that time there is reason to believe that every important group in every
order of the _Mammalia_ was represented. Even the comparatively scanty
Eocene fauna yields examples of the orders _Cheiroptera, Insectivora,
Rodentia_, and _Perissodactyla_; of _Artiodactyla_ under both the
Ruminant and the Porcine modifications; of _Caranivora, Cetacea_, and
_Marsupialia_.
Or, if we go back to the older half of the Mesozoic epoch, how truly
surprising it is to find every order of the _Reptilia_, except the
_Ophidia_, represented; while some groups, such as the _Ornithoseclida_
and the _Pterosauria_, more specialised than any which now exist,
abounded.
There is one division of the _Amphibia_ which offers especially important
evidence upon this point, inasmuch as it bridges over the gap between the
Mesozoic and the Palaeozoic formations (often supposed to be of such
prodigious magnitude), extending, as it does, from the bottom of the
Carboniferous series to the top of the Trias, if not into the Lias. I
refer to the Labyrinthodonts. As the Address of 1862 was passing through
the press, I was able to mention, in a note, the discovery of a large
Labyrinthodont, with well-ossified vertebrae, in the Edinburgh coal-field.
Since that time eight or ten distinct genera of Labyrinthodonts have been
discovered in the Carboniferous rocks of England, Scotland, and Ireland,
not to mention the American forms described by Principal Dawson and
Professor Cope. So that, at the present time, the Labyrinthodont Fauna of
the Carboniferous rocks is more extensive and diversified than that of
the Trias, while its chief types, so far as osteology enables us to
judge, are quite as highly organised. Thus it is certain that a
comparatively highly organised vertebrate type, such as that of the
Labyrinthodonts, is capable of persisting, with no considerable change,
through the period represented by the vast deposits which constitute the
Carboniferous, the Permian, and the Triassic formations.
The very remarkable results which have been brought to light by the
sounding and dredging operations, which have been carried on with such
remarkable success by the expeditions sent out by our own, the American,
and the Swedish Governments, under the supervision of able naturalists,
have a bearing in the same direction. These investigations have
demonstrated the existence, at great depths in the ocean, of living
animals in some cases identical with, in others very similar to, those
which are found fossilised in the white chalk. The _Globigerinoe_,
Cyatholiths, Coccospheres, Discoliths in the one are absolutely identical
with those in the other; there are identical, or closely analogous,
species of Sponges, Echinoderms, and Brachiopods. Off the coast of
Portugal, there now lives a species of _Beryx_, which, doubtless, leaves
its bones and scales here and there in the Atlantic ooze, as its
predecessor left its spoils in the mud of the sea of the Cretaceous
epoch.
Many years ago[1] I ventured to speak of the Atlantic mud as "modern
chalk," and I know of no fact inconsistent with the view which Professor
Wyville Thomson has advocated, that the modern chalk is not only the
lineal descendant of the ancient chalk, but that it remains, so to speak,
in the possession of the ancestral estate; and that from the Cretaceous
period (if not much earlier) to the present day, the deep sea has covered
a large part of what is now the area of the Atlantic. But if
_Globigerina_, and _Terebratula caput-serpentis_ and _Beryx_, not to
mention other forms of animals and of plants, thus bridge over the
interval between the present and the Mesozoic periods, is it possible
that the majority of other living things underwent a "sea-change into
something new and strange" all at once?
[Footnote 1: See an article in the _Saturday Review_, for 1858, on
"Chalk, Ancient and Modern."]
6. Thus far I have endeavoured to expand and to enforce by fresh
arguments, but not to modify in any important respect, the ideas
submitted to you on a former occasion. But when I come to the
propositions touching progressive modification, it appears to me, with
the help of the new light which has broken from various quarters, that
there is much ground for softening the somewhat Brutus-like severity with
which, in 1862, I dealt with a doctrine, for the truth of which I should
have been glad enough to be able to find a good foundation. So far,
indeed, as the _Invertebrata_ and the lower _Vertebrata_ are concerned,
the facts and the conclusions which are to be drawn from them appear to
me to remain what they were. For anything that, as yet, appears to the
contrary, the earliest known Marsupials may have been as highly organised
as their living congeners; the Permian lizards show no signs of
inferiority to those of the present day; the Labyrinthodonts cannot be
placed below the living Salamander and Triton; the Devonian Ganoids are
closely related to _Polypterus_ and to _Lepidosiren_.
But when we turn to the higher _Vertebrata_, the results of recent
investigations, however we may sift and criticise them, seem to me to
leave a clear balance in favour of the doctrine of the evolution of
living forms one from another. Nevertheless, in discussing this question,
it is very necessary to discriminate carefully between the different
kinds of evidence from fossil remains which are brought forward in favour
of evolution.
Every fossil which takes an intermediate place between forms of life
already known, may be said, so far as it is intermediate, to be evidence
in favour of evolution, inasmuch as it shows a possible road by which
evolution may have taken place. But the mere discovery of such a form
does not, in itself, prove that evolution took place by and through it,
nor does it constitute more than presumptive evidence in favour of
evolution in general. Suppose A, B, C to be three forms, while B is
intermediate in structure between A and C. Then the doctrine of evolution
offers four possible alternatives. A may have become C by way of B; or C
may have become A by way of B; or A and C may be independent
modifications of B; or A, B, and C may be independent modifications of
some unknown D. Take the case of the Pigs, the _Anoplothcridoe_, and the
Ruminants. The _Anoplothcridoe_ are intermediate between the first and
the last; but this does not tell us whether the Ruminants have come from
the Pigs, or the Pigs from Ruminants, or both from _Anoplothcridoe_, or
whether Pigs, Ruminants, and _Anoplotlicridoe_ alike may not have
diverged from some common stock.
But if it can be shown that A, B, and C exhibit successive stages in the
degree of modification, or specialisation, of the same type; and if,
further, it can be proved that they occur in successively newer deposits,
A being in the oldest and C in the newest, then the intermediate
character of B has quite another importance, and I should accept it,
without hesitation, as a link in the genealogy of C. I should consider
the burden of proof to be thrown upon any one who denied C to have been
derived from A by way of B, or in some closely analogous fashion; for it
is always probable that one may not hit upon the exact line of filiation,
and, in dealing with fossils, may mistake uncles and nephews for fathers
and sons.
I think it necessary to distinguish between the former and the latter
classes of intermediate forms, as _intercalary types_ and _linear types_.
When I apply the former term, I merely mean to say that as a matter of
fact, the form B, so named, is intermediate between the others, in the
sense in which the _Anoplotherium_ is intermediate between the Pigs and
the Ruminants--without either affirming, or denying, any direct genetic
relation between the three forms involved. When I apply the latter term,
on the other hand, I mean to express the opinion that the forms A, B, and
C constitute a line of descent, and that B is thus part of the lineage of
C.
From the time when Cuvier's wonderful researches upon the extinct Mammals
of the Paris gypsum first made intercalary types known, and caused them
to be recognised as such, the number of such forms has steadily increased
among the higher _Mammalia_. Not only do we now know numerous intercalary
forins of _Ungulata_, but M. Gaudry's great monograph upon the fossils of
Pikermi (which strikes me as one of the most perfect pieces of
palaeontological work I have seen for a long time) shows us, among the
Primates, _Mesopithecus_ as an intercalary form between the
_Semnopitheci_ and the _Macaci_; and among the _Carnivora_, _Hyoenictis_
and _Ictitherium_ as intercalary, or, perhaps, linear types between the
_Viverridoe_ and the _Hyoenidoe_.
Hardly any order of the higher _Mammalia_ stands so apparently separate
and isolated from the rest as that of the _Cetacea_; though a careful
consideration of the structure of the pinnipede _Carnivora_, or Seals,
shows, in them, many an approximation towards the still more completely
marine mammals. The extinct _Zeuglodon_, however, presents us with an
intercalary form between the type of the Seals and that of the Whales.
The skull of this great Eocene sea-monster, in fact, shows by the narrow
and prolonged interorbital region; the extensive union of the parietal
bones in a sagittal suture; the well-developed nasal bones; the distinct
and large incisors implanted in premaxillary bones, which take a full
share in bounding the fore part of the gape; the two-fanged molar teeth
with triangular and serrated crowns, not exceeding five on each side in
each jaw; and the existence of a deciduous dentition--its close relation
with the Seals. While, on the other hand, the produced rostral form of
the snout, the long symphysis, and the low coronary process of the
mandible are approximations to the cetacean form of those parts.
The scapula resembles that of the cetacean _Hyperoodon_, but the supra-
spinous fossa is larger and more seal-like; as is the humerus, which
differs from that of the _Cetacea_ in presenting true articular surfaces
for the free jointing of the bones of the fore-arm. In the apparently
complete absence of hinder limbs, and in the characters of the vertebral
column, the _Zeuglodon_ lies on the cetacean side of the boundary line;
so that upon the whole, the Zeuglodonts, transitional as they are, are
conveniently retained in the cetacean order. And the publication, in
1864, of M. Van Beneden's memoir on the Miocene and Pliocene _Squalodon_,
furnished much better means than anatomists previously possessed of
fitting in another link of the chain which connects the existing
_Cetacea_ with _Zeuglodon_. The teeth are much more numerous, although
the molars exhibit the zeuglodont double fang; the nasal bones are very
short, and the upper surface of the rostrum presents the groove, filled
up during life by the prolongation of the ethmoidal cartilage, which is
so characteristic of the majority of the _Cetacea_.
It appears to me that, just as among the existing _Carnivora_, the
walruses and the eared seals are intercalary forms between the fissipede
Carnivora and the ordinary seals, so the Zeuglodonts are intercalary
between the _Carnivora_, as a whole, and the _Cetacea_. Whether the
Zeuglodonts are also linear types in their relation to these two groups
cannot be ascertained, until we have more definite knowledge than we
possess at present, respecting the relations in time of the _Carnivora_
and _Cetacea_.
Thus far we have been concerned with the intercalary types which occupy
the intervals between Families or Orders of the same class; but the
investigations which have been carried on by Professor Gegenbaur,
Professor Cope, and myself into the structure and relations of the
extinct reptilian forms of the _Ornithoscelida_ (or _Dinosauria_ and
_Compsognatha_) have brought to light the existence of intercalary forms
between what have hitherto been always regarded as very distinct classes
of the vertebrate sub-kingdom, namely _Reptilia_ and _Aves_. Whatever
inferences may, or may not, be drawn from the fact, it is now an
established truth that, in many of these _Ornithoscelida_, the hind limbs
and the pelvis are much more similar to those of Birds than they are to
those of Reptiles, and that these Bird-reptiles, or Reptile-birds, were
more or less completely bipedal.
When I addressed you in 1862, I should have been bold indeed had I
suggested that palaeontology would before long show us the possibility of
a direct transition from the type of the lizard to that of the ostrich.
At the present moment, we have, in the _Ornithoscelida_, the intercalary
type, which proves that transition to be something more than a
possibility; but it is very doubtful whether any of the genera of
_Ornithoscelida_ with which we are at present acquainted are the actual
linear types by which the transition from the lizard to the bird was
effected. These, very probably, are still hidden from us in the older
formations.
Let us now endeavour to find some cases of true linear types, or forms
which are intermediate between others because they stand in a direct
genetic relation to them. It is no easy matter to find clear and
unmistakable evidence of filiation among fossil animals; for, in order
that such evidence should be quite satisfactory, it is necessary that we
should be acquainted with all the most important features of the
organisation of the animals which are supposed to be thus related, and
not merely with the fragments upon which the genera and species of the
palaeontologist are so often based. M. Gaudry has arranged the species of
_Hyoenidoe, Proboscidea, Rhinocerotidoe_, and _Equidoe_ in their order of
filiation from their earliest appearance in the Miocene epoch to the
present time, and Professor Rütimeyer has drawn up similar schemes for
the Oxen and other _Ungulata_--with what, I am disposed to think, is a
fair and probable approximation to the order of nature. But, as no one is
better aware than these two learned, acute, and philosophical biologists,
all such arrangements must be regarded as provisional, except in those
cases in which, by a fortunate accident, large series of remains are
obtainable from a thick and widespread series of deposits. It is easy to
accumulate probabilities--hard to make out some particular case in such a
way that it will stand rigorous criticism.
After much search, however, I think that such a case is to be made out in
favour of the pedigree of the Horses.
The genus _Equus_ is represented as far back as the latter part of the
Miocene epoch; but in deposits belonging to the middle of that epoch its
place is taken by two other genera, _Hipparion_ and _Anchitherium_;[2]
and, in the lowest Miocene and upper Eocene, only the last genus occurs.
A species of _Anchitherium_ was referred by Cuvier to the _Paloeotheria_
under the name of _P. aurelianense_. The grinding-teeth are in fact very
similar in shape and in pattern, and in the absence of any thick layer of
cement, to those of some species of _Paloeotherium_, especially Cuvier's
_Paloeotherium minus_, which has been formed into a separate genus,
_Plagiolophus_, by Pomel. But in the fact that there are only six full-
sized grinders in the lower jaw, the first premolar being very small;
that the anterior grinders are as large as, or rather larger than, the
posterior ones; that the second premolar has an anterior prolongation;
and that the posterior molar of the lower jaw has, as Cuvier pointed out,
a posterior lobe of much smaller size and different form, the dentition
of _Anchitherium_ departs from the type of the _Paloeotherium_, and
approaches that of the Horse.
[Footnote 2: Hermann von Meyer gave the name of _Anchitherium_ to _A.
Ezquerroe_; and in his paper on the subject he takes great pains to
distinguish the latter as the type of a new genus, from Cuvier's
_Paloeotherium d'Orléans_. But it is precisely the _Paloeotherium
d'Orléans_ which is the type of Christol's genus _Hipparitherium_; and
thus, though _Hipparitherium_ is of later date than _Anchitherium_, it
seemed to me to have a sort of equitable right to recognition when this
Address was written. On the whole, however, it seems most convenient to
adopt _Anchitherium_.]
Again, the skeleton of _Anchitherium_ is extremely equine. M. Christol
goes so far as to say that the description of the bones of the horse, or
the ass, current in veterinary works, would fit those of _Anchitherium_.
And, in a general way, this may be true enough; but there are some most
important differences, which, indeed, are justly indicated by the same
careful observer. Thus the ulna is complete throughout, and its shaft is
not a mere rudiment, fused into one bone with the radius. There are three
toes, one large in the middle and one small on each side. The femur is
quite like that of a horse, and has the characteristic fossa above the
external condyle. In the British Museum there is a most instructive
specimen of the leg-bones, showing that the fibula was represented by the
external malleolus and by a flat tongue of bone, which extends up from it
on the outer side of the tibia, and is closely ankylosed with the latter
bone.[3] The hind toes are three, like those of the fore leg; and the
middle metatarsal bone is much less compressed from side to side than
that of the horse.
[Footnote 3: I am indebted to M. Gervais for a specimen which indicates
that the fibula was complete, at any rate, in some cases; and for a very
interesting ramps of a mandible, which shows that, as in the
_Paloeotheria_, the hindermost milk-molar of the lower jaw was devoid of
the posterior lobe which exists in the hindermost true molar.]
In the _Hipparion_, the teeth nearly resemble those of the Horses, though
the crowns of the grinders are not so long; like those of the Horses,
they are abundantly coated with cement. The shaft of the ulna is reduced
to a mere style, ankylosed throughout nearly its whole length with the
radius, and appearing to be little more than a ridge on the surface of
the latter bone until it is carefully examined. The front toes are still
three, but the outer ones are more slender than in _Anchitherium_, and
their hoofs smaller in proportion to that of the middle toe; they are, in
fact, reduced to mere dew-claws, and do not touch the ground. In the leg,
the distal end of the fibula is so completely united with the tibia that
it appears to be a mere process of the latter bone, as in the Horses.
In _Equus_, finally, the crowns of the grinding-teeth become longer, and
their patterns are slightly modified; the middle of the shaft of the ulna
usually vanishes, and its proximal and distal ends ankylose with the
radius. The phalanges of the two outer toes in each foot disappear, their
metacarpal and metatarsal bones being left as the "splints."
The _Hipparion_ has large depressions on the face in front of the orbits,
like those for the "larmiers" of many ruminants; but traces of these are
to be seen in some of the fossil horses from the Sewalik Hills; and, as
Leidy's recent researches show, they are preserved in _Anchitherium_.
When we consider these facts, and the further circumstance that the
Hipparions, the remains of which have been collected in immense numbers,
were subject, as M. Gaudry and others have pointed out, to a great range
of variation, it appears to me impossible to resist the conclusion that
the types of the _Anchitherium_, of the _Hipparion_, and of the ancient
Horses constitute the lineage of the modern Horses, the _Hipparion_ being
the intermediate stage between the other two, and answering to B in my
former illustration.
The process by which the _Anchitherium_ has been converted into _Equus_
is one of specialisation, or of more and more complete deviation from
what might be called the average form of an ungulate mammal. In the
Horses, the reduction of some parts of the limbs, together with the
special modification of those which are left, is carried to a greater
extent than in any other hoofed mammals. The reduction is less and the
specialisation is less in the _Hipparion_, and still less in the
_Anchitherium_; but yet, as compared with other mammals, the reduction
and specialisation of parts in the _Anchitherium_ remain great.
Is it not probable then, that, just as in the Miocene epoch, we find an
ancestral equine form less modified than _Equus_, so, if we go back to
the Eocene epoch, we shall find some quadruped related to the
_Anchitherium_, as _Hipparion_ is related to _Equus_, and consequently
departing less from the average form?
I think that this desideratum is very nearly, if not quite, supplied by
_Plagiolophus_, remains of which occur abundantly in some parts of the
Upper and Middle Eocene formations. The patterns of the grinding-teeth of
_Plagiolophus_ are similar to those of _Anchitherium_, and their crowns
are as thinly covered with cement; but the grinders diminish in size
forwards, and the last lower molar has a large hind lobe, convex outwards
and concave inwards, as in _Palueotherium_. The ulna is complete and much
larger than in any of the _Equidoe_, while it is more slender than in
most of the true _Paloeotheria_; it is fixedly united, but not ankylosed,
with the radius. There are three toes in the fore limb, the outer ones
being slender, but less attenuated than in the _Equidoe_. The femur is
more like that of the _Paloeotheria_ than that of the horse, and has only
a small depression above its outer condyle in the place of the great
fossa which is so obvious in the _Equidoe_. The fibula is distinct, but
very slender, and its distal end is ankylosed with the tibia. There are
three toes on the hind foot having similar proportions to those on the
fore foot. The principal metacarpal and metatarsal bones are flatter than
they are in any of the _Equidoe_; and the metacarpal bones are longer
than the metatarsals, as in the _Paloeotheria_.
In its general form, _Plagiolophus_ resembles a very small and slender
horse,[4] and is totally unlike the reluctant, pig-like creature depicted
in Cuvier's restoration of his _Paloeotherium minus_ in the "Ossemens
Fossiles."
[Footnote 4: Such, at least, is the conclusion suggested by the
proportions of the skeleton figured by Cuvier and De Blainville; but
perhaps something between a Horse and an Agouti would be nearest the
mark.]
It would be hazardous to say that _Plagiolophus_ is the exact radical
form of the Equine quadrupeds; but I do not think there can be any
reasonable doubt that the latter animals have resulted from the
modification of some quadruped similar to _Plagiolophus_.
We have thus arrived at the Middle Eocene formation, and yet have traced
back the Horses only to a three-toed stock; but these three-toed forms,
no less than the Equine quadrupeds themselves, present rudiments of the
two other toes which appertain to what I have termed the "average"
quadruped. If the expectation raised by the splints of the Horses that,
in some ancestor of the Horses, these splints would be found to be
complete digits, has been verified, we are furnished with very strong
reasons for looking for a no less complete verification of the
expectation that the three-toed _Plagiolophus_-like "avus" of the horse
must have had a five-toed "atavus" at some earlier period.
No such five-toed "atavus," however, has yet made its appearance among
the few middle and older Eocene _Mammalia_ which are known.
Another series of closely affiliated forms, though the evidence they
afford is perhaps less complete than that of the Equine series, is
presented to us by the _Dichobune_ of the Eocene epoch, the
_Cainotherium_ of the Miocene, and the _Tragulidoe_, or so-called "Musk-
deer," of the present day.
The _Tragulidoe_; have no incisors in the upper jaw, and only six
grinding-teeth on each side of each jaw; while the canine is moved up to
the outer incisor, and there is a diastema in the lower jaw. There are
four complete toes on the hind foot, but the middle metatarsals usually
become, sooner or later, ankylosed into a cannon bone. The navicular and
the cuboid unite, and the distal end of the fibula is ankylosed with the
tibia.