less and less decisive, in proportion to the complexity of the phænomena
to be explored; and therefore we saw this resource to be less effectual
in chemistry than in physics: and we now find that it is eminently
useful in chemistry in comparison with physiology. _In fact, the nature
of the phænomena seems to offer almost insurmountable impediments to any
extensive and prolific application of such a procedure in
biology._"--Comte, vol i. p. 367.
M. Comte, as his manner is, contradicts himself two pages further on,
but that will hardly relieve him from the responsibility of such a
paragraph as the above.
[6] "Nouvelle Fonction du Foie considéré comme organe producteur de
matière sucrée chez l'Homme et les Animaux," par M. Claude Bernard.
[7] "_Natural Groups given by Type, not by Definition...._ The class is
steadily fixed, though not precisely limited; it is given, though not
circumscribed; it is determined, not by a boundary-line without, but by
a central point within; not by what it strictly excludes, but what it
eminently includes; by an example, not by a precept; in short, instead
of Definition we have a _Type_ for our director. A type is an example of
any class, for instance, a species of a genus, which is considered as
eminently possessing the characters of the class. All the species which
have a greater affinity with this type-species than with any others,
form the genus, and are ranged about it, deviating from it in various
directions and different degrees."--WHEWELL, _The Philosophy of the
Inductive Sciences_, vol. i. pp. 476, 477.
[8] Save for the pleasure of doing so, I need hardly point out my
obligations to Mr. J.S. Mill's "System of Logic," in this view of
scientific method.
VI.
ON THE STUDY OF ZOOLOGY.
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."
Linnæus was a naturalist in this wide sense, and his "Systema Naturæ"
was a work upon natural history, in the broadest acceptation of the
term; in it, that great methodizing spirit embodied all that was known
in his time of the distinctive characters of minerals, animals, and
plants. But the enormous stimulus which Linnæus gave to the
investigation of nature soon rendered it impossible that any one man
should write another "Systema Naturæ," and extremely difficult for any
one to become a naturalist such as Linnæus 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
functions 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 palæontologist. If his mind is more particularly directed to the
description, specific discrimination, classification, and distribution
of animals, he is termed a zoologist.
For the purposes 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 fore
part 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 moveable stalks. Behind these, on the under side of the body, are
two pairs of long feelers, or antennæ, 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
moveable, 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 moveable, 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
_Clenterata_ 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 _Clenterata_, 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.