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 Veränderung seit den ersten Zeiten ihres Ursprunges
erlitten habe, &c."--KANT's _Sämmntliche 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° 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° 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°
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.