Thus we come to the conclusion, strange at first sight, that the MATTER
constituting the living world is identical with that which forms the
inorganic world. And not less true is it that, remarkable as are the
powers or, in other words, as are the FORCES which are exerted by living
beings, yet all these forces are either identical with those which exist
in the inorganic world, or they are convertible into them; I mean in
just the same sense as the researches of physical philosophers have
shown that heat is convertible into electricity, that electricity is
convertible into magnetism, magnetism into mechanical force or chemical
force, and any one of them with the other, each being measurable in
terms of the other,--even so, I say, that great law is applicable to the
living world. Consider why is the skeleton of this horse capable of
supporting the masses of flesh and the various organs forming the living
body, unless it is because of the action of the same forces of cohesion
which combines together the particles of matter composing this piece of
chalk? What is there in the muscular contractile power of the animal
but the force which is expressible, and which is in a certain sense
convertible, into the force of gravity which it overcomes? Or, if you go
to more hidden processes, in what does the process of digestion differ
from those processes which are carried on in the laboratory of the
chemist? Even if we take the most recondite and most complex operations
of animal life--those of the nervous system, these of late years have
been shown to be--I do not say identical in any sense with the
electrical processes--but this has been shown, that they are in some way
or other associated with them; that is to say, that every amount of
nervous action is accompanied by a certain amount of electrical
disturbance in the particles of the nerves in which that nervous action
is carried on. In this way the nervous action is related to electricity
in the same way that heat is related to electricity; and the same sort
of argument which demonstrates the two latter to be related to one
another shows that the nervous forces are correlated to electricity; for
the experiments of M. Dubois Reymond and others have shown that whenever
a nerve is in a state of excitement, sending a message to the muscles or
conveying an impression to the brain, there is a disturbance of the
electrical condition of that nerve which does not exist at other times;
and there are a number of other facts and phenomena of that sort; so
that we come to the broad conclusion that not only as to living matter
itself, but as to the forces that matter exerts, there is a close
relationship between the organic and the inorganic world--the difference
between them arising from the diverse combination and disposition of
identical forces, and not from any primary diversity, so far as we can
see.
I said just now that the Horse eventually died and became converted into
the same inorganic substances from whence all but an inappreciable
fraction of its substance demonstrably originated, so that the actual
wanderings of matter are as remarkable as the transmigrations of the
soul fabled by Indian tradition. But before death has occurred, in the
one sex or the other, and in fact in both, certain products or parts of
the organism have been set free, certain parts of the organisms of the
two sexes have come into contact with one another, and from that
conjunction, from that union which then takes place, there results the
formation of a new being. At stated times the mare, from a particular
part of the interior of her body, called the ovary, gets rid of a minute
particle of matter comparable in all essential respects with that which
we called a cell a little while since, which cell contains a kind of
nucleus in its centre, surrounded by a clear space and by a viscid mass
of protein substance (Fig. 33); and though it is different in appearance
from the eggs which we are mostly acquainted with, it is really an egg.
After a time this minute particle of matter, which may only be a small
fraction of a grain in weight, undergoes a series of changes,--wonderful,
complex changes. Finally, upon its surface there is fashioned a little
elevation, which afterwards becomes divided and marked by a groove. The
lateral boundaries of the groove extend upwards and downwards, and at
length give rise to a double tube. In the upper and smaller tube the
spinal marrow and brain are fashioned; in the lower, the alimentary
canal and heart; and at length two pairs of buds shoot out at the sides
of the body, and they are the rudiments of the limbs. In fact a true
drawing of a section of the embryo in this state would in all essential
respects resemble that diagram of a horse reduced to its simplest
expression, which I first placed before you (Fig. 32).
Slowly and gradually these changes take place. The whole of the body, at
first, can be broken up into "cells," which become in one place
metamorphosed into muscle,--in another place into gristle and bone,--in
another place into fibrous tissue,--and in another into hair; every part
becoming gradually and slowly fashioned, as if there were an artificer
at work in each of these complex structures that I have mentioned. This
embryo, as it is called, then passes into other conditions. I should
tell you that there is a time when the embryos of neither dog, nor
horse, nor porpoise, nor monkey, nor man, can be distinguished by any
essential feature one from the other; there is a time when they each and
all of them resemble this one of the Dog. But as development advances,
all the parts acquire their speciality, till at length you have the
embryo converted into the form of the parent from which it started. So
that, you see, this living animal, this horse, begins its existence as a
minute particle of nitrogenous matter, which, being supplied with
nutriment (derived, as I have shown, from the inorganic world), grows up
according to the special type and construction of its parents, works and
undergoes a constant waste, and that waste is made good by nutriment
derived from the inorganic world; the waste given off in this way being
directly added to the inorganic world. Eventually the animal itself
dies, and, by the process of decomposition, its whole body is returned
to those conditions of inorganic matter in which its substance
originated.
This, then, is that which is true of every living form, from the lowest
plant to the highest animal--to man himself. You might define the life
of every one in exactly the same terms as those which I have now used;
the difference between the highest and the lowest being simply in the
complexity of the developmental changes, the variety of the structural
forms, and the diversity of the physiological functions which are
exerted by each.
If I were to take an oak tree, as a specimen of the plant world, I
should find that it originated in an acorn, which, too, commenced in a
cell; the acorn is placed in the ground, and it very speedily begins to
absorb the inorganic matters I have named, adds enormously to its bulk,
and we can see it, year after year, extending itself upward and
downward, attracting and appropriating to itself inorganic materials,
which it vivifies, and eventually, as it ripens, gives off its own
proper acorns, which again run the same course. But I need not multiply
examples,--from the highest to the lowest the essential features of life
are the same, as I have described in each of these cases.
So much, then, for these particular features of the organic world, which
you can understand and comprehend, so long as you confine yourself to
one sort of living being, and study that only.
But, as you know, horses are not the only living creatures in the
world; and again, horses, like all other animals, have certain
limits--are confined to a certain area on the surface of the earth on
which we live,--and, as that is the simpler matter, I may take that
first. In its wild state, and before the discovery of America, when the
natural state of things was interfered with by the Spaniards, the Horse
was only to be found in parts of the earth which are known to
geographers as the Old World; that is to say, you might meet with horses
in Europe, Asia, or Africa; but there were none in Australia, and there
were none whatsoever in the whole continent of America, from Labrador
down to Cape Horn. This is an empirical fact, and it is what is called,
stated in the way I have given it you, the "Geographical Distribution"
of the Horse.
Why horses should be found in Europe, Asia, and Africa, and not in
America, is not obvious; the explanation that the conditions of life in
America are unfavourable to their existence, and that, therefore, they
had not been created there, evidently does not apply; for when the
invading Spaniards, or our own yeomen farmers, conveyed horses to these
countries for their own use, they were found to thrive well and multiply
very rapidly; and many are even now running wild in those countries, and
in a perfectly natural condition. Now, suppose we were to do for every
animal what we have here done for the Horse,--that is, to mark off and
distinguish the particular district or region to which each belonged;
and supposing we tabulated all these results, that would be called the
Geographical Distribution of animals, while a corresponding study of
plants would yield as a result the Geographical Distribution of plants.
I pass on from that now, as I merely wished to explain to you what I
meant by the use of the term "Geographical Distribution." As I said,
there is another aspect, and a much more important one, and that is, the
relations of the various animals to one another. The Horse is a very
well-defined matter-of-fact sort of animal, and we are all pretty
familiar with its structure. I dare say it may have struck you, that it
resembles very much no other member of the animal kingdom, except
perhaps the Zebra or the Ass. But let me ask you to look along these
diagrams. Here is the skeleton of the Horse, and here the skeleton of
the Dog. You will notice that we have in the Horse a skull, a backbone
and ribs, shoulder-blades and haunch-bones. In the fore-limb, one upper
arm-bone, two fore arm-bones, wrist-bones (wrongly called knee), and
middle hand-bones, ending in the three bones of a finger, the last of
which is sheathed in the horny hoof of the fore-foot: in the hind-limb,
one thigh-bone, two leg-bones, ankle-bones, and middle foot-bones,
ending in the three bones of a toe, the last of which is encased in the
hoof of the hind-foot. Now turn to the Dog's skeleton. We find
identically the same bones, but more of them, there being more toes in
each foot, and hence more toe-bones.
Well, that is a very curious thing! The fact is that the Dog and the
Horse--when one gets a look at them without the outward impediments of
the skin--are found to be made in very much the same sort of fashion.
And if I were to make a transverse section of the Dog, I should find the
same organs that I have already shown you as forming parts of the Horse.
Well, here is another skeleton--that of a kind of Lemur--you see he has
just the same bones; and if I were to make a transverse section of it,
it would be just the same again. In your mind's eye turn him round, so
as to put his backbone in a position inclined obliquely upwards and
forwards, just as in the next three diagrams, which represent the
skeletons of an Orang, a Chimpanzee, and a Gorilla, and you find you
have no trouble in identifying the bones throughout; and lastly turn to
the end of the series, the diagram representing a man's skeleton, and
still you find no great structural feature essentially altered. There
are the same bones in the same relations. From the Horse we pass on and
on, with gradual steps, until we arrive at last at the highest known
forms. On the other hand, take the other line of diagrams, and pass from
the Horse downwards in the scale to this fish; and still, though the
modifications are vastly greater, the essential framework of the
organization remains unchanged. Here, for instance, is a Porpoise; here
is its strong backbone, with the cavity running through it, which
contains the spinal cord; here are the ribs, here the shoulder-blade;
here is the little short upper-arm bone, here are the two forearm
bones, the wrist-bone, and the finger-bones.
Strange, is it not, that the Porpoise should have in this queer-looking
affair--its flapper (as it is called), the same fundamental elements as
the fore-leg of the Horse or the Dog, or the Ape or Man; and here you
will notice a very curious thing,--the hinder limbs are absent. Now, let
us make another jump. Let us go to the Codfish: here you see is the
forearm, in this large pectoral fin--carrying your mind's eye onward
from the flapper of the Porpoise. And here you have the hinder limbs
restored in the shape of these ventral fins. If I were to make a
transverse section of this, I should find just the same organs that we
have before noticed. So that, you see, there comes out this strange
conclusion as the result of our investigations, that the Horse, when
examined and compared with other animals, is found by no means to stand
alone in nature; but that there are an enormous number of other
creatures which have backbones, ribs, and legs, and other parts arranged
in the same general manner, and in all their formation exhibiting the
same broad peculiarities.
I am sure that you cannot have followed me even in this extremely
elementary exposition of the structural relations of animals, without
seeing what I have been driving at all through, which is, to show you
that, step by step, naturalists have come to the idea of a unity of
plan, or conformity of construction, among animals which appeared at
first sight to be extremely dissimilar.
And here you have evidence of such a unity of plan among all the animals
which have backbones, and which we technically call _Vertebrata_. But
there are multitudes of other animals, such as crabs, lobsters, spiders,
and so on, which we term _Annulosa_. In these I could not point out to
you the parts that correspond with those of the Horse,--the backbone,
for instance,--as they are constructed upon a very different principle,
which is also common to all of them; that is to say, the Lobster, the
Spider, and the Centipede, have a common plan running through their
whole arrangement, in just the same way that the Horse, the Dog, and the
Porpoise assimilate to each other.
Yet other creatures--whelks, cuttlefishes, oysters, snails, and all
their tribe (_Mollusca_)--resemble one another in the same way, but
differ from both _Vertebrata_ and _Annulosa_; and the like is true of
the animals called _Coelenterata_ (Polypes) and _Protozoa_
(animalcules and sponges).
Now, by pursuing this sort of comparison, naturalists have arrived at
the conviction that there are,--some think five, and some seven,--but
certainly not more than the latter number--and perhaps it is simpler to
assume five--distinct plans or constructions in the whole of the animal
world; and that the hundreds of thousands of species of creatures on the
surface of the earth, are all reducible to those five, or, at most,
seven, plans of organization.
But can we go no further than that? When one has got so far, one is
tempted to go on a step and inquire whether we cannot go back yet
further and bring down the whole to modifications of one primordial
unit. The anatomist cannot do this; but if he call to his aid the study
of development, he can do it. For we shall find that, distinct as those
plans are, whether it be a porpoise or man, or lobster, or any of those
other kinds I have mentioned, every one begins its existence with one
and the same primitive form,--that of the egg, consisting, as we have
seen, of a nitrogenous substance, having a small particle or nucleus in
the centre of it. Furthermore, the earlier changes of each are
substantially the same. And it is in this that lies that true "unity of
organization" of the animal kingdom which has been guessed at and
fancied for many years; but which it has been left to the present time
to be demonstrated by the careful study of development. But is it
possible to go another step further still, and to show that in the same
way the whole of the organic world is reducible to one primitive
condition of form? Is there among the plants the same primitive form of
organization, and is that identical with that of the animal kingdom? The
reply to that question, too, is not uncertain or doubtful. It is now
proved that every plant begins its existence under the same form; that
is to say, in that of a cell--a particle of nitrogenous matter having
substantially the same conditions. So that if you trace back the oak to
its first germ, or a man, or a horse, or lobster, or oyster, or any
other animal you choose to name, you shall find each and all of these
commencing their existence in forms essentially similar to each other:
and, furthermore, that the first processes of growth, and many of the
subsequent modifications, are essentially the same in principle in
almost all.
In conclusion, let me, in a few words, recapitulate the positions which
I have laid down. And you must understand that I have not been talking
mere theory; I have been speaking of matters which are as plainly
demonstrable as the commonest propositions of Euclid--of facts that must
form the basis of all speculations and beliefs in Biological science. We
have gradually traced down all organic forms, or, in other words, we
have analyzed the present condition of animated nature, until we found
that each species took its origin in a form similar to that under which
all the others commenced their existence. We have found the whole of the
vast array of living forms with which we are surrounded, constantly
growing, increasing, decaying, and disappearing; the animal constantly
attracting, modifying, and applying to its sustenance the matter of the
vegetable kingdom, which derived its support from the absorption and
conversion of inorganic matter. And so constant and universal is this
absorption, waste, and reproduction, that it may be said with perfect
certainty that there is left in no one of our bodies at the present
moment a millionth part of the matter of which they were originally
formed! We have seen, again, that not only is the living matter derived
from the inorganic world, but that the forces of that matter are all of
them correlative with and convertible into those of inorganic nature.
This, for our present purposes, is the best view of the present
condition of organic nature which I can lay before you: it gives you the
great outlines of a vast picture, which you must fill up by your own
study.
In the next lecture I shall endeavour in the same way to go back into
the past, and to sketch in the same broad manner the history of life in
epochs preceding our own.
V
THE PAST CONDITION OF ORGANIC
NATURE.
In the lecture which I delivered last Monday evening, I endeavoured to
sketch in a very brief manner, but as well as the time at my disposal
would permit, the present condition of organic nature, meaning by that
large title simply an indication of the great, broad, and general
principles which are to be discovered by those who look attentively at
the phenomena of organic nature as at present displayed. The general
result of our investigations might be summed up thus: we found that the
multiplicity of the forms of animal life, great as that may be, may be
reduced to a comparatively few primitive plans or types of construction;
that a further study of the development of those different forms
revealed to us that they were again reducible, until we at last brought
the infinite diversity of animal, and even vegetable life, down to the
primordial form of a single cell.
We found that our analysis of the organic world, whether animals or
plants, showed, in the long run, that they might both be reduced into,
and were, in fact, composed of the same constituents. And we saw that
the plant obtained the materials constituting its substance by a
peculiar combination of matters belonging entirely to the inorganic
world; that, then, the animal was constantly appropriating the
nitrogenous matters of the plant to its own nourishment, and returning
them back to the inorganic world, in what we spoke of as its waste; and
that, finally, when the animal ceased to exist, the constituents of its
body were dissolved and transmitted to that inorganic world whence they
had been at first abstracted. Thus we saw in both the blade of grass
and the horse but the same elements differently combined and arranged.
We discovered a continual circulation going on,--the plant drawing in
the elements of inorganic nature and combining them into food for the
animal creation; the animal borrowing from the plant the matter for its
own support, giving off during its life products which returned
immediately to the inorganic world; and that, eventually, the
constituent materials of the whole structure of both animals and plants
were thus returned to their original source: there was a constant
passage from one state of existence to another, and a returning back
again.
Lastly, when we endeavoured to form some notion of the nature of the
forces exercised by living beings, we discovered that they--if not
capable of being subjected to the same minute analysis as the
constituents of those beings themselves--that they were correlative
with--that they were the equivalents of the forces of inorganic
nature--that they were, in the sense in which the term is now used,
convertible with them. That was our general result.
And now, leaving the Present, I must endeavour in the same manner to put
before you the facts that are to be discovered in the Past history of
the living world, in the past conditions of organic nature. We have,
to-night, to deal with the facts of that history--a history involving
periods of time before which our mere human records sink into utter
insignificance--a history the variety and physical magnitude of whose
events cannot even be foreshadowed by the history of human life and
human phenomena--a history of the most varied and complex character.
We must deal with the history, then, in the first place, as we should
deal with all other histories. The historical student knows that his
first business should be to inquire into the validity of his evidence,
and the nature of the record in which the evidence is contained, that he
may be able to form a proper estimate of the correctness of the
conclusions which have been drawn from that evidence. So, here, we must
pass, in the first place, to the consideration of a matter which may
seem foreign to the question under discussion. We must dwell upon the
nature of the records, and the credibility of the evidence they contain;
we must look to the completeness or incompleteness of those records
themselves, before we turn to that which they contain and reveal. The
question of the credibility of the history, happily for us, will not
require much consideration, for, in this history, unlike those of human
origin, there can be no cavilling, no differences as to the reality and
truth of the facts of which it is made up; the facts state themselves,
and are laid out clearly before us.
But, although one of the greatest difficulties of the historical student
is cleared out of our path, there are other difficulties--difficulties
in rightly interpreting the facts as they are presented to us--which may
be compared with the greatest difficulties of any other kinds of
historical study.
What is this record of the past history of the globe, and what are the
questions which are involved in an inquiry into its completeness or
incompleteness? That record is composed of mud; and the question which
we have to investigate this evening resolves itself into a question of
the formation of mud. You may think, perhaps, that this is a vast
step--of almost from the sublime to the ridiculous--from the
contemplation of the history of the past ages of the world's existence
to the consideration of the history of the formation of mud! But, in
nature, there is nothing mean and unworthy of attention; there is
nothing ridiculous or contemptible in any of her works; and this
inquiry, you will soon see, I hope, takes us to the very root and
foundations of our subject.
How, then, is mud formed? Always, with some trifling exception, which I
need not consider now--always, as the result of the action of water,
wearing down and disintegrating the surface of the earth and rocks with
which it comes in contact--pounding and grinding it down, and carrying
the particles away to places where they cease to be disturbed by this
mechanical action, and where they can subside and rest. For the ocean,
urged by winds, washes, as we know, a long extent of coast, and every
wave, loaded as it is with particles of sand and gravel as it breaks
upon the shore, does something towards the disintegrating process. And
thus, slowly but surely, the hardest rocks are gradually ground down to
a powdery substance; and the mud thus formed, coarser or finer, as the
case may be, is carried by the rush of the tides, or currents, till it
reaches the comparatively deeper parts of the ocean, in which it can
sink to the bottom, that is, to parts where there is a depth of about
fourteen or fifteen fathoms, a depth at which the water is, usually,
nearly motionless, and in which, of course, the finer particles of this
detritus, or mud as we call it, sinks to the bottom.
Or, again, if you take a river, rushing down from its mountain sources,
brawling over the stones and rocks that intersect its path, loosening,
removing, and carrying with it in its downward course the pebbles and
lighter matters from its banks, it crushes and pounds down the rocks and
earths in precisely the same way as the wearing action of the sea waves.
The matters forming the deposit are torn from the mountain-side and
whirled impetuously into the valley, more slowly over the plain, thence
into the estuary, and from the estuary they are swept into the sea. The
coarser and heavier fragments are obviously deposited first, that is, as
soon as the current begins to lose its force by becoming amalgamated
with the stiller depths of the ocean, but the finer and lighter
particles are carried further on, and eventually deposited in a deeper
and stiller portion of the ocean.
It clearly follows from this that mud gives us a chronology; for it is
evident that supposing this, which I now sketch, to be the sea bottom,
and supposing this to be a coast-line; from the washing action of the
sea upon the rock, wearing and grinding it down into a sediment of mud,
the mud will be carried down and, at length, deposited in the deeper
parts of this sea-bottom, where it will form a layer; and then, while
that first layer is hardening, other mud which is coming from the same
source will, of course, be carried to the same place; and, as it is
quite impossible for it to get beneath the layer already there, it
deposits itself above it, and forms another layer, and in that way you
gradually have layers of mud constantly forming and hardening one above
the other, and conveying a record of time.
It is a necessary result of the operation of the law of gravitation that
the uppermost layer shall be the youngest and the lowest the oldest, and
that the different beds shall be older at any particular point or spot
in exactly the ratio of their depth from the surface. So that if they
were upheaved afterwards, and you had a series of these different layers
of mud, converted into sandstone, or limestone, as the case might be,
you might be sure that the bottom layer was deposited first, and that
the upper layers were formed afterwards. Here, you see, is the first
step in the history--these layers of mud give us an idea of time.
The whole surface of the earth,--I speak broadly, and leave out minor
qualifications,--is made up of such layers of mud, so hard, the majority
of them, that we call them rock, whether limestone or sandstone, or
other varieties of rock. And, seeing that every part of the crust of the
earth is made up in this way, you might think that the determination of
the chronology, the fixing of the time which it has taken to form this
crust is a comparatively simple matter. Take a broad average, ascertain
how fast the mud is deposited upon the bottom of the sea, or in the
estuary of rivers; take it to be an inch, or two, or three inches a
year, or whatever you may roughly estimate it at; then take the total
thickness of the whole series of stratified rocks, which geologists
estimate at twelve or thirteen miles, or about seventy thousand feet,
make a sum in short division, divide the total thickness by that of the
quantity deposited in one year, and the result will, of course, give you
the number of years which the crust has taken to form.
Truly, that looks a very simple process! It would be so except for
certain difficulties, the very first of which is that of finding how
rapidly sediments are deposited; but the main difficulty--a difficulty
which renders any certain calculations of such a matter out of the
question--is this, the sea-bottom on which the deposit takes place is
continually shifting.
Instead of the surface of the earth being that stable, fixed thing that
it is popularly believed to be, being, in common parlance, the very
emblem of fixity itself, it is incessantly moving, and is, in fact, as
unstable as the surface of the sea, except that its undulations are
infinitely slower and enormously higher and deeper.
Now, what is the effect of this oscillation? Take the case to which I
have previously referred. The finer or coarser sediments that are
carried down by the current of the river will only be carried out a
certain distance, and eventually, as we have already seen, on reaching
the stiller part of the ocean, will be deposited at the bottom.
Let C _y_ (Fig. 35) 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 _y_^1, _x_^1 _y_^1, 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.
[Illustration: FIG. 35.]
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 away, 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. 36.)
[Illustration: FIG. 36.]
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. 35; 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. 35), so that the first deposit is carried no
farther than _a_, forming the bed A^1, and the coarse no farther than
_b_, forming the bed B^1, the result will be the formation of two
continuous beds, one of fine sediment (A A^1) over-lapping another of
coarse sediment (B B^1). Now suppose the whole sea-bottom is raised up,
and a section exposed about the point A^1; 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 B^1; for A
was deposited long before B^1. 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 embedded in the limestone or sandstone which is being thus
formed. You may see in the galleries of the Museum upstairs specimens of
limestones in which such fossil remains of existing animals are
embedded. There are some specimens in which turtles' eggs have been
embedded 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 fossilized.
Not only does this process of embedding and fossilization 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 in 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 1000 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 upheaved, 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 jelly-fishes, 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 embedded 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 embedded, 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 embedded 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, or 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 on what we _do not_ know.
I strongly imagine that if it were not for the peculiar appearance that
fossilized animals have, that 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 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 orders 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 upstairs, 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 ichthyosauri and pterodactyles, and
ammonites, and so forth.
I do not know what Mr. Godwin Austin would say comes next, but probably
rocks containing more ammonites, and more ichthyosauri and plesiosauri,
with a vast number of other things; and under that I should meet with
yet older rocks, containing numbers of strange shells and fishes; and in
thus passing from the surface to the lowest depths of the earth's crust,
the forms of animal life and vegetable life which I should meet with in
the successive beds would, looking at them broadly, be the more
different the further that I went down. Or, in other words, inasmuch as
we started with the clear principle, that in a series of
naturally-disposed mud beds the lowest are the oldest, we should come to
this result, that the farther we go back in time the more difference
exists between the animal and vegetable life of an epoch and that which
now exists. That was the conclusion to which I wished to bring you at
the end of this Lecture.