|
But it by no means follows, because the _Palæotherium_ has much
in
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.
To any one who is acquainted with the
morphology 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
_Palæotherium_. But as further discoveries threw new light upon
its
structure, it was recognised 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 _Palæotherium 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 the 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 palæontological 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 specialised ancestral form. And I found by
correspondence with the
late eminent French anatomist and palæontologist, 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, 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.[3]
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.
All who have occupied themselves with the structure of
_Anchitherium_,
from Cuvier onwards, have acknowledged its many points of
likeness to a
well-known genus of extinct Eocene mammals, _Palæotherium_.
Indeed, as
we have seen, Cuvier regarded his remains of _Anchitherium_ as
those of
a species of _Palæotherium_. Hence, in attempting to trace the
pedigree
of the horse beyond the Miocene epoch and the Anchitheroid form,
I
naturally sought among the various species of Palæotheroid animals
for
its nearest ally, and I was led to conclude that the
_Palæotherium
minus_ (_Plagiolophus_) represented the next step more nearly
than any
form then known.
I think that this opinion was fully
justifiable; but the progress of
investigation has thrown an unexpected light
on the question, and has
brought us much nearer than could have been
anticipated to a knowledge
of the true series of the progenitors of the
horse.
You are all aware that, when your country was first discovered
by
Europeans, there were no traces of the existence of the horse in
any
part of the American continent. The accounts of the conquest of
Mexico
dwell upon the astonishment of the natives of that country when
they
first became acquainted with that astounding phenomenon--a man
seated
upon a horse. Nevertheless, the investigations of American
geologists
have proved that the remains of horses occur in the most
superficial
deposits of both North and South America, just as they do in
Europe.
Therefore, for some reason or other--no feasible suggestion on
that
subject, so far as I know, has been made--the horse must have died
out
on this continent at some period preceding the discovery of America.
Of
late years there has been discovered in your Western Territories
that
marvellous accumulation of deposits, admirably adapted for
the
preservation of organic remains, to which I referred the other
evening,
and which furnishes us with a consecutive series of records of the
fauna
of the older half of the Tertiary epoch, for which we have no
parallel
in Europe. They have yielded fossils in an excellent state
of
conservation and in unexampled number and variety. The researches
of
Leidy and others have shown that forms allied to the _Hipparion_ and
the
_Anchitherium_ are to be found among these remains. But it is
only
recently that the admirably conceived and most thoroughly and
patiently
worked-out investigations of Professor Marsh have given us a just
idea
of the vast fossil wealth, and of the scientific importance, of
these
deposits. I have had the advantage of glancing over the collections
in
Yale Museum; and I can truly say that, so far as my knowledge
extends,
there is no collection from any one region and series of
strata
comparable, for extent, or for the care with which the remains have
been
got together, or for their scientific importance, to the series
of
fossils which he has deposited there. This vast collection has
yielded
evidence bearing upon the question of the pedigree of the horse of
the
most striking character. It tends to show that we must look to
America,
rather than to Europe, for the original seat of the equine series;
and
that the archaic forms and successive modifications of the
horse's
ancestry are far better preserved here than in
Europe.
Professor Marsh's kindness has enabled me to put before you a
diagram,
every figure in which is an actual representation of some specimen
which
is to be seen at Yale at this present time (Fig. 9).
The
succession of forms which he has brought together carries us from
the top to
the bottom of the Tertiaries. Firstly, there is the true
horse. Next we have
the American Pliocene form of the horse
(_Pliohippus_); in the conformation
of its limbs it presents some very
slight deviations from the ordinary horse,
and the crowns of the
grinding teeth are shorter. Then comes the
_Protohippus_, which
represents the European _Hipparion_, having one large
digit and two
small ones on each foot, and the general characters of the
fore-arm and
leg to which I have referred. But it is more valuable than the
European
_Hipparion_, for the reason that it is devoid of some of
the
peculiarities of that form--peculiarities which tend to show that
the
European _Hipparion_ is rather a member of a collateral branch, than
a
form in the direct line of succession. Next, in the backward order
in
time, is the _Miohippus_, which corresponds pretty nearly with
the
_Anchitherium_ of Europe. It presents three complete toes--one
large
median and two smaller lateral ones; and there is a rudiment of
that
digit, which answers to the little finger of the human
hand.
[Illustration: FIG. 9.]
The European record of the pedigree
of the horse stops here; in the
American Tertiaries, on the contrary, the
series of ancestral equine
forms is continued into the Eocene formations. An
older Miocene form,
termed _Mesohippus_, has three toes in front, with a
large splint-like
rudiment representing the little finger; and three toes
behind. The
radius and ulna, the tibia and the fibula, are distinct, and the
short
crowned molar teeth are anchitherold in pattern.
But the most
important discovery of all is the _Orohippus_, which comes
from the Eocene
formation, and is the oldest member of the equine series
as yet known. Here
we find four complete toes on the front limb, three
toes on the hind-limb, a
well-developed ulna, a well-developed fibula,
and short-crowned grinders of
simple pattern.
Thus, thanks to these important researches, it has become
evident that,
so far as our present knowledge extends, the history of the
horse-type
is exactly and precisely that which could have been predicted from
a
knowledge of the principles of evolution. And the knowledge we
now
possess justifies us completely in the anticipation, that when the
still
lower Eocene deposits, and those which belong to the cretaceous
epoch,
have yielded up their remains of ancestral equine animals, we
shall
find, first, a form with four complete toes and a rudiment of
the
innermost or first digit in front, with probably a rudiment of the
fifth
digit in the hind foot;[4] while, in still older forms, the series
of
the digits will be more and more complete, until we come to
the
five-toed animals, in which, if the doctrine of evolution is
well
founded, the whole series must have taken its orgin.
That is what
I mean by demonstrative evidence of evolution. An inductive
hypothesis is
said to be demonstrated when the facts are shown to be in
entire accordance
with it. If that is not scientific proof, there are no
merely inductive
conclusions which can be said to be proved. And the
doctrine of evolution, at
the present time, rests upon exactly as secure
a foundation as the Copernican
theory of the motions of the heavenly
bodies did at the time of its
promulgation. Its logical basis is
precisely of the same character--the
coincidence of the observed facts
with theoretical requirements.
The
only way of escape, if it be a way of escape, from the conclusions
which I
have just indicated, is the supposition that all these different
equine forms
have been created separately at separate epochs of time;
and, I repeat, that
of such an hypothesis as this there neither is, nor
can be, any scientific
evidence; and, assuredly so far as I know, there
is none which is supported,
or pretends to be supported, by evidence or
authority of any other kind. I
can but think that the time will come
when such suggestions as these, such
obvious attempts to escape the
force of demonstration, will be put upon the
same footing as the
supposition made by some writers, who are I believe not
completely
extinct at present, that fossils are mere simulacra, are no
indications
of the former existence of the animals to which they seem to
belong; but
that they are either sports of Nature, or special
creations,
intended--as I heard suggested the other day--to test our
faith.
In fact, the whole evidence is in favour of evolution, and there
is none
against it. And I say this, although perfectly well aware of the
seeming
difficulties which have been built up upon what appears to
the
uninformed to be a solid foundation. I meet constantly with the
argument
that the doctrine of evolution cannot be well founded, because
it
requires the lapse of a very vast period of time; while the duration
of
life upon the earth thus implied is inconsistent with the
conclusions
arrived at by the astronomer and the physicist. I may venture to
say
that I am familiar with those conclusions, inasmuch as some years
ago,
when President of the Geological Society of London, I took the
liberty
of criticising them, and of showing in what respects, as it appeared
to
me, they lacked complete and thorough demonstration. But, putting
that
point aside, suppose that, as the astronomers, or some of them, and
some
physical philosophers, tell us, it is impossible that life could
have
endured upon the earth for as long a period as is required by
the
doctrine of evolution--supposing that to be proved--I desire to
be
informed, what is the foundation for the statement that evolution
does
require so great a time? The biologist knows nothing whatever of
the
amount of time which may be required for the process of evolution. It
is
a matter of fact that the equine forms which I have described to
you
occur, in the order stated, in the Tertiary formations. But I have
not
the slightest means of guessing whether it took a million of years,
or
ten millions, or a hundred millions, or a thousand millions of years,
to
give rise to that series of changes. A biologist has no means
of
arriving at any conclusion as to the amount of time which may be
needed
for a certain quantity of organic change. He takes his time from
the
geologist. The geologist, considering the rate at which deposits
are
formed and the rate at which denudation goes on upon the surface of
the
earth, arrives at more or less justifiable conclusions as to the
time
which is required for the deposit of a certain thickness of rocks;
and
if he tells me that the Tertiary formations required 500,000,000
years
for their deposit, I suppose he has good ground for what he says, and
I
take that as a measure of the duration of the evolution of the
horse
from the _Orohippus_ up to its present condition. And, if he is
right,
undoubtedly evolution is a very slow process and requires a great
deal
of time. But suppose, now, that an astronomer or a
physicist--for
instance, my friend Sir William Thomson--tells me that my
geological
authority is quite wrong; and that he has weighty evidence to show
that
life could not possibly have existed upon the surface of the
earth
500,000,000 years ago, because the earth would have then been too hot
to
allow of life, my reply is: "That is not my affair; settle that with
the
geologist, and when you have come to an agreement among yourselves
I
will adopt your conclusion." We take our time from the geologists
and
physicists; and it is monstrous that having taken our time from
the
physical philosopher's clock, the physical philosopher should turn
round
upon us, and say we are too fast or too slow. What we desire to know
is,
is it a fact that evolution took place? As to the amount of time
which
evolution may have occupied, we are in the hands of the physicist
and
the astronomer, whose business it is to deal with those
questions.
I have now, ladies and gentlemen, arrived at the conclusion of
the task
which I set before myself when I undertook to deliver these
lectures. My
purpose has been, not to enable those among you who have paid
no
attention to these subjects before, to leave this room in a condition
to
decide upon the validity or the invalidity of the hypothesis
of
evolution; but I have desired to put before you the principles
upon
which all hypotheses respecting the history of Nature must be
judged;
and furthermore, to make apparent the nature of the evidence and
the
amount of cogency which is to be expected and may be obtained from
it.
To this end, I have not hesitated to regard you as genuine students
and
persons desirous of knowing the truth. I have not shrunk from taking
you
through long discussions, that I fear may have sometimed tried
your
patience; and I have inflicted upon you details which
were
indispensable, but which may well have been wearisome. But I
shall
rejoice--I shall consider that I have done you the greatest
service
which it was in my power to do--if I have thus convinced you that
the
great question which we have been discussing is not one to be dealt
with
by rhetorical flourishes, or by loose and superficial talk; but that
it
requires the keen attention of the trained intellect and the patience
of
the accurate observer.
ON THE PHYSICAL BASIS OF
LIFE
[1868]
In order to make the title of this discourse
generally intelligible, I
have translated the term "Protoplasm," which is the
scientific name of
the substance of which I am about to speak, by the words
"the physical
basis of life." I suppose that, to many, the idea that there is
such a
thing as a physical basis, or matter, of life may be novel--so
widely
spread is the conception of life as a something which works
through
matter, but is independent of it; and even those who are aware
that
matter and life are inseparably connected, may not be prepared for
the
conclusion plainly suggested by the phrase, "_the_ physical basis
or
matter of life," that there is some one kind of matter which is
common
to all living beings, and that their endless diversities are
bound
together by a physical, as well as an ideal, unity. In fact, when
first
apprehended, such a doctrine as this appears almost shocking to
common
sense.
What, truly, can seem to be more obviously different
from one another,
in faculty, in form, and in substance, than the various
kinds of living
beings? What community of faculty can there be between
the
brightly-coloured lichen, which so nearly resembles a mere
mineral
incrustation of the bare rock on which it grows, and the painter,
to
whom it is instinct with beauty, or the botanist, whom it feeds
with
knowledge?
Again, think of the microscopic fungus--a mere
infinitesimal ovoid
particle, which finds space and duration enough to
multiply into
countless millions in the body of a living fly; and then of the
wealth
of foliage, the luxuriance of flower and fruit, which lies between
this
bald sketch of a plant and the giant pine of California, towering to
the
dimensions of a cathedral spire, or the Indian fig, which covers
acres
with its profound shadow, and endures while nations and empires come
and
go around its vast circumference. Or, turning to the other half of
the
world of life, picture to yourselves the great Finner whale, hugest
of
beasts that live, or have lived, disporting his eighty or ninety feet
of
bone, muscle, and blubber, with easy roll, among waves in which
the
stoutest ship that ever left dockyard would flounder hopelessly;
and
contrast him with the invisible animalcules--mere gelatinous
specks,
multitudes of which could, in fact, dance upon the point of a
needle
with the same ease as the angels of the Schoolmen could, in
imagination.
With these images before your minds, you may well ask, what
community of
form, or structure, is there between the animalcule and the
whale; or
between the fungus and the fig-tree? And, _a fortiori_, between
all
four?
Finally, if we regard substance, or material composition,
what hidden
bond can connect the flower which a girl wears in her hair and
the blood
which courses through her youthful veins; or, what is there in
common
between the dense and resisting mass of the oak, or the strong fabric
of
the tortoise, and those broad disks of glassy jelly which may be
seen
pulsating through the waters of a calm sea, but which drain away
to
mere films in the hand which raises them out of their element?
Such
objections as these must, I think, arise in the mind of every one
who
ponders, for the first time, upon the conception of a single
physical basis
of life underlying all the diversities of vital
existence; but I propose to
demonstrate to you that, notwithstanding
these apparent difficulties, a
threefold unity--namely, a unity of
power or faculty, a unity of form, and a
unity of substantial
composition--does pervade the whole living
world.
No very abstruse argumentation is needed, in the first place, to
prove
that the powers, or faculties, of all kinds of living matter, diverse
as
they may be in degree, are substantially similar in kind.
Goethe
has condensed a survey of all powers of mankind into the
well-known
epigram:--
"Warum treibt sich das Volk so und
schreit?
Es will sich ernahren
Kinder zeugen, und die
nahren so gut es vermag.
* * * *
*
Weiter bringt es kein Mensch, stell' er
sich wie
er auch will."
In physiological language this means, that all the
multifarious and
complicated activities of man are comprehensible under three
categories.
Either they are immediately directed towards the maintenance
and
development of the body, or they effect transitory changes in
the
relative positions of parts of the body, or they tend towards
the
continuance of the species. Even those manifestations of intellect,
of
feeling, and of will, which we rightly name the higher faculties,
are
not excluded from this classification, inasmuch as to every one but
the
subject of them, they are known only as transitory changes in
the
relative positions of parts of the body. Speech, gesture, and
every
other form of human action are, in the long run, resolvable
into
muscular contraction, and muscular contraction is but a
transitory
change in the relative positions of the parts of a muscle. But
the
scheme which is large enough to embrace the activities of the
highest
form of life, covers all those of the lower creatures. The lowest
plant,
or animalcule, feeds, grows, and reproduces its kind. In addition,
all
animals manifest those transitory changes of form which we class
under
irritability and contractility; and, it is more than probable, that
when
the vegetable world is thoroughly explored, we shall find all plants
in
possession of the same powers, at one time or other of their existence. |
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