Discourses - Thomas H. Huxley
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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.
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 Rutimeyer, 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."[5]
[Footnote 5: The paper "On the Form and Distribution of the Landtracts
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 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[6] stated at length the reasons which lead me to
recognise 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 _Arctogoea_, 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 6: "On the Classification and Distribution of the
Alectoromorphoe;" _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 the 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 widespread
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 _Viverridoe_ and _Hyoenidoe_
among other _Carnivora_; with _Edentata_ allied to the Aretogaeal
_Oryeteropus_ 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 _Didelphidoe_, 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, Hyoenarctos_, 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;
_Cantelidoe_ and _Tapirdoe_ are now indigenous in South America as they
are in Arctogaea; and, among the Pliocene Austro-Columbian mammals, the
Arctogaeal 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
(_Homalodotherhon_) 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 depauperised 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 Faunae 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 specialised; while its Miocene ally is more so, and the
specialisation 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 tine 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 _Carivora_, 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 specialised 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.