When it was my duty to consider what subject I would select for the six
lectures 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 nature.
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 the 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, the 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.
In that skeleton there are a number of parts to be recognised. 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 expressions 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).
[Illustration: 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
(_e_); 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 on 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).
[Illustration: 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 are 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 maybe 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.
[Illustration: Inorganic World Fig. 3.]
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 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. 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 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.