Discourses - Thomas H. Huxley
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I may say, therefore, that the sixth segment is like the others in plan,
but that it is modified in its details.
The first segment is like the others, so far as its ring is concerned,
and though its appendages differ from any of those yet examined in the
simplicity of their structure, parts corresponding with the stem and one
of the divisions of the appendages of the other segments can be readily
discerned in them.
Thus it appears that the lobster's tail is composed of a series of
segments which are fundamentally similar, though each presents peculiar
modifications of the plan common to all. But when I turn to the forepart
of the body I see, at first, nothing but a great shield-like shell,
called technically the "carapace," ending in front in a sharp spine, on
either side of which are the curious compound eyes, set upon the ends of
stout movable stalks. Behind these, on the under side of the body, are
two pairs of long feelers, or antennae, followed by six pairs of jaws
folded against one another over the mouth, and five pairs of legs, the
foremost of these being the great pinchers, or claws, of the lobster.
It looks, at first, a little hopeless to attempt to find in this complex
mass a series of rings, each with its pair of appendages, such as I have
shown you in the abdomen, and yet it is not difficult to demonstrate
their existence. Strip off the legs, and you will find that each pair is
attached to a very definite segment of the under wall of the body; but
these segments, instead of being the lower parts of free rings, as in the
tail, are such parts of rings which are all solidly united and bound
together; and the like is true of the jaws, the feelers, and the eye-
stalks, every pair of which is borne upon its own special segment. Thus
the conclusion is gradually forced upon us, that the body of the lobster
is composed of as many rings as there are pairs of appendages, namely,
twenty in all, but that the six hindmost rings remain free and movable,
while the fourteen front rings become firmly soldered together, their
backs forming one continuous shield--the carapace.
Unity of plan, diversity in execution, is the lesson taught by the study
of the rings of the body, and the same instruction is given still more
emphatically by the appendages. If I examine the outermost jaw I find it
consists of three distinct portions, an inner, a middle, and an outer,
mounted upon a common stem; and if I compare this jaw with the legs
behind it, or the jaws in front of it, I find it quite easy to see, that,
in the legs, it is the part of the appendage which corresponds with the
inner division, which becomes modified into what we know familiarly as
the "leg," while the middle division disappears, and the outer division
is hidden under the carapace. Nor is it more difficult to discern that,
in the appendages of the tail, the middle division appears again and the
outer vanishes; while, on the other hand, in the foremost jaw, the so-
called mandible, the inner division only is left; and, in the same way,
the parts of the feelers and of the eye-stalks can be identified with
those of the legs and jaws.
But whither does all this tend? To the very remarkable conclusion that a
unity of plan, of the same kind as that discoverable in the tail or
abdomen of the lobster, pervades the whole organisation of its skeleton,
so that I can return to the diagram representing any one of the rings of
the tail, which I drew upon the board, and by adding a third division to
each appendage, I can use it as a sort of scheme or plan of any ring of
the body. I can give names to all the parts of that figure, and then if I
take any segment of the body of the lobster, I can point out to you
exactly, what modification the general plan has undergone in that
particular segment; what part has remained movable, and what has become
fixed to another; what has been excessively developed and metamorphosed
and what has been suppressed.
But I imagine I hear the question, How is all this to be tested? No doubt
it is a pretty and ingenious way of looking at the structure of any
animal; but is it anything more? Does Nature acknowledge, in any deeper
way, this unity of plan we seem to trace?
The objection suggested by these questions is a very valid and important
one, and morphology was in an unsound state so long as it rested upon the
mere perception of the analogies which obtain between fully formed parts.
The unchecked ingenuity of speculative anatomists proved itself fully
competent to spin any number of contradictory hypotheses out of the same
facts, and endless morphological dreams threatened to supplant scientific
theory.
Happily, however, there is a criterion of morphological truth, and a sure
test of all homologies. Our lobster has not always been what we see it;
it was once an egg, a semifluid mass of yolk, not so big as a pin's head,
contained in a transparent membrane, and exhibiting not the least trace
of any one of those organs, the multiplicity and complexity of which, in
the adult, are so surprising. After a time, a delicate patch of cellular
membrane appeared upon one face of this yolk, and that patch was the
foundation of the whole creature, the clay out of which it would be
moulded. Gradually investing the yolk, it became subdivided by transverse
constrictions into segments, the forerunners of the rings of the body.
Upon the ventral surface of each of the rings thus sketched out, a pair
of bud-like prominences made their appearance--the rudiments of the
appendages of the ring. At first, all the appendages were alike, but, as
they grew, most of them became distinguished into a stem and two terminal
divisions, to which, in the middle part of the body, was added a third
outer division; and it was only at a later period, that by the
modification, or absorption, of certain of these primitive constituents,
the limbs acquired their perfect form.
Thus the study of development proves that the doctrine of unity of plan
is not merely a fancy, that it is not merely one way of looking at the
matter, but that it is the expression of deep-seated natural facts. The
legs and jaws of the lobster may not merely be regarded as modifications
of a common type,--in fact and in nature they are so,--the leg and the
jaw of the young animal being, at first, indistinguishable.
These are wonderful truths, the more so because the zoologist finds them
to be of universal application. The investigation of a polype, of a
snail, of a fish, of a horse, or of a man, would have led us, though by a
less easy path, perhaps, to exactly the same point. Unity of plan
everywhere lies hidden under the mask of diversity of structure--the
complex is everywhere evolved out of the simple. Every animal has at
first the form of an egg, and every animal and every organic part, in
reaching its adult state, passes through conditions common to other
animals and other adult parts; and this leads me to another point. I have
hitherto spoken as if the lobster were alone in the world, but, as I need
hardly remind you, there are myriads of other animal organisms. Of these,
some, such as men, horses, birds, fishes, snails, slugs, oysters, corals,
and sponges, are not in the least like the lobster. But other animals,
though they may differ a good deal from the lobster, are yet either very
like it, or are like something that is like it. The cray fish, the rock
lobster, and the prawn, and the shrimp, for example, however different,
are yet so like lobsters, that a child would group them as of the lobster
kind, in contradistinction to snails and slugs; and these last again
would form a kind by themselves, in contradistinction to cows, horses,
and sheep, the cattle kind.
But this spontaneous grouping into "kinds" is the first essay of the
human mind at classification, or the calling by a common name of those
things that are alike, and the arranging them in such a manner as best to
suggest the sum of their likenesses and unlikenesses to other things.
Those kinds which include no other subdivisions than the sexes, or
various breeds, are called, in technical language, species. The English
lobster is a species, our cray fish is another, our prawn is another. In
other countries, however, there are lobsters, cray fish, and prawns, very
like ours, and yet presenting sufficient differences to deserve
distinction. Naturalists, therefore, express this resemblance and this
diversity by grouping them as distinct species of the same "genus." But
the lobster and the cray fish, though belonging to distinct genera, have
many features in common, and hence are grouped together in an assemblage
which is called a family. More distant resemblances connect the lobster
with the prawn and the crab, which are expressed by putting all these
into the same order. Again, more remote, but still very definite,
resemblances unite the lobster with the woodlouse, the king crab, the
water flea, and the barnacle, and separate them from all other animals;
whence they collectively constitute the larger group, or class,
_Crustacea_. But the _Crustacea_ exhibit many peculiar features in common
with insects, spiders, and centipedes, so that these are grouped into the
still larger assemblage or "province" _Articulata_; and, finally, the
relations which these have to worms and other lower animals, are
expressed by combining the whole vast aggregate into the sub-kingdom of
_Annulosa_.
If I had worked my way from a sponge instead of a lobster, I should have
found it associated, by like ties, with a great number of other animals
into the sub-kingdom _Protozoa_; if I had selected a fresh-water polype
or a coral, the members of what naturalists term the sub-kingdom
_Coelenterata_, would have grouped themselves around my type; had a snail
been chosen, the inhabitants of all univalve and bivalve, land and water,
shells, the lamp shells, the squids, and the sea-mat would have gradually
linked themselves on to it as members of the same sub-kingdom of
_Mollusca_; and finally, starting from man, I should have been compelled
to admit first, the ape, the rat, the horse, the dog, into the same
class; and then the bird, the crocodile, the turtle, the frog, and the
fish, into the same sub-kingdom of _Vertebrata_.
And if I had followed out all these various lines of classification
fully, I should discover in the end that there was no animal, either
recent or fossil, which did not at once fall into one or other of these
sub-kingdoms. In other words, every animal is organised upon one or other
of the five, or more, plans, the existence of which renders our
classification possible. And so definitely and precisely marked is the
structure of each animal, that, in the present state of our knowledge,
there is not the least evidence to prove that a form, in the slightest
degree transitional between any of the two groups _Vertebrata, Annulosa,
Mollusca_, and _Coelenterata_, either exists, or has existed, during that
period of the earth's history which is recorded by the geologist.[1]
Nevertheless, you must not for a moment suppose, because no such
transitional forms are known, that the members of the sub-kingdoms are
disconnected from, or independent of, one another. On the contrary, in
their earliest condition they are all similar, and the primordial germs
of a man, a dog, a bird, a fish, a beetle, a snail, and a polype are, in
no essential structural respects, distinguishable.
[Footnote 1: The different grouping necessitated by later knowledge does
not affect the principle of the argument.--1894.]
In this broad sense, it may with truth be said, that all living animals,
and all those dead faunae which geology reveals, are bound together by an
all-pervading unity of organisation, of the same character, though not
equal in degree, to that which enables us to discern one and the same
plan amidst the twenty different segments of a lobster's body. Truly it
has been said, that to a clear eye the smallest fact is a window through
which the Infinite may be seen.
Turning from these purely morphological considerations, let us now
examine into the manner in which the attentive study of the lobster
impels us into other lines of research.
Lobsters are found in all the European seas; but on the opposite shores
of the Atlantic and in the seas of the southern hemisphere they do not
exist. They are, however, represented in these regions by very closely
allied, but distinct forms--the _Homarus Americanus_ and the _Homarus
Capensis:_ so that we may say that the European has one species of
_Homuarus_; the American, another; the African, another; and thus the
remarkable facts of geographical distribution begin to dawn upon us.
Again, if we examine the contents of the earth's crust, we shall find in
the latter of those deposits, which have served as the great burying
grounds of past ages, numberless lobster-like animals, but none so
similar to our living lobster as to make zoologists sure that they
belonged even to the same genus. If we go still further back in time, we
discover, in the oldest rocks of all, the remains of animals, constructed
on the same general plan as the lobster, and belonging to the same great
group of _Crustacea_; but for the most part totally different from the
lobster, and indeed from any other living form of crustacean; and thus we
gain a notion of that successive change of the animal population of the
globe, in past ages, which is the most striking fact revealed by geology.
Consider, now, where our inquiries have led us. We studied our type
morphologically, when we determined its anatomy and its development, and
when comparing it, in these respects, with other animals, we made out its
place in a system of classification. If we were to examine every animal
in a similar manner, we should establish a complete body of zoological
morphology.
Again, we investigated the distribution of our type in space and in time,
and, if the like had been done with every animal, the sciences of
geographical and geological distribution would have attained their limit.
But you will observe one remarkable circumstance, that, up to this point,
the question of the life of these organisms has not come under
consideration. Morphology and distribution might be studied almost as
well, if animals and plants were a peculiar kind of crystals, and
possessed none of those functions which distinguish living beings so
remarkably. But the facts of morphology and distribution have to be
accounted for, and the science, the aim of which it is to account for
them, is Physiology.
Let us return to our lobster once more. If we watched the creature in its
native element, we should see it climbing actively the submerged rocks,
among which it delights to live, by means of its strong legs; or swimming
by powerful strokes of its great tail, the appendages of the sixth joint
of which are spread out into a broad fan-like Propeller: seize it, and it
will show you that its great claws are no mean weapons of offence;
suspend a piece of carrion among its haunts, and it will greedily devour
it, tearing and crushing the flesh by means of its multitudinous jaws.
Suppose that we had known nothing of the lobster but as an inert mass, an
organic crystal, if I may use the phrase, and that we could suddenly see
it exerting all these powers, what wonderful new ideas and new questions
would arise in our minds! The great new question would be, "How does all
this take place?" the chief new idea would be, the idea of adaptation to
purpose,--the notion, that the constituents of animal bodies are not mere
unconnected parts, but organs working together to an end. Let us consider
the tail of the lobster again from this point of view. Morphology has
taught us that it is a series of segments composed of homologous parts,
which undergo various modifications--beneath and through which a common
plan of formation is discernible. But if I look at the same part
physiologically, I see that it is a most beautifully constructed organ of
locomotion, by means of which the animal can swiftly propel itself either
backwards or forwards.
But how is this remarkable propulsive machine made to perform its
functions? If I were suddenly to kill one of these animals and to take
out all the soft parts, I should find the shell to be perfectly inert, to
have no more power of moving itself than is possessed by the machinery of
a mill when disconnected from its steam-engine or water-wheel. But if I
were to open it, and take out the viscera only, leaving the white flesh,
I should perceive that the lobster could bend and extend its tail as well
as before. If I were to cut off the tail, I should cease to find any
spontaneous motion in it; but on pinching any portion of the flesh, I
should observe that it underwent a very curious change--each fibre
becoming shorter and thicker. By this act of contraction, as it is
termed, the parts to which the ends of the fibre are attached are, of
course, approximated; and according to the relations of their points of
attachment to the centres of motions of the different rings, the bending
or the extension of the tail results. Close observation of the newly-
opened lobster would soon show that all its movements are due to the same
cause--the shortening and thickening of these fleshy fibres, which are
technically called muscles.
Here, then, is a capital fact. The movements of the lobster are due to
muscular contractility. But why does a muscle contract at one time and
not at another? Why does one whole group of muscles contract when the
lobster wishes to extend his tail, and another group when he desires to
bend it? What is it originates, directs, and controls the motive power?
Experiment, the great instrument for the ascertainment of truth in
physical science, answers this question for us. In the head of the
lobster there lies a small mass of that peculiar tissue which is known as
nervous substance. Cords of similar matter connect his brain of the
lobster, directly or indirectly, with the muscles. Now, if these
communicating cords are cut, the brain remaining entire, the power of
exerting what we call voluntary motion in the parts below the section is
destroyed; and, on the other hand, if, the cords remaining entire, the
brain mass be destroyed, the same voluntary mobility is equally lost.
Whence the inevitable conclusion is, that the power of originating these
motions resides in the brain and is propagated along the nervous cords.
In the higher animals the phenomena which attend this transmission have
been investigated, and the exertion of the peculiar energy which resides
in the nerves has been found to be accompanied by a disturbance of the
electrical state of their molecules.
If we could exactly estimate the signification of this disturbance; if we
could obtain the value of a given exertion of nerve force by determining
the quantity of electricity, or of heat, of which it is the equivalent;
if we could ascertain upon what arrangement, or other condition of the
molecules of matter, the manifestation of the nervous and muscular
energies depends (and doubtless science will some day or other ascertain
these points), physiologists would have attained their ultimate goal in
this direction; they would have determined the relation of the motive
force of animals to the other forms of force found in nature; and if the
same process had been successfully performed for all the operations which
are carried on in, and by, the animal frame, physiology would be perfect,
and the facts of morphology and distribution would be deducible from the
laws which physiologists had established, combined with those determining
the condition of the surrounding universe.
There is not a fragment of the organism of this humble animal whose study
would not lead us into regions of thought as large as those which I have
briefly opened up to you; but what I have been saying, I trust, has not
only enabled you to form a conception of the scope and purport of
zoology, but has given you an imperfect example of the manner in which,
in my opinion, that science, or indeed any physical science, may be best
taught. The great matter is, to make teaching real and practical, by
fixing the attention of the student on particular facts; but at the same
time it should be rendered broad and comprehensive, by constant reference
to the generalisations of which all particular facts are illustrations.
The lobster has served as a type of the whole animal kingdom, and its
anatomy and physiology have illustrated for us some of the greatest
truths of biology. The student who has once seen for himself the facts
which I have described, has had their relations explained to him, and has
clearly comprehended them, has, so far, a knowledge of zoology, which is
real and genuine, however limited it may be, and which is worth more than
all the mere reading knowledge of the science he could ever acquire. His
zoological information is, so far, knowledge and not mere hearsay.
And if it were nay business to fit you for the certificate in zoological
science granted by this department, I should pursue a course precisely
similar in principle to that which I have taken to-night. I should select
a fresh-water sponge, a fresh-water polype or a _Cyanoea_, a fresh-water
mussel, a lobster, a fowl, as types of the five primary divisions of the
animal kingdom. I should explain their structure very fully, and show how
each illustrated the great principles of zoology. Having gone very
carefully and fully over this ground, I should feel that you had a safe
foundation, and I should then take you in the same way, but less
minutely, over similarly selected illustrative types of the classes; and
then I should direct your attention to the special forms enumerated under
the head of types, in this syllabus, and to the other facts there
mentioned.
That would, speaking generally, be my plan. But I have undertaken to
explain to you the best mode of acquiring and communicating a knowledge
of zoology, and you may therefore fairly ask me for a more detailed and
precise account of the manner in which I should propose to furnish you
with the information I refer to.
My own impression is, that the best model for all kinds of training in
physical science is that afforded by the method of teaching anatomy, in
use in the medical schools. This method consists of three elements--
lectures, demonstrations, and examinations.
The object of lectures is, in the first place, to awaken the attention
and excite the enthusiasm of the student; and this, I am sure, may be
effected to a far greater extent by the oral discourse and by the
personal influence of a respected teacher than in any other way.
Secondly, lectures have the double use of guiding the student to the
salient points of a subject, and at the same time forcing him to attend
to the whole of it, and not merely to that part which takes his fancy.
And lastly, lectures afford the student the opportunity of seeking
explanations of those difficulties which will, and indeed ought to, arise
in the course of his studies.
What books shall I read? is a question constantly put by the student to
the teacher. My reply usually is, "None: write your notes out carefully
and fully; strive to understand them thoroughly; come to me for the
explanation of anything you cannot understand; and I would rather you did
not distract your mind by reading." A properly composed course of
lectures ought to contain fully as much matter as a student can
assimilate in the time occupied by its delivery; and the teacher should
always recollect that his business is to feed, and not to cram the
intellect. Indeed, I believe that a student who gains from a course of
lectures the simple habit of concentrating his attention upon a
definitely limited series of facts, until they are thoroughly mastered,
has made a step of immeasurable importance.
But, however good lectures may be, and however extensive the course of
reading by which they are followed up, they are but accessories to the
great instrument of scientific teaching--demonstration. If I insist
unweariedly, nay fanatically, upon the importance of physical science as
an educational agent, it is because the study of any branch of science,
if properly conducted, appears to me to fill up a void left by all other
means of education. I have the greatest respect and love for literature;
nothing would grieve me more than to see literary training other than a
very prominent branch of education: indeed, I wish that real literary
discipline were far more attended to than it is; but I cannot shut my
eyes to the fact, that there is a vast difference between men who have
had a purely literary, and those who have had a sound scientific,
training.
Seeking for the cause of this difference, I imagine I can find it in the
fact that, in the world of letters, learning and knowledge are one, and
books are the source of both; whereas in science, as in life, learning
and knowledge are distinct, and the study of things, and not of books, is
the source of the latter.