_Colcochaete_, which, without being truly parasitic, grow upon certain
water-weeds, as lichens grow upon trees. The little plant has the form of
an elegant green star, the branching arms of which are divided into
cells. Its greenness is due to its chlorophyll, and it undoubtedly has
the manufacturing power in full degree, decomposing carbonic acid and
setting oxygen free, under the influence of sunlight. But the
protoplasmic contents of some of the cells of which the plant is made up
occasionally divide, by a method similar to that which effects the
division of the contents of the _Peronospora_ spore; and the severed
portions are then set free as active monad-like zoospores. Each is oval
and is provided at one extremity with two long active cilia. Propelled by
these, it swims about for a longer or shorter time, but at length comes
to a state of rest and gradually grows into a _Coleochaete_. Moreover, as
in the _Peronospora_, conjugation may take place and result in an
oospore; the contents of which divide and are set free as monadiform
germs.
If the whole history of the zoospores of _Peronospora_ and of
_Coleochaete_ were unknown, they would undoubtedly be classed among
"Monads" with the same right as _Heteromita_; why then may not
_Heteromita_ be a plant, even though the cycle of forms through which it
passes shows no terms quite so complex as those which occur in
_Peronospora_ and _Coleochaete_? And, in fact, there are some green
organisms, in every respect characteristically plants, such as
_Chlamydomonas_, and the common _Volvox_, or so-called "Globe
animalcule," which run through a cycle of forms of just the same simple
character as those of _Heteromita_.
The name of _Chlamydomonas_ is applied to certain microscopic green
bodies, each of which consists of a protoplasmic central substance
invested by a structureless sac. The latter contains cellulose, as in
ordinary plants; and the chlorophyll which gives the green colour enables
the _Chlamydomonas_ to decompose carbonic acid and fix carbon as they do.
Two long cilia protrude through the cell-wall, and effect the rapid
locomotion of this "monad," which, in all respects except its mobility,
is characteristically a plant. Under ordinary circumstances, the
_Chlamydomonas_ multiplies by simple fission, each splitting into two or
into four parts, which separate and become independent organisms.
Sometimes, however, the _Chlamydomonas_ divides into eight parts, each of
which is provided with four instead of two cilia. These "zoospores"
conjugate in pairs, and give rise to quiescent bodies, which multiply by
division, find eventually pass into the active state.
Thus, so far as outward form and the general character of the cycle of
modifications, through which the organism passes in the course of its
life, are concerned, the resemblance between _Chlamydomonas_ and
_Heteromita_ is of the closest description. And on the face of the matter
there is no ground for refusing to admit that _Heteromita_ may be related
to _Chlamydomonas_, as the colourless fungus is to the green alga.
_Volvox_ may be compared to a hollow sphere, the wall of which is made up
of coherent Chlamydomonads; and which progresses with a rotating motion
effected by the paddling of the multitudinous pairs of cilia which
project from its surface. Each _Volvox_-monad, moreover, possesses a red
pigment spot, like the simplest form of eye known among animals. The
methods of fissive multiplication and of conjugation observed in the
monads of this locomotive globe are essentially similar to those observed
in _Chlamydomonas_; and, though a hard battle has been fought over it,
_Volvox_ is now finally surrendered to the Botanists.
Thus there is really no reason why _Heteromita_ may not be a plant; and
this conclusion would be very satisfactory, if it were not equally easy
to show that there is really no reason why it should not be an animal.
For there are numerous organisms presenting the closest resemblance to
_Heteromita_, and, like it, grouped under the general name of "Monads,"
which, nevertheless, can be observed to take in solid nutriment, and
which, therefore, have a virtual, if not an actual, mouth and digestive
cavity, and thus come under Cuvier's definition of an animal. Numerous
forms of such animals have been described by Ehrenberg, Dujardin, H.
James Clark, and other writers on the _Infusoria_. Indeed, in another
infusion of hay in which my _Heteromita lens_ occurred, there were
innumerable such infusorial animalcules belonging to the well-known
species _Colpoda cucullus_.[6]
[Footnote 6: Excellently described by Stein, almost all of whose
statements I have verified.]
Full-sized specimens of this animalcule attain a length of between 1/300
or 1/400 of an inch, so that it may have ten times the length and a
thousand times the mass of a _Heteromita_. In shape, it is not altogether
unlike _Heteromita_. The small end, however, is not produced into one
long cilium, but the general surface of the body is covered with small
actively vibrating ciliary organs, which are only longest at the small
end. At the point which answers to that from which the two cilia arise in
_Heteromita_, there is a conical depression, the mouth; and, in young
specimens, a tapering filament, which reminds one of the posterior cilium
of _Heteromita_, projects from this region.
The body consists of a soft granular protoplasmic substance, the middle
of which is occupied by a large oval mass called the "nucleus"; while, at
its hinder end, is a "contractile vacuole," conspicuous by its regular
rhythmic appearances and disappearances. Obviously, although the
_Colpoda_ is not a monad, it differs from one only in subordinate
details. Moreover, under certain conditions, it becomes quiescent,
incloses itself in a delicate case or _cyst_, and then divides into two,
four, or more portions, which are eventually set free and swim about as
active _Colpodoe_.
But this creature is an unmistakable animal, and full-sized _Colpodoe_
may be fed as easily as one feeds chickens. It is only needful to diffuse
very finely ground carmine through the water in which they live, and, in
a very short time, the bodies of the _Colpodoe_ are stuffed with the
deeply-coloured granules of the pigment.
And if this were not sufficient evidence of the animality of _Colpoda_,
there comes the fact that it is even more similar to another well-known
animalcule, _Paramoecium_, than it is to a monad. But _Paramoecium_ is so
huge a creature compared with those hitherto discussed--it reaches 1/120
of an inch or more in length--that there is no difficulty in making out
its organisation in detail; and in proving that it is not only an animal,
but that it is an animal which possesses a somewhat complicated
organisation. For example, the surface layer of its body is different in
structure from the deeper parts. There are two contractile vacuoles, from
each of which radiates a system of vessel-like canals; and not only is
there a conical depression continuous with a tube, which serve as mouth
and gullet, but the food ingested takes a definite course, and refuse is
rejected from a definite region. Nothing is easier than to feed these
animals, and to watch the particles of indigo or carmine accumulate at
the lower end of the gullet. From this they gradually project, surrounded
by a ball of water, which at length passes with a jerk, oddly simulating
a gulp, into the pulpy central substance of the body, there to circulate
up one side and down the other, until its contents are digested and
assimilated. Nevertheless, this complex animal multiplies by division, as
the monad does, and, like the monad, undergoes conjugation. It stands in
the same relation to _Heteromita_ on the animal side, as _Coleochaete_
does on the plant side. Start from either, and such an insensible series
of gradations leads to the monad that it is impossible to say at any
stage of the progress where the line between the animal and the plant
must be drawn.
There is reason to think that certain organisms which pass through a
monad stage of existence, such as the _Myxomycetes_, are, at one time of
their lives, dependent upon external sources for their protein matter, or
are animals; and, at another period, manufacture it, or are plants. And
seeing that the whole progress of modern investigation is in favour of
the doctrine of continuity, it is a fair and probable speculation--though
only a speculation--that, as there are some plants which can manufacture
protein out of such apparently intractable mineral matters as carbonic
acid, water, nitrate of ammonia, metallic and earthy salts; while others
need to be supplied with their carbon and nitrogen in the somewhat less
raw form of tartrate of ammonia and allied compounds; so there may be yet
others, as is possibly the case with the true parasitic plants, which can
only manage to put together materials still better prepared--still more
nearly approximated to protein--until we arrive at such organisms as the
_Psorospermioe_ and the _Panhistophyton_, which are as much animal as
vegetable in structure, but are animal in their dependence on other
organisms for their food.
The singular circumstance observed by Meyer, that the _Torula_ of yeast,
though an indubitable plant, still flourishes most vigorously when
supplied with the complex nitrogenous substance, pepsin; the probability
that the _Peronospora_ is nourished directly by the protoplasm of the
potato-plant; and the wonderful facts which have recently been brought to
light respecting insectivorous plants, all favour this view; and tend to
the conclusion that the difference between animal and plant is one of
degree rather than of kind, and that the problem whether, in a given
case, an organism is an animal or a plant, may be essentially insoluble.
VII
A LOBSTER; OR, THE STUDY OF ZOOLOGY
[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 is 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 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.