Discourses
by
Thomas H. Huxley
Part 5 out of 5
The third part contains an "Attempt to give a sound explanation of the
ancient history of the earth."
I suppose that it would be very easy to pick holes in the details of
Kant's speculations, whether cosmological, or specially telluric, in
their application. But for all that, he seems to me to have been the
first person to frame a complete system of geological speculation by
founding the doctrine of evolution.
With as much truth as Hutton, Kant could say, "I take things just as I
find them at present, and, from these, I reason with regard to that which
must have been." Like Hutton, he is never tired of pointing out that "in
Nature there is wisdom, system, and consistency." And, as in these great
principles, so in believing that the cosmos has a reproductive operation
"by which a ruined constitution may be repaired," he forestalls Hutton;
while, on the other hand, Kant is true to science. He knows no bounds to
geological speculation but those of the intellect. He reasons back to a
beginning of the present state of things; he admits the possibility of an
end.
I have said that the three schools of geological speculation which I have
termed Catastrophism, Uniformitarianism, and Evolutionism, are commonly
supposed to be antagonistic to one another; and I presume it will have
become obvious that in my belief, the last is destined to swallow up the
other two. But it is proper to remark that each of the latter has kept
alive the tradition of precious truths.
CATASTROPHISM has insisted upon the existence of a practically unlimited
bank of force, on which the theorist might draw; and it has cherished the
idea of the development of the earth from a state in which its form, and
the forces which it exerted, were very different from those we now know.
That such difference of form and power once existed is a necessary part
of the doctrine of evolution.
UNIFORMITARIANISM, on the other hand, has with equal justice insisted
upon a practically unlimited bank of time, ready to discount any quantity
of hypothetical paper. It has kept before our eyes the power of the
infinitely little, time being granted, and has compelled us to exhaust
known causes, before flying to the unknown.
To my mind there appears to be no sort of necessary theoretical
antagonism between Catastrophism and Uniformitarianism. On the contrary,
it is very conceivable that catastrophes may be part and parcel of
uniformity. Let me illustrate my case by analogy. The working of a clock
is a model of uniform action; good time-keeping means uniformity of
action. But the striking of the clock is essentially a catastrophe; the
hammer might be made to blow up a barrel of gunpowder, or turn on a
deluge of water; and, by proper arrangement, the clock, instead of
marking the hours, might strike at all sorts of irregular periods, never
twice alike, in the intervals, force, or number of its blows.
Nevertheless, all these irregular, and apparently lawless, catastrophes
would be the result of an absolutely uniformitarian action; and we might
have two schools of clock-theorists, one studying the hammer and the
other the pendulum.
Still less is there any necessary antagonists between either of these
doctrines and that of Evolution, which embraces all that is sound in both
Catastrophism and Uniformitarianism, while it rejects the arbitrary
assumptions of the one and the, as arbitrary, limitations of the other.
Nor is the value of the doctrine of Evolution to the philosophic thinker
diminished by the fact that it applies the same method to the living and
the not-living world; and embraces, in one stupendous analogy, the growth
of a solar system from molecular chaos, the shaping of the earth from the
nebulous cub-hood of its youth, through innumerable changes and
immeasurable ages, to its present form; and the development of a living
being from the shapeless mass of protoplasm we term a germ.
I do not know whether Evolutionism can claim that amount of currency
which would entitle it to be called British popular geology; but, more or
less vaguely, it is assuredly present in the minds of most geologists.
Such being the three phases of geological speculation, we are now in
position to inquire which of these it is that Sir William Thomson calls
upon us to reform in the passages which I have cited.
It is obviously Uniformitarianism which the distinguished physicist takes
to be the representative of geological speculation in general. And thus a
first issue is raised, inasmuch as many persons (and those not the least
thoughtful among the younger geologists) do not accept strict
Uniformitarianism as the final form of geological speculation. We should
say, if Hutton and Playfair declare the course of the world to have been
always the same, point out the fallacy by all means; but, in so doing, do
not imagine that you are proving modern geology to be in opposition to
natural philosophy. I do not suppose that, at the present day, any
geologist would be found to maintain absolute Uniformitarianism, to deny
that the rapidity of the rotation of the earth _may_ be diminishing, that
the sun _may_ be waxing dim, or that the earth itself _may_ be cooling.
Most of us, I suspect, are Gallios, "who care for none of these things,"
being of opinion that, true or fictitious, they have made no practical
difference to the earth, during the period of which a record is preserved
in stratified deposits.
The accusation that we have been running counter to the _principles_ of
natural philosophy, therefore, is devoid of foundation. The only question
which can arise is whether we have, or have not, been tacitly making
assumptions which are in opposition to certain conclusions which may be
drawn from those principles. And this question subdivides itself into
two:--the first, are we really contravening such conclusions? the second,
if we are, are those conclusions so firmly based that we may not
contravene them? I reply in the negative to both these questions, and I
will give you my reasons for so doing. Sir William Thomson believes that
he is able to prove, by physical reasonings, "that the existing state of
things on the earth, life on the earth--all geological history showing
continuity of life--must be limited within some such period of time as
one hundred million years" (_loc. cit._ p. 25).
The first inquiry which arises plainly is, has it ever been denied that
this period _may_ be enough for the purposes of geology?
The discussion of this question is greatly embarrassed by the vagueness
with which the assumed limit is, I will not say defined, but indicated,--
"some such period of past time as one hundred million years." Now does
this mean that it may have been two, or three, or four hundred million
years? Because this really makes all the difference.[15]
[Footnote 15: Sir William Thomson implies (_loc. cit_. p. 16) that the
precise time is of no consequence: "the principle is the same"; but, as
the principle is admitted, the whole discussion turns on its practical
results.]
I presume that 100,000 feet may be taken as a full allowance for the
total thickness of stratified rocks containing traces of life; 100,000
divided by 100,000,000 = 0.001. Consequently, the deposit of 100,000 feet
of stratified rock in 100,000,000 years means that the deposit has taken
place at the rate of 1/1000 of a foot, or, say, 1/83 of an inch, per
annum.
Well, I do not know that any one is prepared to maintain that, even
making all needful allowances, the stratified rocks may not have been
formed, on the average, at the rate of 1/83 of an inch per annum. I
suppose that if such could be shown to be the limit of world-growth, we
could put up with the allowance without feeling that our speculations had
undergone any revolution. And perhaps, after all, the qualifying phrase
"some such period" may not necessitate the assumption of more than 1/166
or 1/249 or 1/332 of an inch of deposit per year, which, of course, would
give us still more ease and comfort.
But, it may be said, that it is biology, and not geology, which asks for
so much time--that the succession of life demands vast intervals; but
this appears to me to be reasoning in a circle. Biology takes her time
from geology. The only reason we have for believing in the slow rate of
the change in living forms is the fact that they persist through a series
of deposits which, geology informs us, have taken a long while to make.
If the geological clock is wrong, all the naturalist will have to do is
to modify his notions of the rapidity of change accordingly. And I
venture to point out that, when we are told that the limitation of the
period during which living beings have inhabited this planet to one, two,
or three hundred million years requires a complete revolution in
geological speculation, the _onus probandi_ rests on the maker of the
assertion, who brings forward not a shadow of evidence in its support.
Thus, if we accept the limitation of time placed before us by Sir W.
Thomson, it is not obvious, on the face of the matter, that we shall have
to alter, or reform, our ways in any appreciable degree; and we may
therefore proceed with much calmness, and indeed much indifference, as to
the result, to inquire whether that limitation is justified by the
arguments employed in its support.
These arguments are three in number.--
I. The first is based upon the undoubted fact that the tides tend to
retard the rate of the earth's rotation upon its axis. That this must be
so is obvious, if one considers, roughly, that the tides result from the
pull which the sun and the moon exert upon the sea, causing it to act as
a sort of break upon the rotating solid earth.
Kant, who was by no means a mere "abstract philosopher," but a good
mathematician and well versed in the physical science of his time, not
only proved this in an essay of exquisite clearness and intelligibility,
now more than a century old,[16] but deduced from it some of its more
important consequences, such as the constant turning of one face of the
moon towards the earth.
[Footnote 16: "Untersuchung der Frage oh die Erde in ihrer Umdrehung um
die Achse, wodurch sie die Abwechselung des Tages und der Nacht
hervorbringt, einige Veraenderung seit den ersten Zeiten ihres Ursprunges
erlitten habe, &c."--KANT's _Saemmntliche Werke_, Bd. i. p. 178.]
But there is a long step from the demonstration of a tendency to the
estimation of the practical value of that tendency, which is all with
which we are at present concerned. The facts bearing on this point appear
to stand as follows:--
It is a matter of observation that the moon's mean motion is (and has for
the last 3,000 years been) undergoing an acceleration, relatively to the
rotation of the earth. Of course this may result from one of two causes:
the moon may really have been moving more swiftly in its orbit; or the
earth may have been rotating more slowly on its axis.
Laplace believed he had accounted for this phenomenon by the fact that
the eccentricity of the earth's orbit has been diminishing throughout
these 3,000 years. This would produce a diminution of the mean attraction
of the sun on the moon; or, in other words, an increase in the attraction
of the earth on the moon; and, consequently, an increase in the rapidity
of the orbital motion of the latter body. Laplace, therefore, laid the
responsibility of the acceleration upon the moon, and if his views were
correct, the tidal retardation must either be insignificant in amount, or
be counteracted by some other agency.
Our great astronomer, Adams, however, appears to have found a flaw in
Laplace's calculation, and to have shown that only half the observed
retardation could be accounted for in the way he had suggested. There
remains, therefore, the other half to be accounted for; and here, in the
absence of all positive knowledge, three sets of hypotheses have been
suggested.
(_a_.) M. Delaunay suggests that the earth is at fault, in consequence of
the tidal retardation. Messrs. Adams, Thomson, and Tait work out this
suggestion, and, "on a certain assumption as to the proportion of
retardations due to the sun and moon," find the earth may lose twenty-two
seconds of time in a century from this cause.[17]
[Footnote 17: Sir W. Thomson, _loc. cit_. p. 14.]
(_b_.) But M. Dufour suggests that the retardation of the earth (which is
hypothetically assumed to exist) may be due in part, or wholly, to the
increase of the moment of inertia of the earth by meteors falling upon
its surface. This suggestion also meets with the entire approval of Sir
W. Thomson, who shows that meteor-dust, accumulating at the rate of one
foot in 4,000 years, would account for the remainder of retardation.[18]
[Footnote 18: _Ibid._ p. 27.]
(_c_.) Thirdly, Sir W. Thomson brings forward an hypothesis of his own
with respect to the cause of the hypothetical retardation of the earth's
rotation:--
"Let us suppose ice to melt from the polar regions (20 deg. round each pole,
we may say) to the extent of something more than a foot thick, enough to
give 1.1 foot of water over those areas, or 0.006 of a foot of water if
spread over the whole globe, which would, in reality, raise the sea-level
by only some such undiscoverable difference as three-fourths of an inch
or an inch. This, or the reverse, which we believe might happen any year,
and could certainly not be detected without far more accurate
observations and calculations for the mean sea-level than any hitherto
made, would slacken or quicken the earth's rate as a timekeeper by one-
tenth of a second per year."[19]
[Footnote 19: _Ibid._]
I do not presume to throw the slightest doubt upon the accuracy of any of
the calculations made by such distinguished mathematicians as those who
have made the suggestions I have cited. On the contrary, it is necessary
to my argument to assume that they are all correct. But I desire to point
out that this seems to be one of the many cases in which the admitted
accuracy of mathematical process is allowed to throw a wholly
inadmissible appearance of authority over the results obtained by them.
Mathematics may be compared to a mill of exquisite workmanship, which
grinds you stuff of any degree of fineness; but, nevertheless, what you
get out depends upon what you put in; and as the grandest mill in the
world will not extract wheat-flour from peascods, so pages of formulae
will not get a definite result out of loose data.
In the present instance it appears to be admitted:--
1. That it is not absolutely certain, after all, whether the moon's mean
motion is undergoing acceleration, or the earth's rotation
retardation.[20] And yet this is the key of the whole position.
[Footnote 20: It will be understood that I do not wish to deny that the
earth's rotation _may be_ undergoing retardation.]
2. If the rapidity of the earth's rotation is diminishing, it is not
certain how much of that retardation is due to tidal friction, how much
to meteors, how much to possible excess of melting over accumulation of
polar ice, during the period covered by observation, which amounts, at
the outside, to not more than 2,600 years.
3. The effect of a different distribution of land and water in modifying
the retardation caused by tidal friction, and of reducing it, under some
circumstances, to a minimum, does not appear to be taken into account.
4. During the Miocene epoch the polar ice was certainly many feet thinner
than it has been during, or since, the Glacial epoch. Sir W. Thomson
tells us that the accumulation of something more than a foot of ice
around the poles (which implies the withdrawal of, say, an inch of water
from the general surface of the sea) will cause the earth to rotate
quicker by one-tenth of a second per annum. It would appear, therefore,
that the earth may have been rotating, throughout the whole period which
has elapsed from the commencement of the Glacial epoch down to the
present time, one, or more, seconds per annum quicker than it rotated
during the Miocene epoch.
But, according to Sir W. Thomson's calculation, tidal retardation will
only account for a retardation of 22" in a century, or 22/100 (say 1/5)
of a second per annum.
Thus, assuming that the accumulation of polar ice since the Miocene epoch
has only been sufficient to produce ten times the effect of a coat of ice
one foot thick, we shall have an accelerating cause which covers all the
loss from tidal action, and leaves a balance of 4/5 of a second per annum
in the way of acceleration.
If tidal retardation can be thus checked and overthrown by other
temporary conditions, what becomes of the confident assertion, based upon
the assumed uniformity of tidal retardation, that ten thousand million
years ago the earth must have been rotating more than twice as fast as at
present, and, therefore, that we geologists are "in direct opposition to
the principles of Natural Philosophy" if we spread geological history
over that time?
II. The second argument is thus stated by Sir W. Thomson:--"An article,
by myself, published in 'Macmillan's Magazine' for March 1862, on the age
of the sun's heat, explains results of investigation into various
questions as to possibilities regarding the amount of heat that the sun
could have, dealing with it as you would with a stone, or a piece of
matter, only taking into account the sun's dimensions, which showed it to
be possible that the sun may have already illuminated the earth for as
many as one hundred million years, but at the same time rendered it
almost certain that he had not illuminated the earth for five hundred
millions of years. The estimates here are necessarily very vague; but
yet, vague as they are, I do not know that it is possible, upon any
reasonable estimate founded on known properties of matter, to say that we
can believe the sun has really illuminated the earth for five hundred
million years."[21]
[Footnote 21: _Loc. cit._ p. 20.]
I do not wish to "Hansardise" Sir William Thomson by laying much stress
on the fact that, only fifteen years ago he entertained a totally
different view of the origin of the sun's heat, and believed that the
energy radiated from year to year was supplied from year to year--a
doctrine which would have suited Hutton perfectly. But the fact that so
eminent a physical philosopher has, thus recently, held views opposite to
those which he now entertains, and that he confesses his own estimates to
be "very vague," justly entitles us to disregard those estimates, if any
distinct facts on our side go against them. However, I am not aware that
such facts exist. As I have already said, for anything I know, one, two,
or three hundred millions of years may serve the needs of geologists
perfectly well.
III. The third line of argument is based upon the temperature of the
interior of the earth. Sir W. Thomson refers to certain investigations
which prove that the present thermal condition of the interior of the
earth implies either a heating of the earth within the last 20,000 years
of as much as 100 deg. F., or a greater heating all over the surface at some
time further back than 20,000 years, and then proceeds thus:--
"Now, are geologists prepared to admit that, at some time within the last
20,000 years, there has been all over the earth so high a temperature as
that? I presume not; no geologist--no _modern_ geologist--would for a
moment admit the hypothesis that the present state of underground heat is
due to a heating of the surface at so late a period as 20,000 years ago.
If that is not admitted we are driven to a greater heat at some time more
than 20,000 years ago. A greater heating all over the surface than 100 deg.
Fahrenheit would kill nearly all existing plants and animals, I may
safely say. Are modern geologists prepared to say that all life was
killed off the earth 50,000, 100,000, or 200,000 years ago? For the
uniformity theory, the further back the time of high surface-temperature
is put the better; but the further back the time of heating, the hotter
it must have been. The best for those who draw most largely on time is
that which puts it furthest back; and that is the theory that the heating
was enough to melt the whole. But even if it was enough to melt the
whole, we must still admit some limit, such as fifty million years, one
hundred million years, or two or three hundred million years ago. Beyond
that we cannot go."[22]
[Footnote 22: _Loc. cit._ p. 24.]
It will be observed that the "limit" is once again of the vaguest,
ranging from 50,000,000 years to 300,000,000. And the reply is, once
more, that, for anything that can be proved to the contrary, one or two
hundred million years might serve the purpose, even of a thoroughgoing
Huttonian uniformitarian, very well.
But if, on the other hand, the 100,000,000 or 200,000,000 years appear to
be insufficient for geological purposes, we must closely criticise the
method by which the limit is reached. The argument is simple enough.
_Assuming_ the earth to be nothing but a cooling mass, the quantity of
heat lost per year, _supposing_ the rate of cooling to have been uniform,
multiplied by any given number of years, will be given the minimum
temperature that number of years ago.
But is the earth nothing but a cooling mass, "like a hot-water jar such
as is used in carriages," or "a globe of sandstone," and has its cooling
been uniform? An affirmative answer to both these questions seems to be
necessary to the validity of the calculations on which Sir W. Thomson
lays so much stress.
Nevertheless it surely may be urged that such affirmative answers are
purely hypothetical, and that other suppositions have an equal right to
consideration.
For example, is it not possible that, at the prodigious temperature which
would seem to exist at 100 miles below the surface, all the metallic
bases may behave as mercury does at a red heat, when it refuses to
combine with oxygen; while, nearer the surface, and therefore at a lower
temperature, they may enter into combination (as mercury does with oxygen
a few degrees below its boiling-point), and so give rise to a heat
totally distinct from that which they possess as cooling bodies? And has
it not also been proved by recent researches that the quality of the
atmosphere may immensely affect its permeability to heat; and,
consequently, profoundly modify the rate of cooling the globe as a whole?
I do not think it can be denied that such conditions may exist, and may
so greatly affect the supply, and the loss, of terrestrial heat as to
destroy the value of any calculations which leave them out of sight.
My functions as your advocate are at an end. I speak with more than the
sincerity of a mere advocate when I express the belief that the case
against us has entirely broken down. The cry for reform which has been
raised without, is superfluous, inasmuch as we have long been reforming
from within, with all needful speed. And the critical examination of the
grounds upon which the very grave charge of opposition to the principles
of Natural Philosophy has been brought against us, rather shows that we
have exercised a wise discrimination in declining, for the present, to
meddle with our foundations.
XI
PALAEONTOLOGY AND THE DOCTRINE OF EVOLUTION
[1870]
It is now eight years since, in the absence of the late Mr. Leonard
Horner, who then presided over us, it fell to my lot, as one of the
Secretaries of this Society, to draw up the customary Annual Address. I
availed myself of the opportunity to endeavour to "take stock" of that
portion of the science of biology which is commonly called
"palaeontology," as it then existed; and, discussing one after another the
doctrines held by palaeontologists, I put before you the results of my
attempts to sift the well-established from the hypothetical or the
doubtful. Permit me briefly to recall to your minds what those results
were:--
1. The living population of all parts of the earth's surface which have
yet been examined has undergone a succession of changes which, upon the
whole, have been of a slow and gradual character.
2. When the fossil remains which are the evidences of these successive
changes, as they have occurred in any two more or less distant parts of
the surface of the earth, are compared, they exhibit a certain broad and
general parallelism. In other words, certain forms of life in one
locality occur in the same general order of succession as, or are
_homotaxial_ with, similar forms in the other locality.
3. Homotaxis is not to be held identical with synchronism without
independent evidence. It is possible that similar, or even identical,
faunae and florae in two different localities may be of extremely different
ages, if the term "age" is used in its proper chronological sense. I
stated that "geographical provinces, or zones, may have been as
distinctly marked in the Palaeozoic epoch as at present; and those
seemingly sudden appearances of new genera and species which we ascribe
to new creation, may be simple results of migration."
4. The opinion that the oldest known fossils are the earliest forms of
life has no solid foundation.
5. If we confine ourselves to positively ascertained facts, the total
amount of change in the forms of animal and vegetable life, since the
existence of such forms is recorded, is small. When compared with the
lapse of time since the first appearance of these forms, the amount of
change is wonderfully small. Moreover, in each great group of the animal
and vegetable kingdoms, there are certain forms which I termed PERSISTENT
TYPES, which have remained, with but very little apparent change, from
their first appearance to the present time.
6. In answer to the question "What, then, does an impartial survey of the
positively ascertained truths of palaeontology testify in relation to the
common doctrines of progressive modification, which suppose that
modification to have taken place by a necessary progress from more to
less embryonic forms, from more to less generalised types, within the
limits of the period represented by the fossiliferous rocks?" I reply,
"It negatives these doctrines; for it either shows us no evidence of such
modification, or demonstrates such modification as has occurred to have
been very slight; and, as to the nature of that modification, it yields
no evidence whatsoever that the earlier members of any long-continued
group were more generalised in structure than the later ones."
I think that I cannot employ my last opportunity of addressing you,
officially, more properly--I may say more dutifully--than in revising
these old judgments with such help as further knowledge and reflection,
and an extreme desire to get at the truth, may afford me.
1. With respect to the first proposition, I may remark that whatever may
be the case among the physical geologists, catastrophic palaeontologists
are practically extinct. It is now no part of recognised geological
doctrine that the species of one formation all died out and were replaced
by a brand-new set in the next formation. On the contrary, it is
generally, if not universally, agreed that the succession of life has
been the result of a slow and gradual replacement of species by species;
and that all appearances of abruptness of change are due to breaks in the
series of deposits, or other changes in physical conditions. The
continuity of living forms has been unbroken from the earliest times to
the present day.
2, 3. The use of the word "homotaxis" instead of "synchronism" has not,
so far as I know, found much favour in the eyes of geologists. I hope,
therefore, that it is a love for scientific caution, and not mere
personal affection for a bantling of my own, which leads me still to
think that the change of phrase is of importance, and that the sooner it
is made, the sooner shall we get rid of a number of pitfalls which beset
the reasoner upon the facts and theories of geology.
One of the latest pieces of foreign intelligence which has reached us is
the information that the Austrian geologists have, at last, succumbed to
the weighty evidence which M. Barrande has accumulated, and have admitted
the doctrine of colonies. But the admission of the doctrine of colonies
implies the further admission that even identity of organic remains is no
proof of the synchronism of the deposits which contain them.
4. The discussions touching the _Eozoon,_ which commenced in 1864, have
abundantly justified the fourth proposition. In 1862, the oldest record
of life was in the Cambrian rocks; but if the _Eozoon_ be, as Principal
Dawson and Dr. Carpenter have shown so much reason for believing, the
remains of a living being, the discovery of its true nature carried life
back to a period which, as Sir William Logan has observed, is as remote
from that during which the Cambrian rocks were deposited, as the Cambrian
epoch itself is from the tertiaries. In other words, the ascertained
duration of life upon the globe was nearly doubled at a stroke.
5. The significance of persistent types, and of the small amount of
change which has taken place even in those forms which can be shown to
have been modified, becomes greater and greater in my eyes, the longer I
occupy myself with the biology of the past.
Consider how long a time has elapsed since the Miocene epoch. Yet, at
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 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 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'Orleans_. But it is precisely the _Paloeotherium
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.[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.
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 _Tragulidoe_. If _Dichobune_ has
a fore-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 _Hyopopotamus_ and _Choeropotamus_; but _Hyopotamus_ appears to have
had only two toes.
Again, all the great groups of the Ruminants, the _Bovidoe, Antilopidoe,
Camelopardalidoe_, and _Cervidoe_, are represented in the Miocene epoch,
and so are the Camels. The Upper Eocene _Anoplotherium_, which is
intercalary between the Pigs and the _Tragulidoe_, 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,
Hyracotherium_, 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.
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.
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.
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