On the Study of Zoology
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Thomas H. Huxley >> On the Study of Zoology
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ON THE STUDY OF ZOOLOGY*
by Thomas H. Huxley
[footnote] *A Lecture delivered at the South Kensington
Museum in 1861.
NATURAL HISTORY is the name familiarly applied to the study of the
properties of such natural bodies as minerals, plants, and animals; the
sciences which embody the knowledge man has acquired upon these
subjects are commonly termed Natural Sciences, in contradistinction to
other so-called "physical" sciences; and those who devote themselves
especially to the pursuit of such sciences have been and are commonly
termed "Naturalists."
Linnaeus was a naturalist in this wide sense, and his 'Systema Naturae'
was a work upon natural history, in the broadest acceptation of the
term; in it, that great methodising spirit embodied all that was known
in his time of the distinctive characters of minerals, animals, and
plants. But the enormous stimulus which Linnaeus gave to the
investigation of nature soon rendered it impossible that any one man
should write another 'Systema Naturae,' and extremely difficult for any
one to become even a naturalist such as Linnaeus was.
Great as have been the advances made by all the three branches of
science, of old included under the title of natural history, there can
be no doubt that zoology and botany have grown in an enormously greater
ratio than mineralogy; and hence, as I suppose, the name of "natural
history" has gradually become more and more definitely attached to these
prominent divisions of the subject, and by "naturalist" people have
meant more and more distinctly to imply a student of the structure and
function of living beings.
However this may be, it is certain that the advance of knowledge has
gradually widened the distance between mineralogy and its old
associates, while it has drawn zoology and botany closer together; so
that of late years it has been found convenient (and indeed necessary)
to associate the sciences which deal with vitality and all its
phenomena under the common head of "biology"; and the biologists have
come to repudiate any blood-relationship with their foster-brothers,
the mineralogists.
Certain broad laws have a general application throughout both the animal
and the vegetable worlds, but the ground common to these kingdoms of
nature is not of very wide extent, and the multiplicity of details is
so great, that the student of living beings finds himself obliged to
devote his attention exclusively either to the one or the other. If he
elects to study plants, under any aspect, we know at once what to call
him. He is a botanist, and his science is botany. But if the
investigation of animal life be his choice, the name generally applied
to him will vary according to the kind of animals he studies, or the
particular phenomena of animal life to which he confines his
attention. If the study of man is his object, he is called an
anatomist, or a physiologist, or an ethnologist; but if he dissects
animals, or examines into the mode in which their functions are
performed, he is a comparative anatomist or comparative physiologist.
If he turns his attention to fossil animals, he is a palaeontologist.
If his mind is more particularly directed to the specific description,
discrimination, classification, and distribution of animals, he is
termed a zoologist.
For the purpose of the present discourse, however, I shall recognise
none of these titles save the last, which I shall employ as the
equivalent of botanist, and I shall use the term zoology as denoting
the whole doctrine of animal life, in contradistinction to botany, which
signifies the whole doctrine of vegetable life.
Employed in this sense, zoology, like botany, is divisible into three
great but subordinate sciences, morphology, physiology, and
distribution, each of which may, to a very great extent, be studied
independently of the other.
Zoological morphology is the doctrine of animal form or structure.
Anatomy is one of its branches; development is another; while
classification is the expression of the relations which different
animals bear to one another, in respect of their anatomy and their
development.
Zoological distribution is the study of animals in relation to the
terrestrial conditions which obtain now, or have obtained at any
previous epoch of the earth's history.
Zoological physiology, lastly, is the doctrine of the functions or
actions of animals. It regards animal bodies as machines impelled by
certain forces, and performing an amount of work which can be expressed
in terms of the ordinary forces of nature. The final object of
physiology is to deduce the facts of morphology, on the one hand, and
those of distribution on the other, from the laws of the molecular
forces of matter.
Such is the scope of zoology. But if I were to content myself with the
enunciation of these dry definitions, I should ill exemplify that
method of teaching this branch of physical science, which it is my
chief business to-night to recommend. Let us turn away then from
abstract definitions. Let us take some concrete living thing, some
animal, the commoner the better, and let us see how the application of
common sense and common logic to the obvious facts it presents,
inevitably leads us into all these branches of zoological science.
I have before me a lobster. When I examine it, what appears to be the
most striking character it presents? Why, I observe that this part
which we call the tail of the lobster, is made up of six distinct hard
rings and a seventh terminal piece. If I separate one of the middle
rings, say the third, I find it carries upon its under surface a pair
of limbs or appendages, each of which consists of a stalk and two
terminal pieces. So that I can represent a transverse section of the
ring and its appendages upon the diagram board in this way.
If I now take the fourth ring, I find it has the same structure, and so
have the fifth and the second; so that, in each of these divisions of
the tail, I find parts which correspond with one another, a ring and
two appendages; and in each appendage a stalk and two end pieces.
These corresponding parts are called, in the technical language of
anatomy, "homologous parts." The ring of the third division is the
"homologue" of the ring of the fifth, the appendage of the former is
the homologue of the appendage of the latter. And, as each division
exhibits corresponding parts in corresponding places, we say that all
the divisions are constructed upon the same plan. But now let us
consider the sixth division. It is similar to, and yet different from,
the others. The ring is essentially the same as in the other divisions;
but the appendages look at first as if they were very different; and
yet when we regard them closely, what do we find? A stalk and two
terminal divisions, exactly as in the others, but the stalk is very
short and very thick, the terminal divisions are very broad and flat,
and one of them is divided into two pieces.
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 organization 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, whose multiplicity and
complexity, 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 organized upon one or
other of the five, or more, plans, whose existence 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. 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 alike, 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.
In this broad sense, it may with truth be said, that all living animals,
and all those dead creations which geology reveals, are bound together
by an all-pervading unity of organization, 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
'Homarus'; 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, whose aim 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 whose
sixth joint 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 this 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.