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Darwiniana

T >> Thomas Henry Huxley >> Darwiniana




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 organisation 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
organisation.

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 organisation" 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 organisation,
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 analysed 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.



II. 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 exceptions, 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.

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