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The Present Condition of Organic Nature by Thomas H. Huxley

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Of the great thinkers of the nineteenth century, Thomas Henry Huxley,
son of an Ealing schoolmaster, was undoubtedly the most noteworthy. His
researches in biology, his contributions to scientific controversy, his
pungent criticisms of conventional beliefs and thoughts have probably
had greater influence than the work of any other English scientist. And
yet he was a "self-made" intellectualist. In spite of the fact that his
father was a schoolmaster he passed through no regular course of
education. "I had," he said, "two years of a pandemonium of a school
(between eight and ten) and after that neither help nor sympathy in any
intellectual direction till I reached manhood." When he was twelve a
craving for reading found satisfaction in Hutton's "Geology," and when
fifteen in Hamilton's "Logic."

At seventeen Huxley entered as a student at Charing Cross Hospital, and
three years later he was M.B. and the possessor of the gold medal for
anatomy and physiology. An appointment as surgeon in the navy proved
to be the entry to Huxley's great scientific career, for he was
gazetted to the "Rattlesnake", commissioned for surveying work in
Torres Straits. He was attracted by the teeming surface life of
tropical seas and his study of it was the commencement of that
revolution in scientific knowledge ultimately brought about by his

Thomas Henry Huxley was born at Ealing on May 4, 1825, and died at
Eastbourne June 29, 1895.




The Publisher of these interesting Lectures, having made an arrangement
for their publication with Mr. J. A. Mays, the Reporter, begs to append
the following note from Professor Huxley:--

"Mr. J. Aldous Mays, who is taking shorthand notes of my 'Lectures to
Working Men,' has asked me to allow him, on his own account, to print
those Notes for the use of my audience. I willingly accede to this
request, on the understanding that a notice is prefixed to the effect
that I have no leisure to revise the Lectures, or to make alterations
in them, beyond the correction of any important error in a matter of


When it was my duty to consider what subject I would select for the six
lectures [*To Working Men, at the Museum of Practical Geology, 1863.]
which I shall now have the pleasure of delivering to you, it occurred
to me that I could not do better than endeavour to put before you in a
true light, or in what I might perhaps with more modesty call, that
which I conceive myself to be the true light, the position of a book
which has been more praised and more abused, perhaps, than any book
which has appeared for some years;--I mean Mr. Darwin's work on the
"Origin of Species". That work, I doubt not, many of you have read;
for I know the inquiring spirit which is rife among you. At any rate,
all of you will have heard of it,--some by one kind of report and some
by another kind of report; the attention of all and the curiosity of
all have been probably more or less excited on the subject of that
work. All I can do, and all I shall attempt to do, is to put before
you that kind of judgment which has been formed by a man, who, of
course, is liable to judge erroneously; but, at any rate, of one whose
business and profession it is to form judgments upon questions of this

And here, as it will always happen when dealing with an extensive
subject, the greater part of my course--if, indeed, so small a number
of lectures can be properly called a course--must be devoted to
preliminary matters, or rather to a statement of those facts and of
those principles which the work itself dwells upon, and brings more or
less directly before us. I have no right to suppose that all or any of
you are naturalists; and even if you were, the misconceptions and
misunderstandings prevalent even among naturalists on these matters
would make it desirable that I should take the course I now propose to
take,--that I should start from the beginning,--that I should endeavour
to point out what is the existing state of the organic world,--that I
should point out its past condition,--that I should state what is the
precise nature of the undertaking which Mr. Darwin has taken in hand;
that I should endeavour to show you what are the only methods by which
that undertaking can be brought to an issue, and to point out to you
how far the author of the work in question has satisfied those
conditions, how far he has not satisfied them, how far they are
satisfiable by man, and how far they are not satisfiable by man.

To-night, in taking up the first part of this question, I shall
endeavour to put before you a sort of broad notion of our knowledge of
the condition of the living world. There are many ways of doing this.
I might deal with it pictorially and graphically. Following the
example of Humboldt in his "Aspects of Nature", I might endeavour to
point out the infinite variety of organic life in every mode of its
existence, with reference to the variations of climate and the like;
and such an attempt would be fraught with interest to us all; but
considering the subject before us, such a course would not be that best
calculated to assist us. In an argument of this kind we must go
further and dig deeper into the matter; we must endeavour to look into
the foundations of living Nature, if I may so say, and discover the
principles involved in some of her most secret operations. I propose,
therefore, in the first place, to take some ordinary animal with which
you are all familiar, and, by easily comprehensible and obvious
examples drawn from it, to show what are the kind of problems which
living beings in general lay before us; and I shall then show you that
the same problems are laid open to us by all kinds of living beings.
But first, let me say in what sense I have used the words "organic
nature." In speaking of the causes which lead to our present knowledge
of organic nature, I have used it almost as an equivalent of the word
"living," and for this reason,--that in almost all living beings you
can distinguish several distinct portions set apart to do particular
things and work in a particular way. These are termed "organs," and
the whole together is called "organic." And as it is universally
characteristic of them, this term "organic" has been very conveniently
employed to denote the whole of living nature,--the whole of the plant
world, and the whole of the animal world.

Few animals can be more familiar to you than that whose skeleton is
shown on our diagram. You need not bother yourselves with this "Equus
caballus" written under it; that is only the Latin name of it, and does
not make it any better. It simply means the common Horse. Suppose we
wish to understand all about the Horse. Our first object must be to
study the structure of the animal. The whole of his body is inclosed
within a hide, a skin covered with hair; and if that hide or skin be
taken off, we find a great mass of flesh, or what is technically called
muscle, being the substance which by its power of contraction enables
the animal to move. These muscles move the hard parts one upon the
other, and so give that strength and power of motion which renders the
Horse so useful to us in the performance of those services in which we
employ him.

And then, on separating and removing the whole of this skin and flesh,
you have a great series of bones, hard structures, bound together with
ligaments, and forming the skeleton which is represented here.

[FIGURE 1. (Section through a horse.)

FIGURE 2. (Section through a cell.)]

In that skeleton there are a number of parts to be recognized. The
long series of bones, beginning from the skull and ending in the tail,
is called the spine, and those in front are the ribs; and then there
are two pairs of limbs, one before and one behind; and there are what
we all know as the fore-legs and the hind-legs. If we pursue our
researches into the interior of this animal, we find within the
framework of the skeleton a great cavity, or rather, I should say, two
great cavities,--one cavity beginning in the skull and running through
the neck-bones, along the spine, and ending in the tail, containing the
brain and the spinal marrow, which are extremely important organs. The
second great cavity, commencing with the mouth, contains the gullet,
the stomach, the long intestine, and all the rest of those internal
apparatus which are essential for digestion; and then in the same great
cavity, there are lodged the heart and all the great vessels going from
it; and, besides that, the organs of respiration-- the lungs: and then
the kidneys, and the organs of reproduction, and so on. Let us now
endeavour to reduce this notion of a horse that we now have, to some
such kind of simple expression as can be at once, and without
difficulty, retained in the mind, apart from all minor details. If I
make a transverse section, that is, if I were to saw a dead horse
across, I should find that, if I left out the details, and supposing I
took my section through the anterior region, and through the
fore-limbs, I should have here this kind of section of the body (Fig.
1). Here would be the upper part of the animal--that great mass of
bones that we spoke of as the spine (a, Fig. 1). Here I should have
the alimentary canal (b, Fig. 1). Here I should have the heart (c,
Fig. 1); and then you see, there would be a kind of double tube, the
whole being inclosed within the hide; the spinal marrow would be placed
in the upper tube (a, Fig. 1), and in the lower tube (d d, Fig. 1),
there would be the alimentary canal (b), and the heart (c); and here I
shall have the legs proceeding from each side. For simplicity's sake,
I represent them merely as stumps (e e, Fig. 1). Now that is a
horse--as mathematicians would say--reduced to its most simple
expression. Carry that in your minds, if you please, as a simplified
idea of the structure of the Horse. The considerations which I have
now put before you belong to what we technically call the 'Anatomy' of
the Horse. Now, suppose we go to work upon these several parts,--flesh
and hair, and skin and bone, and lay open these various organs with our
scalpels, and examine them by means of our magnifying- glasses, and see
what we can make of them. We shall find that the flesh is made up of
bundles of strong fibres. The brain and nerves, too, we shall find,
are made up of fibres, and these queer-looking things that are called
ganglionic corpuscles. If we take a slice of the bone and examine it,
we shall find that it is very like this diagram of a section of the
bone of an ostrich, though differing, of course, in some details; and
if we take any part whatsoever of the tissue, and examine it, we shall
find it all has a minute structure, visible only under the microscope.
All these parts constitute microscopic anatomy or 'Histology.' These
parts are constantly being changed; every part is constantly growing,
decaying, and being replaced during the life of the animal. The tissue
is constantly replaced by new material; and if you go back to the young
state of the tissue in the case of muscle, or in the case of skin, or
any of the organs I have mentioned, you will find that they all come
under the same condition. Every one of these microscopic filaments and
fibres (I now speak merely of the general character of the whole
process)-- every one of these parts--could be traced down to some
modification of a tissue which can be readily divided into little
particles of fleshy matter, of that substance which is composed of the
chemical elements, carbon, hydrogen, oxygen, and nitrogen, having such
a shape as this (Fig. 2). These particles, into which all primitive
tissues break up, are called cells. If I were to make a section of a
piece of the skin of my hand, I should find that it was made up of
these cells. If I examine the fibres which form the various organs of
all living animals, I should find that all of them, at one time or
other, had been formed out of a substance consisting of similar
elements; so that you see, just as we reduced the whole body in the
gross to that sort of simple expression given in Fig. 1, so we may
reduce the whole of the microscopic structural elements to a form of
even greater simplicity; just as the plan of the whole body may be so
represented in a sense (Fig. 1), so the primary structure of every
tissue may be represented by a mass of cells (Fig. 2).

Having thus, in this sort of general way, sketched to you what I may
call, perhaps, the architecture of the body of the Horse (what we term
technically its Morphology), I must now turn to another aspect. A
horse is not a mere dead structure: it is an active, living, working
machine. Hitherto we have, as it were, been looking at a steam-engine
with the fires out, and nothing in the boiler; but the body of the
living animal is a beautifully-formed active machine, and every part
has its different work to do in the working of that machine, which is
what we call its life. The Horse, if you see him after his day's work
is done, is cropping the grass in the fields, as it may be, or munching
the oats in his stable. What is he doing? His jaws are working as a
mill--and a very complex mill too--grinding the corn, or crushing the
grass to a pulp. As soon as that operation has taken place, the food
is passed down to the stomach, and there it is mixed with the chemical
fluid called the gastric juice, a substance which has the peculiar
property of making soluble and dissolving out the nutritious matter in
the grass, and leaving behind those parts which are not nutritious; so
that you have, first, the mill, then a sort of chemical digester; and
then the food, thus partially dissolved, is carried back by the
muscular contractions of the intestines into the hinder parts of the
body, while the soluble portions are taken up into the blood. The
blood is contained in a vast system of pipes, spreading through the
whole body, connected with a force pump,--the heart,--which, by its
position and by the contractions of its valves, keeps the blood
constantly circulating in one direction, never allowing it to rest; and
then, by means of this circulation of the blood, laden as it is with
the products of digestion, the skin, the flesh, the hair, and every
other part of the body, draws from it that which it wants, and every
one of these organs derives those materials which are necessary to
enable it to do its work.

The action of each of these organs, the performance of each of these
various duties, involve in their operation a continual absorption of
the matters necessary for their support, from the blood, and a constant
formation of waste products, which are returned to the blood, and
conveyed by it to the lungs and the kidneys, which are organs that have
allotted to them the office of extracting, separating, and getting rid
of these waste products; and thus the general nourishment, labour, and
repair of the whole machine is kept up with order and regularity. But
not only is it a machine which feeds and appropriates to its own
support the nourishment necessary to its existence--it is an engine for
locomotive purposes. The Horse desires to go from one place to
another; and to enable it to do this, it has those strong contractile
bundles of muscles attached to the bones of its limbs, which are put in
motion by means of a sort of telegraphic apparatus formed by the brain
and the great spinal cord running through the spine or backbone; and to
this spinal cord are attached a number of fibres termed nerves, which
proceed to all parts of the structure. By means of these the eyes,
nose, tongue, and skin--all the organs of perception--transmit
impressions or sensations to the brain, which acts as a sort of great
central telegraph-office, receiving impressions and sending messages to
all parts of the body, and putting in motion the muscles necessary to
accomplish any movement that may be desired. So that you have here an
extremely complex and beautifully-proportioned machine, with all its
parts working harmoniously together towards one common object--the
preservation of the life of the animal.

Now, note this: the Horse makes up its waste by feeding, and its food
is grass or oats, or perhaps other vegetable products; therefore, in
the long run, the source of all this complex machinery lies in the
vegetable kingdom. But where does the grass, or the oat, or any other
plant, obtain this nourishing food-producing material? At first it is
a little seed, which soon begins to draw into itself from the earth and
the surrounding air matters which in themselves contain no vital
properties whatever; it absorbs into its own substance water, an
inorganic body; it draws into its substance carbonic acid, an inorganic
matter; and ammonia, another inorganic matter, found in the air; and
then, by some wonderful chemical process, the details of which chemists
do not yet understand, though they are near foreshadowing them, it
combines them into one substance, which is known to us as 'Protein,' a
complex compound of carbon, hydrogen, oxygen, and nitrogen, which alone
possesses the property of manifesting vitality and of permanently
supporting animal life. So that, you see, the waste products of the
animal economy, the effete materials which are continually being thrown
off by all living beings, in the form of organic matters, are
constantly replaced by supplies of the necessary repairing and
rebuilding materials drawn from the plants, which in their turn
manufacture them, so to speak, by a mysterious combination of those
same inorganic materials.

Let us trace out the history of the Horse in another direction. After
a certain time, as the result of sickness or disease, the effect of
accident, or the consequence of old age, sooner or later, the animal
dies. The multitudinous operations of this beautiful mechanism flag in
their performance, the Horse loses its vigour, and after passing
through the curious series of changes comprised in its formation and
preservation, it finally decays, and ends its life by going back into
that inorganic world from which all but an inappreciable fraction of
its substance was derived. Its bones become mere carbonate and
phosphate of lime; the matter of its flesh, and of its other parts,
becomes, in the long run, converted into carbonic acid, into water, and
into ammonia. You will now, perhaps, understand the curious relation
of the animal with the plant, of the organic with the inorganic world,
which is shown in this diagram (Fig. 3).

[FIGURE 3. (Diagram showing material relationship of the Vegetable,
Animal and Inorganic Worlds.)]

The plant gathers these inorganic materials together and makes them up
into its own substance. The animal eats the plant and appropriates the
nutritious portions to its own sustenance, rejects and gets rid of the
useless matters; and, finally, the animal itself dies, and its whole
body is decomposed and returned into the inorganic world. There is
thus a constant circulation from one to the other, a continual
formation of organic life from inorganic matters, and as constant a
return of the matter of living bodies to the inorganic world; so that
the materials of which our bodies are composed are largely, in all
probability, the substances which constituted the matter of long
extinct creations, but which have in the interval constituted a part of
the inorganic world.

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. 2); 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
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, which 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. 1).

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 we 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; and 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, 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

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, anklebones, 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, 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 commence 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.

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