Discourses - Thomas H. Huxley
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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[7] in which case Australia must at
that time have been in continuity with the Arctogaeal continent.
[Footnote 7: Since this Address was read, Mr. Krefft has sent us news of
the discovery in Australia of a freshwater fish of strangely Palaeozoic
aspect, and apparently a Ganoid intermediate between _Dipterus_ and
_Lepidosiren_. [The now well-known _Ceratodus_. 1894.]]
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 to 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.
Again, let me say that I am making no gratuitous assumption of
inconceivable changes. It is clear that the enormous area of Polynesia
is, on the whole, an area over which depression has taken place to an
immense extent; consequently a great continent, or assemblage of
subcontinental masses of land must have existed at some former time, and
that at a recent period, geologically speaking, in the area of the
Pacific. But if that continent had contained Mammals, some of them must
have remained to tell the tale; and as it is well known that these
islands have no indigenous _Mammalia_, it is safe to assume that none
existed. Thus, midway between Australia and South America, each of which
possesses an abundant and diversified mammalian fauna, a mass of land,
which may have been as large as both put together, must have existed
without a mammalian inhabitant. Suppose that the shores of this great
land were fringed, as those of tropical Australia are now, with belts of
mangroves, which would extend landwards on the one side, and be buried
beneath littoral deposits on the other side, as depression went on; and
great beds of mangrove lignite might accumulate over the sinking land.
Let upheaval of the whole now take place, in such a manner as to bring
the emerging land into continuity with the South-American or Australian
continent, and, in course of time, it would be peopled by an extension of
the fauna of one of these two regions--just as I imagine the European
Permian dry land to have been peopled.
I see nothing whatever against the supposition that distributional
provinces of terrestrial life existed in the Devonian epoch, inasmuch as
M. Barrande has proved that they existed much earlier. I am aware of no
reason for doubting that, as regards the grades of terrestrial life
contained in them, one of these may have been related to another as New
Zealand is to Australia, or as Australia is to India, at the present day.
Analogy seems to me to be rather in favour of, than against, the
supposition that while only Ganoid fishes inhabited the fresh waters of
our Devonian land, _Amphibia_ and _Reptilia_, or even higher forms, may
have existed, though we have not yet found them. The earliest
Carboniferous _Amphibia_ now known, such as _Anthracosaurus_, are so
highly specialised that I can by no means conceive that they have been
developed out of piscine forms in the interval between the Devonian and
the Carboniferous periods, considerable as that is. And I take refuge in
one of two alternatives: either they existed in our own area during the
Devonian epoch and we have simply not yet found them; or they formed part
of the population of some other distributional province of that day, and
only entered our area by migration at the end of the Devonian epoch.
Whether _Reptilia_ and _Mammalia_ existed along with them is to me, at
present, a perfectly open question, which is just as likely to receive an
affirmative as a negative answer from future inquirers.
Let me now gather together the threads of my argumentation into the form
of a connected hypothetical view of the manner in which the distribution
of living and extinct animals has been brought about.
I conceive that distinct provinces of the distribution of terrestrial
life have existed since the earliest period at which that life is
recorded, and possibly much earlier; and I suppose, with Mr. Darwin, that
the progress of modification of terrestrial forms is more rapid in areas
of elevation than in areas of depression. I take it to be certain that
Labyrinthodont _Amphibia_ existed in the distributional province which
included the dry land depressed during the Carboniferous epoch; and I
conceive that, in some other distributional provinces of that day, which
remained in the condition of stationary or of increasing dry land, the
various types of the terrestrial _Sauropsida_ and of the _Mammalia_ were
gradually developing.
The Permian epoch marks the commencement of a new movement of upheaval in
our area, which dry land existed in North America, Europe, Asia, and
Africa, as it does now. Into this great new continental area the Mammals,
Birds, and Reptiles developed during the Palaeozoic epoch spread, and
formed the great Triassic Arctogaeal province. But, at the end of the
Triassic period, the movement of depression recommenced in our area,
though it was doubtless balanced by elevation elsewhere; modification and
development, checked in the one province, went on in that "elsewhere";
and the chief forms of Mammals, Birds and Reptiles, as we know them, were
evolved and peopled the Mesozoic continent. I conceive Australia to have
become separated from the continent as early as the end of the Triassic
epoch, or not much later. The Mesozoic continent must, I conceive, have
lain to the east, about the shores of the North Pacific and Indian
Oceans; and I am inclined to believe that it continued along the eastern
side of the Pacific area to what is now the province of Austro-Columbia,
the characteristic fauna of which is probably a remnant of the population
of the latter part of this period.
Towards the latter part of the Mesozoic period the movement of upheaval
around the shores of the Atlantic once more recommenced, and was very
probably accompanied by a depression around those of the Pacific. The
Vertebrate fauna elaborated in the Mesozoic continent moved westward and
took possession of the new lands, which gradually increased in extent up
to, and in some directions after, the Miocene epoch.
It is in favour of this hypothesis, I think, that it is consistent with
the persistence of a general uniformity in the positions of the great
masses of land and water. From the Devonian period, or earlier, to the
present day, the four great oceans, Atlantic, Pacific, Arctic, and
Antarctic, may have occupied their present positions, and only their
coasts and channels of communication have undergone an incessant
alteration. And, finally, the hypothesis I have put before you requires
no supposition that the rate of change in organic life has been either
greater or less in ancient times than it is now; nor any assumption,
either physical or biological, which has not its justification in
analogous phenomena of existing nature.
I have now only to discharge the last duty of my office, which is to
thank you, not only for the patient attention with which you have
listened to me so long to-day, but also for the uniform kindness with
which, for the past two years, you have rendered my endeavours to perform
the important, and often laborious, functions of your President a
pleasure instead of a burden.