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Critiques and Addresses by Thomas Henry Huxley

T >> Thomas Henry Huxley >> Critiques and Addresses




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 _Anoplotheridae_, and the Ruminants. The _Anoplotheridae_
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 _Anoplotheridae_, or whether Pigs,
Ruminants, and _Anoplotheridae_ 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 specialization, 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 anyone 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 recognized as such, the number of such forms has
steadily increased among the higher _Mammalia_. Not only do we now
know numerous intercalary forms 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, Hyaenictis_ and _Ictitherium_ as intercalary, or, perhaps,
linear types between the _Viverridae_ and the _Hyaenidae_.

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
organization 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 _Hyaenidae, Proboscidea, Rhinocerotidae_, and _Equidae_ in their
order of filiation from their earliest appearance in the Miocene epoch
to the present time, and Professor Ruetimeyer 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 wide-spread
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_[1]; and, in the lowest Miocene and upper Eocene, only
the last genus occurs. A species of _Anchitherium_ was referred by
Cuvier to the _Palaeotheria_ 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 _Palaeotherium_, especially Cuvier's _Palaeotherium 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 _Palaeotherium_, and approaches that of
the Horse.

[Footnote 1: Hermann von Meyer gave the name of _Anchitherium_ to _A.
Ezguerrae_; 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
_Palaeotherium d'Orleans._ But it is precisely the _Palaeotherium
d'Orleans_ 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.[1] 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 1: 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 ramus of a mandible, which shows that, as
in the _Palaeotheria_, 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 specialization, 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 specialization is less in the _Hipparion_,
and still less in the _Anchitherium_; but yet, as compared with
other mammals, the reduction and specialization 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
_Palceotherium_. The ulna is complete and much larger than in any
of the _Equidae_, while it is more slender than in most of the true
_Palaeotheria_; 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 _Equidae_. The femur is more
like that of the _Palaeotheria_ 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 _Equidae_. 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 _Equidae_; and the metacarpal
bones are longer than the metatarsals, as in the _Palaeotheria_.

In its general form, _Plagiolophus_ resembles a very small and slender
horse[1], and is totally unlike the reluctant, pig-like creature
depicted in Cuvier's restoration of his _Palaeotherium minus_ in the
"Os semens Fossils."

[Footnote 1: 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 _Tragulidae_, or so-called
"Musk-deer," of the present day.

The _Tragulidae_ 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.

In _Cainotherium_ and _Dichobune_ the upper incisors are fully
developed. There are seven grinders; the teeth form a continuous
series without a diastema. The metatarsals, the navicular and cuboid,
and the distal end of the fibula, remain free. In the _Cainotherium_,
also, the second metacarpal is developed, but is much shorter than the
third, while the fifth is absent or rudimentary. In this respect it
resembles _Anoplotherium secundarium_. This circumstance, and the
peculiar pattern of the upper molars in _Cainotherium_, lead me
to hesitate in considering it as the actual ancestor of the modern
_Tragulidae_. If _Dichobune_ has a four-toed fore foot (though I am
inclined to suspect that it resembles _Cainotherium_), it will be a
better representative of the oldest forms of the Traguline series; but
_Dichobune_ occurs in the Middle-Eocene, and is, in fact, the oldest
known artiodactyle mammal. Where, then, must we look for its five-toed
ancestor?

If we follow down other lines of recent and tertiary _Ungulata_, the
same question presents itself. The Pigs are traceable back through
the Miocene epoch to the Upper Eocene, where they appear in the
two well-marked forms of _Hyopotamus_ and _Chaeropotamus_; but
_Hyopotamus_ appears to have had only two toes.

Again, all the great groups of the Ruminants, the _Bovidae,
Antilopidae, Camelopardalidae_, and _Cervidae_, are represented in
the Miocene epoch, and so are the Camels. The Upper Eocene
_Anoplotherium_, which is intercalary between the Pigs and the
_Tragulidae_, has only two or, at most, three toes. Among the scanty
mammals of the Lower Eocene formation we have the perissodactyle
_Ungulata_ represented by _Coryphodon, Hyra-cotherium_, and
_Pliolophus_. Suppose for a moment, for the sake of following out
the argument, that _Pliolophus_ represents the primary stock of the
Perissodactyles, and _Dichobune_ that of the Artiodactyles (though
I am far from saying that such is the case), then we find, in the
earliest fauna of the Eocene epoch to which our investigations carry
us, the two divisions of the _Ungulata_ completely differentiated, and
no trace of any common stock of both, or of five-toed predecessors to
either. With the case of the Horses before us, justifying a belief in
the production of new animal forms by modification of old ones, I see
no escape from the necessity of seeking for these ancestors of the
_Ungulata_ beyond the limits of the Tertiary formations.

I could as soon admit special creation, at once, as suppose that the
Perissodactyles and Artiodactyles had no five-toed ancestors. And when
we consider how large a portion of the Tertiary period elapsed before
_Anchitherium_ was converted into _Equus_, it is difficult to escape
the conclusion that a large proportion of time anterior to the
Tertiary period must have been expended in converting the common stock
of the _Ungulata_ into Perissodactyles and Artiodactyles.

The same moral is inculcated by the study of every other order of
Tertiary monodelphous _Mammalia_. Each of these orders is represented
in the Miocene epoch: the Eocene formation, as I have already said,
contains _Cheiroptera, Insectivora, Rodentia, Ungulata, Carnivora,_
and _Cetacea_. But the _Cheiroptera_ are extreme modifications of the
_Insectivora_, just as the _Cetacea_ are extreme modifications of
the Carnivorous type; and therefore it is to my mind incredible
that monodelphous _Insectivora_ and _Carnivora_ should not have been
abundantly developed, along with _Ungulata_, in the Mesozoic epoch.
But if this be the case, how much further back must we go to find the
common stock of the monodelphous _Mammalia_? As to the _Didelphia_,
if we may trust the evidence which seems to be afforded by their
very scanty remains, a Hypsiprymnoid form existed at the epoch of the
Trias, contemporaneously with a Carnivorous form. At the epoch of the
Trias, therefore, the _Marsupialia_ must have, already existed long
enough to have become differentiated into carnivorous and herbivorous
forms. But the _Monotremata_ are lower forms than the _Didelphia,_
which last are intercalary between the _Ornithodelphia_ and the
_Monodelphia_. To what point of the Palaeozoic epoch, then, must we,
upon any rational estimate, relegate the origin of the _Monotremata_?

The investigation of the occurrence of the classes and of the orders
of the _Sauropsida_ in time points in exactly the same direction.
If, as there is great reason to believe, true Birds existed in the
Triassic epoch, the ornithoscelidous forms by which Reptiles passed
into Birds must have preceded them. In fact there is, even at present,
considerable ground for suspecting the existence of _Dinosauria_ in
the Permian formations; but, in that case, lizards must be of still
earlier date. And if the very small differences which are observable
between the _Crocodilia_ of the older Mesozoic formations and those of
the present day furnish any sort of approximation towards an estimate
of the average rate of change among the _Sauropsida_, it is almost
appalling to reflect how far back in Palaeozoic times we must go,
before we can hope to arrive at that common stock from which the
_Crocodilia, Lacertilia, Ornithoscelida_, and _Plesiosauria_, which
had attained so great a development in the Triassic epoch, must have
been derived.

The _Amphibia_ and _Pisces_ tell the same story. There is not a
single class of vertebrated animals which, when it first appears,
is represented by analogues of the lowest known members of the same
class. Therefore, if there is any truth in the doctrine of evolution,
every class must be vastly older than the first record of its
appearance upon the surface of the globe. But if considerations of
this kind compel us to place the origin of vertebrated animals at
a period sufficiently distant from the Upper Silurian, in which the
first Elasmobranchs and Ganoids occur, to allow of the evolution of
such fishes as these from a Vertebrate as simple as the _Amphioxus_,
I can only repeat that it is appalling to speculate upon the extent to
which that origin must have preceded the epoch of the first recorded
appearance of vertebrate life.