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Darwiniana

T >> Thomas Henry Huxley >> Darwiniana




[Illustration: Fig. 4.]

Let C _y_ (Fig. 4) be the sea-bottom, _y_ D the shore, _x y_
the sea-level, then the coarser deposit will subside over the region B, the
finer over A, while beyond A there will be no deposit at all; and,
consequently, no record will be kept, simply because no deposit is going
on. Now, suppose that the whole land, C, D, which we have regarded as
stationary, goes down, as it does so, both A and B go further out from the
shore, which will be at _y1_; _x1_, _y1_, being the new
sea-level. The consequence will be that the layer of mud (A), being now,
for the most part, further than the force of the current is strong enough
to convey even the finest _débris_, will, of course, receive no more
deposits, and having attained a certain thickness will now grow no thicker.

We should be misled in taking the thickness of that layer, whenever it may
be exposed to our view, as a record of time in the manner in which we are
now regarding this subject, as it would give us only an imperfect and
partial record: it would seem to represent too short a period of time.

Suppose, on the other hand, that the land (C D) had gone on rising slowly
and gradually--say an inch or two inches in the course of a century,--what
would be the practical effect of that movement? Why, that the sediment A
and B which has been already deposited, would eventually be brought nearer
to the shore-level and again subjected to the wear and tear of the sea; and
directly the sea begins to act upon it, it would of course soon cut up and
carry it way, to a greater or less extent, to be re-deposited further out.

Well, as there is, in all probability, not one single spot on the whole
surface of the earth, which has not been up and down in this way a great
many times, it follows that the thickness of the deposits formed at any
particular spot cannot be taken (even supposing we had at first obtained
correct data as to the rate at which they took place), as affording
reliable information as to the period of time occupied in its deposit. So
that you see it is absolutely necessary from these facts, seeing that our
record entirely consists of accumulations of mud, superimposed one on the
other; seeing in the next place that any particular spots on which
accumulations have occurred, have been constantly moving up and down, and
sometimes out of the reach of a deposit, and at other times its own deposit
broken up and carried away, it follows that our record must be in the
highest degree imperfect, and we have hardly a trace left of thick
deposits, or any definite knowledge of the area that they occupied, in a
great many cases. And mark this! That supposing even that the whole surface
of the earth had been accessible to the geologist,--that man had had access
to every part of the earth, and had made sections of the whole, and put
them all together,--even then his record must of necessity be imperfect.

But to how much has man really access? If you will look at this map you
will see that it represents the proportion of the sea to the earth: this
coloured part indicates all the dry land, and this other portion is the
water. You will notice at once that the water covers three-fifths of the
whole surface of the globe, and has covered it in the same manner ever
since man has kept any record of his own observations, to say nothing of
the minute period during which he has cultivated geological inquiry. So
that three-fifths of the surface of the earth is shut out from us because
it is under the sea. Let us look at the other two-fifths, and see what are
the countries in which anything that may be termed searching geological
inquiry has been carried out: a good deal of France, Germany, and Great
Britain and Ireland, bits of Spain, of Italy, and of Russia, have been
examined, but of the whole great mass of Africa, except parts of the
southern extremity, we know next to nothing; little bits of India, but of
the greater part of the Asiatic continent nothing; bits of the Northern
American States and of Canada, but of the greater part of the continent of
North America, and in still larger proportion, of South America, nothing!

Under these circumstances, it follows that even with reference to that kind
of imperfect information which we can possess, it is only of about the
ten-thousandth part of the accessible parts of the earth that has been
examined properly. Therefore, it is with justice that the most thoughtful
of those who are concerned in these inquiries insist continually upon the
imperfection of the geological record; for, I repeat, it is absolutely
necessary, from the nature of things, that that record should be of the
most fragmentary and imperfect character. Unfortunately this circumstance
has been constantly forgotten. Men of science, like young colts in a fresh
pasture, are apt to be exhilarated on being turned into a new field of
inquiry, to go off at a hand-gallop, in total disregard of hedges and
ditches, to lose sight of the real limitation of their inquiries, and to
forget the extreme imperfection of what is really known. Geologists have
imagined that they could tell us what was going on at all parts of the
earth's surface during a given epoch; they have talked of this deposit
being contemporaneous with that deposit, until, from our little local
histories of the changes at limited spots of the earth's surface, they have
constructed a universal history of the globe as full of wonders and
portents as any other story of antiquity.

But what does this attempt to construct a universal history of the globe
imply? It implies that we shall not only have a precise knowledge of the
events which have occurred at any particular point, but that we shall be
able to say what events, at any one spot, took place at the same time with
those at other spots.

Let us see how far that is in the nature of things practicable. Suppose
that here I make a section of the Lake of Killarney, and here the section
of another lake--that of Loch Lomond in Scotland for instance. The rivers
that flow into them are constantly carrying down deposits of mud, and beds,
or strata, are being as constantly formed, one above the other, at the
bottom of those lakes. Now, there is not a shadow of doubt that in these
two lakes the lower beds are all older than the upper--there is no doubt
about that; but what does _this_ tell us about the age of any given
bed in Loch Lomond, as compared with that of any given bed in the Lake of
Killarney? It is, indeed, obvious that if any two sets of deposits are
separated and discontinuous, there is absolutely no means whatever given
you by the nature of the deposit of saying whether one is much younger or
older than the other; but you may say, as many have said and think, that
the case is very much altered if the beds which we are comparing are
continuous. Suppose two beds of mud hardened into rock,--A and B--are seen
in section. (Fig. 5.)

[Illustration: Fig. 5.]

Well, you say, it is admitted that the lowermost bed is always the older.
Very well; B, therefore, is older than A. No doubt, _as a whole_, it
is so; or if any parts of the two beds which are in the same vertical line
are compared, it is so. But suppose you take what seems a very natural step
further, and say that the part _a_ of the bed A is younger than the
part _b_ of the bed B. Is this sound reasoning? If you find any record
of changes taking place at _b_, did they occur before any events which
took place while _a_ was being deposited? It looks all very plain
sailing, indeed, to say that they did; and yet there is no proof of
anything of the kind. As the former Director of this Institution, Sir H. De
la Beche, long ago showed, this reasoning may involve an entire fallacy. It
is extremely possible that _a_ may have been deposited ages before
_b_. It is very easy to understand how that can be. To return to Fig.
4; when A and B were deposited, they were _substantially_
contemporaneous; A being simply the finer deposit, and B the coarser of the
same detritus or waste of land. Now suppose that that sea-bottom goes down
(as shown in Fig. 4), so that the first deposit is carried no farther than
_a_, forming the bed A1, and the coarse no farther than _b_,
forming the bed B1, the result will be the formation of two continuous
beds, one of fine sediment (A A1) over-lapping another of coarse sediment
(B B1). Now suppose the whole sea-bottom is raised up, and a section
exposed about the point A1; no doubt, _at this spot_, the upper bed is
younger than the lower. But we should obviously greatly err if we concluded
that the mass of the upper bed at A was younger than the lower bed at B;
for we have just seen that they are contemporaneous deposits. Still more
should we be in error if we supposed the upper bed at A to be younger than
the continuation of the lower bed at B1; for A was deposited long before
B1. In fine, if, instead of comparing immediately adjacent parts of two
beds, one of which lies upon another, we compare distant parts, it is quite
possible that the upper may be any number of years older than the under,
and the under any number of years younger than the upper.

Now you must not suppose that I put this before you for the purpose of
raising a paradoxical difficulty; the fact is, that the great mass of
deposits have taken place in sea-bottoms which are gradually sinking, and
have been formed under the very conditions I am here supposing.

Do not run away with the notion that this subverts the principle I laid
down at first. The error lies in extending a principle which is perfectly
applicable to deposits in the same vertical line to deposits which are not
in that relation to one another.

It is in consequence of circumstances of this kind, and of others that I
might mention to you, that our conclusions on and interpretations of the
record are really and strictly only valid so long as we confine ourselves
to one vertical section. I do not mean to tell you that there are no
qualifying circumstances, so that, even in very considerable areas, we may
safely speak of conformably superimposed beds being older or younger than
others at many different points. But we can never be quite sure in coming
to that conclusion, and especially we cannot be sure if there is any break
in their continuity, or any very great distance between the points to be
compared.

Well now, so much for the record itself,--so much for its
imperfections,--so much for the conditions to be observed in interpreting
it, and its chronological indications, the moment we pass beyond the limits
of a vertical linear section.

Now let us pass from the record to that which it contains,--from the book
itself to the writing and the figures on its pages. This writing and these
figures consist of remains of animals and plants which, in the great
majority of cases, have lived and died in the very spot in which we now
find them, or at least in the immediate vicinity. You must all of you be
aware--and I referred to the fact in my last lecture--that there are vast
numbers of creatures living at the bottom of the sea. These creatures, like
all others, sooner or later die, and their shells and hard parts lie at the
bottom; and then the fine mud which is being constantly brought down by
rivers and the action of the wear and tear of the sea, covers them over and
protects them from any further change or alteration; and, of course, as in
process of time the mud becomes hardened and solidified, the shells of
these animals are preserved and firmly imbedded in the limestone or
sandstone which is being thus formed. You may see in the galleries of the
Museum up stairs specimens of limestones in which such fossil remains of
existing animals are imbedded. There are some specimens in which turtles'
eggs have been imbedded in calcareous sand, and before the sun had hatched
the young turtles, they became covered over with calcareous mud, and thus
have been preserved and fossilised.

Not only does this process of imbedding and fossilisation occur with marine
and other aquatic animals and plants, but it affects those land animals and
plants which are drifted away to sea, or become buried in bogs or morasses;
and the animals which have been trodden down by their fellows and crushed
in the mud at the river's bank, as the herd have come to drink. In any of
these cases, the organisms may be crushed or be mutilated, before or after
putrefaction, in such a manner that perhaps only a part will be left in the
form in which it reaches us. It is, indeed, a most remarkable fact, that it
is quite an exceptional case to find a skeleton of any one of all the
thousands of wild land animals that we know are constantly being killed, or
dying in the course of nature: they are preyed on and devoured by other
animals, or die in places where their bodies are not afterwards protected
by mud. There are other animals existing on the sea, the shells of which
form exceedingly large deposits. You are probably aware that before the
attempt was made to lay the Atlantic telegraphic cable, the Government
employed vessels in making a series of very careful observations and
soundings of the bottom of the Atlantic; and although, as we must all
regret, that up to the present time that project has not succeeded, we have
the satisfaction of knowing that it yielded some most remarkable results to
science. The Atlantic Ocean had to be sounded right across, to depths of
several miles in some places, and the nature of its bottom was carefully
ascertained. Well, now, a space of about 1,000 miles wide from east to
west, and I do not exactly know how many from north to south, but at any
rate 600 or 700 miles, was carefully examined, and it was found that over
the whole of that immense area an excessively fine chalky mud is being
deposited; and this deposit is entirely made up of animals whose hard parts
are deposited in this part of the ocean, and are doubtless gradually
acquiring solidity and becoming metamorphosed into a chalky limestone.
Thus, you see, it is quite possible in this way to preserve unmistakable
records of animal and vegetable life. Whenever the sea-bottom, by some of
those undulations of the earth's crust that I have referred to, becomes
up-heaved, and sections or borings are made, or pits are dug, then we
become able to examine the contents and constituents of these ancient
sea-bottoms, and find out what manner of animals lived at that period.

Now it is a very important consideration in its bearing on the completeness
of the record, to inquire how far the remains contained in these
fossiliferous limestones are able to convey anything like an accurate or
complete account of the animals which were in existence at the time of its
formation. Upon that point we can form a very clear judgment, and one in
which there is no possible room for any mistake. There are of course a
great number of animals--such as jellyfishes, and other animals--without
any hard parts, of which we cannot reasonably expect to find any traces
whatever: there is nothing of them to preserve. Within a very short time,
you will have noticed, after they are removed from the water, they dry up
to a mere nothing; certainly they are not of a nature to leave any very
visible traces of their existence on such bodies as chalk or mud. Then
again, look at land animals; it is, as I have said, a very uncommon thing
to find a land animal entire after death. Insects and other carnivorous
animals very speedily pull them to pieces, putrefaction takes place, and
so, out of the hundreds of thousands that are known to die every year, it
is the rarest thing in the world to see one imbedded in such a way that its
remains would be preserved for a lengthened period. Not only is this the
case, but even when animal remains have been safely imbedded, certain
natural agents may wholly destroy and remove them.

Almost all the hard parts of animals--the bones and so on--are composed
chiefly of phosphate of lime and carbonate of lime. Some years ago, I had
to make an inquiry into the nature of some very curious fossils sent to me
from the North of Scotland. Fossils are usually hard bony structures that
have become imbedded in the way I have described, and have gradually
acquired the nature and solidity of the body with which they are
associated; but in this case I had a series of _holes_ in some pieces
of rock, and nothing else. Those holes, however, had a certain definite
shape about them, and when I got a skilful workman to make castings of the
interior of these holes, I found that they were the impressions of the
joints of a backbone and of the armour of a great reptile, twelve or more
feet long. This great beast had died and got buried in the sand; the sand
had gradually hardened over the bones, but remained porous. Water had
trickled through it, and that water being probably charged with a
superfluity of carbonic acid, had dissolved all the phosphate and carbonate
of lime, and the bones themselves had thus decayed and entirely
disappeared; but as the sandstone happened to have consolidated by that
time, the precise shape of the bones was retained. If that sandstone had
remained soft a little longer, we should have known nothing whatsoever of
the existence of the reptile whose bones it had encased.

How certain it is that a vast number of animals which have existed at one
period on this earth have entirely perished, and left no trace whatever of
their forms, may be proved to you by other considerations. There are large
tracts of sandstone in various parts of the world, in which nobody has yet
found anything but footsteps. Not a bone of any description, but an
enormous number of traces of footsteps. There is no question about them.
There is a whole valley in Connecticut covered with these footsteps, and
not a single fragment of the animals which made them have yet been found.
Let me mention another case while upon that matter, which is even more
surprising than those to which I have yet referred. There is a limestone
formation near Oxford, at a place called Stonesfield, which has yielded the
remains of certain very interesting mammalian animals, and up to this time,
if I recollect rightly, there have been found seven specimens of its lower
jaws, and not a bit of anything else, neither limb-bones nor skull, nor any
part whatever; not a fragment of the whole system! Of course, it would be
preposterous to imagine that the beasts had nothing else but a lower jaw!
The probability is, as Dr. Buckland showed, as the result of his
observations on dead dogs in the river Thames, that the lower jaw, not
being secured by very firm ligaments to the bones of the head, and being a
weighty affair, would easily be knocked off, or might drop away from the
body as it floated in water in a state of decomposition. The jaw would thus
be deposited immediately, while the rest of the body would float and drift
away altogether, ultimately reaching the sea, and perhaps becoming
destroyed. The jaw becomes covered up and preserved in the river silt, and
thus it comes that we have such a curious circumstance as that of the lower
jaws in the Stonesfield slates. So that, you see, faulty as these layers of
stone in the earth's crust are, defective as they necessarily are as a
record, the account of contemporaneous vital phenomena presented by them
is, by the necessity of the case, infinitely more defective and
fragmentary.

It was necessary that I should put all this very strongly before you,
because, otherwise, you might have been led to think differently of the
completeness of our knowledge by the next facts I shall state to you.

The researches of the last three-quarters of a century have, in truth,
revealed a wonderful richness of organic life in those rocks. Certainly not
fewer than thirty or forty thousand different species of fossils have been
discovered. You have no more ground for doubting that these creatures
really lived and died at or near the places in which we find them than you
have for like scepticism about a shell on the sea-shore. The evidence is as
good in the one case as in the other.

Our next business is to look at the general character of these fossil
remains, and it is a subject which will be requisite to consider carefully;
and the first point for us is to examine how much the extinct _Flora_
and _Fauna_ as a _whole_--disregarding altogether the
_succession_ of their constituents, of which I shall speak
afterwards--differ from the _Flora_ and _Fauna_ of the present
day;--how far they differ in what we _do_ know about them, leaving
altogether out of consideration speculations based upon what we _do
not_ know.

I strongly imagine that if it were not for the peculiar appearance that
fossilised animals have, any of you might readily walk through a museum
which contains fossil remains mixed up with those of the present forms of
life, and I doubt very much whether your uninstructed eyes would lead you
to see any vast or wonderful difference between the two. If you looked
closely, you would notice, in the first place, a great many things very
like animals with which you are acquainted now: you would see differences
of shape and proportion, but on the whole a close similarity.

I explained what I meant by ORDERS the other day, when I described the
animal kingdom as being divided into sub-kingdoms, classes and orders. If
you divide the animal kingdom into orders you will find that there are
above one hundred and twenty. The number may vary on one side or the other,
but this is a fair estimate. That is the sum total of the orders of all the
animals which we know now, and which have been known in past times, and
left remains behind.

Now, how many of those are absolutely extinct? That is to say, how many of
these orders of animals have lived at a former period of the world's
history but have at present no representatives? That is the sense in which
I meant to use the word "extinct." I mean that those animals did live on
this earth at one time, but have left no one of their kind with us at the
present moment. So that estimating the number of extinct animals is a sort
of way of comparing the past creation as a whole with the present as a
whole. Among the mammalia and birds there are none extinct; but when we
come to the reptiles there is a most wonderful thing: out of the eight
orders, or thereabouts, which you can make among reptiles, one-half are
extinct. These diagrams of the plesiosaurus, the ichthyosaurus, the
pterodactyle, give you a notion of some of these extinct reptiles. And here
is a cast of the pterodactyle and bones of the ichthyosaurus and the
plesiosaurus, just as fresh-looking as if it had been recently dug up in a
churchyard. Thus, in the reptile class, there are no less than half of the
orders which are absolutely extinct. If we turn to the _Amphibia_,
there was one extinct order, the Labyrinthodonts, typified by the large
salamander-like beast shown in this diagram.

No order of fishes is known to be extinct. Every fish that we find in the
strata--to which I have been referring--can be identified and placed in one
of the orders which exist at the present day. There is not known to be a
single ordinal form of insect extinct. There are only two orders extinct
among the _Crustacea_. There is not known to be an extinct order of
these creatures, the parasitic and other worms; but there are two, not to
say three, absolutely extinct orders of this class, the
_Echinodermata_; out of all the orders of the _Coelenterata_ and
_Protozoa_ only one, the Rugose Corals.

So that, you see, out of somewhere about 120 orders of animals, taking them
altogether, you will not, at the outside estimate, find above ten or a
dozen extinct. Summing up all the order of animals which have left remains
behind them, you will not find above ten or a dozen which cannot be
arranged with those of the present day; that is to say, that the difference
does not amount to much more than ten per cent.: and the proportion of
extinct orders of plants is still smaller. I think that that is a very
astounding a most astonishing fact: seeing the enormous epochs of time
which have elapsed during the constitution of the surface of the earth as
it at present exists, it is, indeed, a most astounding thing that the
proportion of extinct ordinal types should be so exceedingly small.

But now, there is another point of view in which we must look at this past
creation. Suppose that we were to sink a vertical pit through the floor
beneath us, and that I could succeed in making a section right through in
the direction of New Zealand, I should find in each of the different beds
through which I passed the remains of animals which I should find in that
stratum and not in the others. First, I should come upon beds of gravel or
drift containing the bones of large animals, such as the elephant,
rhinoceros, and cave tiger. Rather curious things to fall across in
Piccadilly! If I should dig lower still, I should come upon a bed of what
we call the London clay, and in this, as you will see in our galleries up
stairs, are found remains of strange cattle, remains of turtles, palms, and
large tropical fruits; with shell-fish such as you see the like of now only
in tropical regions. If I went below that, I should come upon the chalk,
and there I should find something altogether different, the remains of
ichthyosauria and pterodactyles, and ammonites, and so forth.

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