two cases are very great.
[Illustration: FIG. 6.--BIRD. ORNITHOSCELIDAN. CROCODILE.
(The letters have the same signification in all the figures. _Il._,
Ilium; _a_, anterior end; _b_, posterior end _Is._, ischium; _Pb._,
pubis; _T_, tibia; _F_, fibula; _As._, astragalus; _Ca._, calcaneum;
_i_, distal portion of the tarsus; i., ii., iii., iv., metatarsal
bones.)]
But the most striking contrast between the two lies in the bones of the
leg and of that part of the foot termed the tarsus, which follows upon
the leg. In the crocodile, the fibula _(F)_ is relatively large and its
lower end is complete. The tibia _(T)_ has no marked crest at its upper
end, and its lower end is narrow and not pulley-shaped. There are two
rows of separate tarsal bones _(As., Ca., &c.)_ and four distinct
metatarsal bones, with a rudiment of a fifth.
In the bird the fibula is small and its lower end diminishes to a point.
The tibia has a strong crest at its upper end and its lower extremity
passes into a broad pulley. There seem at first to be no tarsal bones;
and only one bone, divided at the end into three heads for the three
toes which are attached to it, appears in the place of the metatarsus.
In a young bird, however, the pulley-shaped apparent end of the tibia is
a distinct bone, which represents the bones marked _As., Ca._, in the
crocodile; while the apparently single metatarsal bone consists of three
bones, which early unite with one another and with an additional bone,
which represents the lower row of bones in the tarsus of the crocodile.
In other words it can be shown by the study of development that the
bird's pelvis and hind limb are simply extreme modifications of the same
fundamental plan as that upon which these parts are modelled in
reptiles.
On comparing the pelvis and hind limb of the ornithoscelidan with that
of the crocodile, on the one side, and that of the bird, on the other
(Fig. 6), it is obvious that it represents a middle term between the
two. The pelvic bones approach the form of those of the birds, and the
direction of the pubis and ischium is nearly that which is
characteristic of birds; the thigh bone, from the direction of its head,
must have lain close to the body; the tibia has a great crest; and,
immovably fitted on to its lower end, there is a pulley-shaped bone,
like that of the bird, but remaining distinct. The lower end of the
fibula is much more slender, proportionally, than in the crocodile. The
metatarsal bones have such a form that they fit together immovably,
though they do not enter into bony union; the third toe is, as in the
bird, longest and strongest. In fact, the ornithoscelidan limb is
comparable to that of an unhatched chick.
[Illustration: FIG. 7.--RESTORATION OF COMPSOGNATHUS LONGIPES.]
Taking all these facts together, it is obvious that the view, which was
entertained by Mantell and the probability of which was demonstrated by
your own distinguished anatomist, Leidy, while much additional evidence
in the same direction has been furnished by Professor Cope, that some of
these animals may have walked upon their hind legs, as birds do,
acquires great weight. In fact, there can be no reasonable doubt that
one of the smaller forms of the _Ornithoscelida, Compsognathus_, the
almost entire skeleton of which has been discovered in the Solenhofen
slates, was a bipedal animal. The parts of this skeleton are somewhat
twisted out of their natural relations, but the accompanying figure
gives a just view of the general form of _Compsognathus_ and of the
proportions of its limbs; which, in some respects, are more completely
bird-like than those of other _Ornithoscelida_.
We have had to stretch the definition of the class of birds so as to
include birds with teeth and birds with paw-like fore-limbs and long
tails. There is no evidence that _Compsognathus_ possessed feathers;
but, if it did, it would be hard indeed to say whether it should be
called a reptilian bird or an avian reptile.
As _Compsognathus_ walked upon its hind legs, it must have made tracks
like those of birds. And as the structure of the limbs of several of the
gigantic _Ornithoscelida_, such as _Iguandon_, leads to the conclusion
that they also may have constantly, or occasionally, assumed the same
attitude, a peculiar interest attaches to the fact that, in the Wealden
strata of England, there are to be found gigantic footsteps, arranged in
order like those of the _Brontozoum_, and which there can be no
reasonable doubt were made by some of the _Ornithoscelida_, the remains
of which are found in the same rocks. And, knowing that reptiles that
walked upon their hind legs and shared many of the anatomical characters
of birds did once exist, it becomes a very important question whether
the tracks in the Trias of Massachusetts, to which I referred some time
ago, and which formerly used to be unhesitatingly ascribed to birds may
not all have been made by Ornithoscelidan reptiles; and whether, if we
could obtain the skeletons of the animals which made these tracks, we
should not find in them the actual steps of the evolutional process by
which reptiles gave rise to birds.
The evidential value of the facts I have brought forward in this Lecture
must be neither over nor under estimated. It is not historical proof of
the occurrence of the evolution of birds from reptiles, for we have no
safe ground for assuming that true birds had not made their appearance
at the commencement of the Mesozoic epoch. It is in fact, quite possible
that all these more or less aviform reptiles of the Mesozoic epoch are
not terms in the series of progression from birds to reptiles at all,
but simply the more or less modified descendants of Palæozoic forms
through which that transition was actually effected.
[Illustration: FIG. 8.--PTERODACTYLUS SPECTABILIS (Von Meyer).]
We are not in a position to say that the known _Ornithoscelida_ are
intermediate in the order of their appearance on the earth between
reptiles and birds. All that can be said is that, if independent
evidence of the actual occurrence of evolution is producible, then these
intercalary forms remove every difficulty in the way of understanding
what the actual steps of the process, in the case of birds, may have
been.
That intercalary forms should have existed in ancient times is a
necessary consequence of the truth of the hypothesis of evolution; and,
hence, the evidence I have laid before you in proof of the existence of
such forms, is, so far as it goes, in favour of that hypothesis.
There is another series of extinct reptiles which may be said to be
intercalary between reptiles and birds, in so far as they combine some
of the characters of these groups; and which, as they possessed the
power of flight, may seem, at first sight, to be nearer representatives
of the forms by which the transition from the reptile to the bird was
effected, than the _Ornithoscelida_.
These are the _Pterosauria_, or Pterodactyles, the remains of which are
met with throughout the series of Mesozoic rocks, from the lias to the
chalk, and some of which attain a great size, their wings having a span
of eighteen or twenty feet. These animals, in the form and proportions
of the head and neck relatively to the body, and in the fact that the
ends of the jaws were often, if not always, more or less extensively
ensheathed in horny beaks, remind us of birds. Moreover, their bones
contained air cavities, rendering them specifically lighter, as is the
case in most birds. The breast-bone was large and keeled, as in most
birds and in bats, and the shoulder girdle is strikingly similar to that
of ordinary birds. But it seems to me that the special resemblance of
pterodactyles to birds ends here, unless I may add the entire absence of
teeth which characterises the great pterodactyles (_Pteranodon_)
discovered by Professor Marsh. All other known pterodactyles have teeth
lodged in sockets. In the vertebral column and the hind-limbs there are
no special resemblances to birds, and when we turn to the wings they are
found to be constructed on a totally different principle from those of
birds.
There are four fingers. These four fingers are large, and three of them,
those which answer to the thumb and two following fingers in my
hand--are terminated by claws, while the fourth is enormously prolonged
and converted into a great jointed style. You see at once, from what I
have stated about a bird's wing, that there could be nothing less like a
bird's wing than this is. It was concluded by general reasoning that
this finger had the office of supporting a web which extended between it
and the body. An existing specimen proves that such was really the case,
and that the pterodactyles were devoid of feathers, but that the fingers
supported a vast web like that of a bat's wing; in fact, there can be no
doubt that this ancient reptile flew after the fashion of a bat.
Thus, though the pterodactyle is a reptile which has become modified in
such a manner as to enable it to fly, and therefore, as might be
expected, presents some points of resemblance to other animals which
fly; it has, so to speak, gone off the line which leads directly from
reptiles to birds, and has become disqualified for the changes which
lead to the characteristic organisation of the latter class. Therefore,
viewed in relation to the classes of reptiles and birds, the
pterodactyles appear to me to be, in a limited sense, intercalary forms;
but they are not even approximately linear, in the sense of exemplifying
those modifications of structure through which the passage from the
reptile to the bird took place.
III
THE DEMONSTRATIVE EVIDENCE OF EVOLUTION
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 palæontology 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 ensure 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 _Equidæ_, 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 those 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-arm contains distinct bones called the radius and
the ulna. The corresponding region in the horse seems 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 apposed 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 grinder. 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.