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Critiques and Addresses by Thomas Henry Huxley

T >> Thomas Henry Huxley >> Critiques and Addresses




Such is the further commentary which I have to offer upon the
statement of the chief results of palaeontology which I formerly
ventured to lay before you.

But the growth of knowledge in the interval makes me conscious of
an omission of considerable moment in that statement, inasmuch as it
contains no reference to the bearings of palaeontology upon the theory
of the distribution of life; nor takes note of the remarkable manner
in which the facts of distribution, in present and past times, accord
with the doctrine of evolution, especially in regard to land animals.

That connection between palaeontology and geology and the present
distribution of terrestrial animals, which so strikingly impressed
Mr. Darwin, thirty years ago, as to lead him to speak of a "law of
succession of types," and of the wonderful relationship on the same
continent between the dead and the living, has recently received much
elucidation from the researches of Gaudry, of Ruetimeyer, of Leidy,
and of Alphonse Milne-Edwards, taken in connection with the
earlier labours of our lamented colleague Falconer; and it has been
instructively discussed in the thoughtful and ingenious work of Mr.
Andrew Murray "On the Geographical Distribution of Mammals."[1]

[Footnote 1: The paper "On the Form and Distribution of the
Land-tracts during the Secondary and Tertiary Periods respectively;
and on the Effect upon Animal Life which great Changes in Geographical
Configuration have probably produced," by Mr. Searles V. Wood, jun.,
which was published in the _Philosophical Magazine_, in 1862, was
unknown to me when this Address was written. It is well worthy of the
most careful study.]

I propose to lay before you, as briefly as I can, the ideas to which a
long consideration of the subject has given rise in my own mind.

If the doctrine of evolution is sound, one of its immediate
consequences clearly is, that the present distribution of life
upon the globe is the product of two factors, the one being the
distribution which obtained in the immediately preceding epoch, and
the other the character and the extent of the changes which have taken
place in physical geography between the one epoch and the other; or,
to put the matter in another way, the Fauna and Flora of any given
area, in any given epoch, can consist only of such forms of life as
are directly descended from those which constituted the Fauna and
Flora of the same area in the immediately preceding epoch, unless the
physical geography (under which I include climatal conditions) of
the area has been so altered as to give rise to immigration of living
forms from some other area.

The evolutionist, therefore, is bound to grapple with the following
problem whenever it is clearly put before him:--Here are the Faunae of
the same area during successive epochs. Show good cause for believing
either that these Faunae have been derived from one another by gradual
modification, or that the Faunae have reached the area in question
by migration from some area in which they have undergone their
development.

I propose to attempt to deal with this problem, so far as it is
exemplified by the distribution of the terrestrial _Vertebrata_, and I
shall endeavour to show you that it is capable of solution in a sense
entirely favourable to the doctrine of evolution.

I have elsewhere[1] stated at length the reasons which lead me to
recognize four primary distributional provinces for the terrestrial
_Vertebrata_ in the present world, namely,--first, the _Novozelanian_,
or New-Zealand province; secondly, the _Australian_ province,
including Australia, Tasmania, and the Negrito Islands; thirdly,
_Austro-Columbia_, or South America _plus_ North America as far as
Mexico; and fourthly, the rest of the world, or _Arctogaea_, in which
province America north of Mexico constitutes one sub-province, Africa
south of the Sahara a second, Hindostan a third, and the remainder of
the Old World, a fourth.

[Footnote 1: "On the Classification and Distribution of the
Alectoromorphae;" Proceedings of the Zoological Society, 1868.]

Now the truth which Mr. Darwin perceived and promulgated as "the law
of the succession of types" is, that, in all these provinces, the
animals found in Pliocene or later deposits are closely affined to
those which now inhabit the same provinces; and that, conversely, the
forms characteristic of other provinces are absent. North and South
America, perhaps, present one or two exceptions to the last rule, but
they are readily susceptible of explanation. Thus, in Australia, the
later Tertiary mammals are marsupials (possibly with exception of the
Dog and a Rodent or two, as at present). In Austro-Columbia the later
Tertiary fauna exhibits numerous and varied forms of Platyrrhine
Apes, Rodents, Cats, Dogs, Stags, _Edentata_, and Opossums; but, as
at present, no Catarrhine Apes, no Lemurs, no _Insectivora_, Oxen,
Antelopes, Rhinoceroses, nor _Didelphia_ other than Opossums. And in
the wide-spread Arctogaeal province, the Pliocene and later mammals
belong to the same groups as those which now exist in the province.
The law of succession of types, therefore, holds good for the present
epoch as compared with its predecessor. Does it equally well apply to
the Pliocene fauna when we compare it with that of the Miocene epoch?
By great good fortune, an extensive mammalian fauna of the latter
epoch has now become known, in four very distant portions of the
Arctogaeal province which do not differ greatly in latitude. Thus
Falconer and Cautley have made known the fauna of the sub-Himalayas
and the Perim Islands; Gaudry that of Attica; many observers that of
Central Europe and France; and Leidy that of Nebraska, on the eastern
flank of the Rocky Mountains. The results are very striking. The total
Miocene fauna comprises many genera, and species of Catarrhine Apes,
of Bats, of _Insectivora_; of Arctogaeal types of _Rodentia_; of
_Proboscidea_; of equine, rhinocerotic, and tapirine quadrupeds; of
cameline, bovine, antilopine, cervine, and traguline Ruminants; of
Pigs and Hippopotamuses; of _Viverridae_ and _Hyaenidae_ among other
_Carnivora_; with _Edentata_ allied to the Arctogaeal _Orycteropus_
and _Manis_, and not to the Austro-Columbian Edentates. The only type
present in the Miocene, but absent in the existing, fauna of Eastern
Arctogaea, is that of the _Didelphidae_, which, however, remains in
North America.

But it is very remarkable that while the Miocene fauna of the
Arctogaeal province, as a whole, is of the same character as the
existing fauna of the same province, as a whole, the component
elements of the fauna were differently associated. In the Miocene
epoch, North America possessed Elephants, Horses, Rhinoceroses, and
a great number and variety of Ruminants and Pigs, which are absent
in the present indigenous fauna; Europe had its Apes, Elephants,
Rhinoceroses, Tapirs, Musk-deer, Giraffes, Hyaenas, great Cats,
Edentates, and Opossum-like Marsupials, which have equally vanished
from its present fauna; and in Northern India, the African types of
Hippopotamuses, Giraffes, and Elephants were mixed up with what
are now the Asiatic types of the latter, and with Camels, and
Semnopithecine and Pithecine Apes of no less distinctly Asiatic forms.

In fact the Miocene mammalian fauna of Europe and the Himalayan
regions contains, associated together, the types which are at present
separately located in the South-African and Indian sub-provinces of
Arctogaea. Now there is every reason to believe, on other grounds,
that both Hindostan, south of the Ganges, and Africa, south of the
Sahara, were separated by a wide sea from Europe and North Asia during
the Middle and Upper Eocene epochs. Hence it becomes highly probable
that the well-known similarities, and no less remarkable differences,
between the present Faunae of India and South Africa have arisen
in some such fashion as the following. Some time during the Miocene
epoch, possibly when the Himalayan chain was elevated, the bottom of
the nummulitic sea was upheaved and converted into dry land, in the
direction of a line extending from Abyssinia to the mouth of the
Ganges. By this means, the Dekhan on the one hand, and South Africa
on the other, became connected with the Miocene dry land and with one
another. The Miocene mammals spread gradually over this intermediate
dry land; and if the condition of its eastern and western ends offered
as wide contrasts as the valleys of the Ganges and Arabia do now, many
forms which made their way into Africa must have been different from
those which reached the Dekhan, while others might pass into both
these sub-provinces.

That there was a continuity of dry land between Europe and North
America during the Miocene epoch, appears to me to be a necessary
consequence of the fact that many genera of terrestrial mammals, such
as _Castor_, _Hystrix_, _Elephas_, _Mastodon_, _Equus_, _Hipparion_,
_Anchitherium_, _Rhinoceros_, _Cervus_, _Amphicyon_, _Hyaenarctos_,
and _Machairodus_, are common to the Miocene formations of the two
areas, and have as yet been found (except perhaps _Anchitherium_) in
no deposit of earlier age. Whether this connection took place by the
east, or by the west, or by both sides of the Old World, there is at
present no certain evidence, and the question is immaterial to the
present argument; but, as there are good grounds for the belief that
the Australian province and the Indian and South-African sub-provinces
were separated by sea from the rest of Arctogaea before the Miocene
epoch, so it has been rendered no less probable, by the investigations
of Mr. Carrick Moore and Professor Duncan, that Austro-Columbia was
separated by sea from North America during a large part of the Miocene
epoch.

It is unfortunate that we have no knowledge of the Miocene mammalian
fauna of the Australian and Austro-Columbian provinces; but, seeing
that not a trace of a Platyrrhine Ape, of a Procyonine Carnivore, of a
characteristically South-American Rodent, of a Sloth, an Armadillo,
or an Ant-eater has yet been found in Miocene deposits of Arctogaea, I
cannot doubt that they already existed in the Miocene Austro-Columbian
province.

Nor is it less probable that the characteristic types of Australian
Mammalia were already developed in that region in Miocene times.

But Austro-Columbia presents difficulties from which Australia is
free; _Camelidae_ and _Tapiridae_ are now indigenous in South America
as they are in Arctogaea; and, among the Pliocene Austro-Columbian
mammals, the Austro-Columbian genera _Equus_, _Mastodon_, and
_Machairodus_ are numbered. Are these Postmiocene immigrants, or
Praemiocene natives?

Still more perplexing are the strange and interesting forms _Toxodon_,
_Macrauchenia_, _Typotherium_, and a new Anoplotherioid mammal
(_Homalodotherium_) which Dr. Cunningham sent over to me some time ago
from Patagonia. I confess I am strongly inclined to surmise that these
last, at any rate, are remnants of the population of Austro-Columbia
before the Miocene epoch, and were not derived from Arctogaea by way
of the north and east.

The fact that this immense fauna of Miocene Arctogaea is now fully
and richly represented only in India and in South Africa, while it
is shrunk and depauperized in North Asia, Europe, and North America,
becomes at once intelligible, if we suppose that India and South
Africa had but a scanty mammalian population before the Miocene
immigration, while the conditions were highly favourable to the new
comers. It is to be supposed that these new regions offered themselves
to the Miocene Ungulates, as South America and Australia offered
themselves to the cattle, sheep, and horses of modern colonists. But,
after these great areas were thus peopled, came the Glacial epoch,
during which the excessive cold, to say nothing of depression and
ice-covering, must have almost depopulated all the northern parts of
Arctogaea, destroying all the higher mammalian forms, except those
which, like the Elephant and Rhinoceros, could adjust their coats to
the altered conditions. Even these must have been driven away from the
greater part of the area; only those Miocene mammals which had passed
into Hindostan and into South Africa would escape decimation by such
changes in the physical geography of Arctogaea. And when the northern
hemisphere passed into its present condition, these lost tribes of the
Miocene Fauna were hemmed by the Himalayas, the Sahara, the Red Sea,
and the Arabian deserts, within their present boundaries. Now, on the
hypothesis of evolution, there is no sort of difficulty in admitting
that the differences between the Miocene forms of the mammalian
Fauna and those which exist at present are the results of gradual
modification; and, since such differences in distribution as obtain
are readily explained by the changes which have taken place in the
physical geography of the world since the Miocene epoch, it is clear
that the result of the comparison of the Miocene and present Fauna is
distinctly in favour of evolution. Indeed I may go further. I may
say that the hypothesis of evolution explains the facts of Miocene,
Pliocene, and Recent distribution, and that no other supposition even
pretends to account for them. It is, indeed, a conceivable supposition
that every species of Rhinoceros and every species of Hyaena, in the
long succession of forms between the Miocene and the present species,
was separately constructed out of dust, or out of nothing, by
supernatural power; but until I receive distinct evidence of the fact,
I refuse to run the risk of insulting any sane man by supposing that
he seriously holds such a notion.

Let us now take a step further back in time, and inquire into the
relations between the Miocene Fauna and its predecessor of the Upper
Eocene formation.

Here it is to be regretted that our materials for forming a judgment
are nothing to be compared in point of extent or variety with those
which are yielded by the Miocene strata. However, what we do know
of this Upper Eocene Fauna of Europe gives sufficient positive
information to enable us to draw some tolerably safe inferences. It
has yielded representatives of _Insectivora_, of _Cheiroptera_,
of _Rodentia_, of _Carnivora_, of artiodactyle and perissodactyle
_Ungulata_, and of opossum-like Marsupials. No Australian type of
Marsupial has been discovered in the Upper Eocene strata, nor any
Edentate mammal. The genera (except perhaps in the case of some of the
_Insectivora_, _Cheiroptera_, and _Rodentia_) are different from those
of the Miocene epoch, but present a remarkable general similarity to
the Miocene and recent genera. In several cases, as I have already
shown, it has now been clearly made out that the relation between
the Eocene and Miocene forms is such that the Eocene form is the less
specialized; while its Miocene ally is more so, and the specialization
reaches its maximum in the recent forms of the same type.

So far as the Upper Eocene and the Miocene Mammalian Faunae are
comparable, their relations are such as in no way to oppose the
hypothesis that the older are the progenitors of the more recent
forms, while, in some cases, they distinctly favour that hypothesis.
The period in time and the changes in physical geography represented
by the nummulitic deposits are undoubtedly very great, while the
remains of Middle Eocene and Older Eocene Mammals are comparatively
few. The general facies of the Middle Eocene Fauna, however, is quite
that of the Upper. The Older Eocene pre-nummulitic mammalian Fauna
contains Bats, two genera of _Carnivora_, three genera of _Ungulata_
(probably all perissodactyle), and a didelphid Marsupial; all these
forms, except perhaps the Bat and the Opossum, belong to genera
which are not known to occur out of the Lower Eocene formation. The
_Coryphodon_ appears to have been allied to the Miocene and later
Tapirs, while _Pliolophus_, in its skull and dentition, curiously
partakes of both artiodactyle and perissodactyle characters; the third
trochanter upon its femur, and its three-toed hind foot, however,
appear definitely to fix its position in the latter division.

There is nothing, then, in what is known of the older Eocene mammals
of the Arctogaeal province to forbid the supposition that they stood
in an ancestral relation to those of the Calcaire Grossier and the
Gypsum of the Paris basin, and that our present fauna, therefore, is
directly derived from that which already existed in Arctogaea at the
commencement of the Tertiary period. But if we now cross the frontier
between the Cainozoic and the Mesozoic faunae, as they are preserved
within the Arctogaeal area, we meet with an astounding change, and
what appears to be a complete and unmistakable break in the line of
biological continuity.

Among the twelve or fourteen species of _Mammalia_ which are said to
have been found in the Purbecks, not one is a member of the orders
_Cheiroptera_, _Rodentia_, _Ungulata_, or _Carnivora_, which are so
well represented in the Tertiaries. No _Insectivora_ are certainly
known, nor any opossum-like Marsupials. Thus there is a vast negative
difference between the Cainozoic and the Mesozoic mammalian faunae
of Europe. But there is a still more important positive difference,
inasmuch as all these Mammalia appear to be Marsupials belonging to
Australian groups, and thus appertaining to a different distributional
province from the Eocene and Miocene marsupials, which are
Austro-Columbian. So far as the imperfect materials which exist enable
a judgment to be formed, the same law appears to have held good for
all the earlier Mesozoic _Mammalia_. Of the Stonesfield slate mammals,
one, _Amphitherium_, has a definitely Australian character; one,
_Phascolotherium_, may be either Dasyurid or Didelphine; of a third,
_Stereognathus_, nothing can at present be said. The two mammals of
the Trias, also, appear to belong to Australian groups.

Every one is aware of the many curious points of resemblance between
the marine fauna of the European Mesozoic rocks and that which now
exists in Australia. But if there was this Australian facies about
both the terrestrial and the marine faunae of Mesozoic Europe, and
if there is this unaccountable and immense break between the fauna
of Mesozoic and that of Tertiary Europe, is it not a very obvious
suggestion that, in the Mesozoic epoch, the Australian province
included Europe, and that the Arctogaeal province was contained within
other limits? The Arctogaeal province is at present enormous, while
the Australian is relatively small. Why should not these proportions
have been different during the Mesozoic epoch?

Thus I am led to think that by far the simplest and most rational
mode of accounting for the great change which took place in the living
inhabitants of the European area at the end of the Mesozoic epoch, is
the supposition that it arose from a vast alteration of the physical
geography of the globe; whereby an area long tenanted by Cainozoic
forms was brought into such relations with the European area that
migration from the one to the other became possible, and took place on
a great scale.

This supposition relieves us, at once, from the difficulty in which we
were left, some time ago, by the arguments which I used to demonstrate
the necessity of the existence of all the great types of the Eocene
epoch in some antecedent period.

It is this Mesozoic continent (which may well have lain in the
neighbourhood of what are now the shores of the North Pacific Ocean)
which I suppose to have been occupied by the Mesozoic _Monodelphia_;
and it is in this region that I conceive they must have gone through
the long series of changes by which they were specialized into the
forms which we refer to different orders. I think it very probable
that what is now South America may have received the characteristic
elements of its mammalian fauna during the Mesozoic epoch; and there
can be little doubt that the general nature of the change which took
place at the end of the Mesozoic epoch in Europe was the upheaval of
the eastern and northern regions of the Mesozoic sea-bottom into a
westward extension of the Mesozoic continent, over which the mammalian
fauna, by which it was already peopled, gradually spread. This
invasion of the land was prefaced by a previous invasion of the
Cretaceous sea by modern forms of mollusca and fish.

It is easy to imagine how an analogous change might come about in the
existing world. There is, at present, a great difference between the
fauna of the Polynesian Islands and that of the west coast of America.
The animals which are leaving their spoils in the deposits now forming
in these localities are widely different. Hence, if a gradual
shifting of the deep sea, which at present bars migration between the
easternmost of these islands and America, took place to the westward,
while the American side of the sea-bottom was gradually upheaved,
the palaeontologist of the future would find, over the Pacific area,
exactly such a change as I am supposing to have occurred in the
North-Atlantic area at the close of the Mesozoic period. An Australian
fauna would be found underlying an American fauna, and the transition
from the one to the other would be as abrupt as that between the Chalk
and lower Tertiaries; and as the drainage-area of the newly formed
extension of the American continent gave rise to rivers and lakes, the
mammals mired in their mud would differ from those of like deposits on
the Australian side, just as the Eocene mammals differ from those of
the Purbecks.

How do similar reasonings apply to the other great change of
life--that which took place at the end of the Palaeozoic period?

In the Triassic epoch, the distribution of the dry land and of
terrestrial vertebrate life appears to have been, generally, similar
to that which existed in the Mesozoic epoch; so that the Triassic
continents and their faunae seem to be related to the Mesozoic lands
and their faunae, just as those of the Miocene epoch are related to
those of the present day. In fact, as I have recently endeavoured
to prove to the Society, there was an Arctogaeal continent and an
Arctogaeal province of distribution in Triassic times as there is now;
and the _Sauropsida_ and _Marsupialia_ which constituted that
fauna were, I doubt not, the progenitors of the _Sauropsida_ and
_Marsupialia_ of the whole Mesozoic epoch.

Looking at the present terrestrial fauna of Australia, it appears to
me to be very probable that it is essentially a remnant of the
fauna of the Triassic, or even of an earlier, age[1]; in which
case Australia must at that time have been in continuity with the
Arctogaeal continent.

[Footnote 1: Since this Address was read, Mr. Krefft has sent us
news of the discovery in Australia of a fresh-water fish of strangely
Palaeozoic aspect, and apparently a Ganoid intermediate between
_Dipterus_ and _Lepidosiren_.]

But now comes the further inquiry. Where was the highly differentiated
Sauropsidan fauna of the Trias in Palaeozoic times? The supposition
that the Dinosaurian, Crocodilian, Dicynodontian, and Plesiosaurian
types were suddenly created at the end of the Permian epoch may
be dismissed, without further consideration, as a monstrous and
unwarranted assumption. The supposition that all these types were
rapidly differentiated out of _Lacertilia_, in the time represented by
the passage from the Palaeozoic to the Mesozoic formation, appears to
me to be hardly more credible, to say nothing of the indications of
the existence of Dinosaurian forms in the Permian rocks which have
already been obtained.

For my part, I entertain no sort of doubt that the Reptiles, Birds,
and Mammals of the Trias are the direct descendants of Reptiles,
Birds, and Mammals which existed in the latter part of the Palaeozoic
epoch, but not in any area of the present dry land which has yet been
explored by the geologist.

This may seem a bold assumption, but it will not appear unwarrantable
to those who reflect upon the very small extent of the earth's surface
which has hitherto exhibited the remains of the great Mammalian fauna
of the Eocene times. In this respect, the Permian land Vertebrate
fauna appears to me to be related to the Triassic much as the Eocene
is to the Miocene. Terrestrial reptiles have been found in Permian
rocks only in three localities; in some spots of France, and recently
of England, and over a more extensive area in Germany. Who can suppose
that the few fossils yet found in these regions give any sufficient
representation of the Permian fauna?

It may be said that the Carboniferous formations demonstrate the
existence of a vast extent of dry land in the present dry-land area,
and that the supposed terrestrial Palaeozoic Vertebrate Fauna ought to
have left its remains in the Coal-measures, especially as there is now
reason to believe that much of the coal was formed by the accumulation
of spores and sporangia on dry land. But if we consider the matter
more closely, I think that this apparent objection loses its force. It
is clear that, during the Carboniferous epoch, the vast area of land
which is now covered by Coal-measures must have been undergoing a
gradual depression. The dry land thus depressed must, therefore, have
existed, as such, before the Carboniferous epoch--in other words, in
Devonian times--and its terrestrial population may never have been
other than such as existed during the Devonian, or some previous
epoch, although much higher forms may have been developed elsewhere.