Discourses - Thomas H. Huxley
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During these processes of multiplication by fission, the _Heteromita_
remains active; but sometimes another mode of fission occurs. The body
becomes rounded and quiescent, or nearly so; and, while in this resting
state, divides into two portions, each of which is rapidly converted into
an active _Heteromita_.
A still more remarkable phenomenon is that kind of multiplication which
is preceded by the union of two monads, by a process which is termed
_conjugation_. Two active _Heteromitoe_ become applied to one another,
and then slowly and gradually coalesce into one body. The two nuclei run
into one; and the mass resulting from the conjugation of the two
_Heteromitoe_, thus fused together, has a triangular form. The two pairs
of cilia are to be seen, for some time, at two of the angles, which
answer to the small ends of the conjoined monads; but they ultimately
vanish, and the twin organism, in which all visible traces of
organisation have disappeared, falls into a state of rest. Sudden wave-
like movements of its substance next occur; and, in a short time, the
apices of the triangular mass burst, and give exit to a dense yellowish,
glairy fluid, filled with minute granules. This process, which, it will
be observed, involves the actual confluence and mixture of the substance
of two distinct organisms, is effected in the space of about two hours.
The authors whom I quote say that they "cannot express" the excessive
minuteness of the granules in question, and they estimate their diameter
at less than 1/200000 of an inch. Under the highest powers of the
microscope, at present applicable, such specks are hardly discernible.
Nevertheless, particles of this size are massive when compared to
physical molecules; whence there is no reason to doubt that each, small
as it is, may have a molecular structure sufficiently complex to give
rise to the phenomena of life. And, as a matter of fact, by patient
watching of the place at which these infinitesimal living particles were
discharged, our observers assured themselves of their growth and
development into new monads. In about four hours from their being set
free, they had attained a sixth of the length of the parent, with the
characteristic cilia, though at first they were quite motionless; and, in
four hours more, they had attained the dimensions and exhibited all the
activity of the adult. These inconceivably minute particles are therefore
the germs of the _Heteromita_; and from the dimensions of these germs it
is easily shown that the body formed by conjugation may, at a low
estimate, have given exit to thirty thousand of them; a result of a
matrimonial process whereby the contracting parties, without a metaphor,
"become one flesh," enough to make a Malthusian despair of the future of
the Universe.
I am not aware that the investigators from whom I have borrowed this
history have endeavoured to ascertain whether their monads take solid
nutriment or not; so that though they help us very much to fill up the
blanks in the history of my _Heteromita_, their observations throw no
light on the problem we are trying to solve--Is it an animal or is it a
plant?
Undoubtedly it is possible to bring forward very strong arguments in
favour of regarding _Heteromita_ as a plant.
For example, there is a Fungus, an obscure and almost microscopic mould,
termed _Peronospora infestans_. Like many other Fungi, the _Peronosporoe_
are parasitic upon other plants; and this particular _Peronospora_
happens to have attained much notoriety and political importance, in a
way not without a parallel in the career of notorious politicians,
namely, by reason of the frightful mischief it has done to mankind. For
it is this _Fungus_ which is the cause of the potato disease; and,
therefore, _Peronospora infestans_ (doubtless of exclusively Saxon
origin, though not accurately known to be so) brought about the Irish
famine. The plants afflicted with the malady are found to be infested by
a mould, consisting of fine tubular filaments, termed _hyphoe_, which
burrow through the substance of the potato plant, and appropriate to
themselves the substance of their host; while, at the same time, directly
or indirectly, they set up chemical changes by which even its woody
framework becomes blackened, sodden, and withered.
In structure, however, the _Peronospora_ is as much a mould as the common
_Penicillium_; and just as the _Penicillium_ multiplies by the breaking
up of its hyphoe into separate rounded bodies, the spores; so, in the
_Peronospora_, certain of the hyphoe grow out into the air through the
interstices of the superficial cells of the potato plant, and develop
spores. Each of these hyphoe usually gives off several branches. The ends
of the branches dilate and become closed sacs, which eventually drop off
as spores. The spores falling on some part of the same potato plant, or
carried by the wind to another, may at once germinate, throwing out
tubular prolongations which become hyphoe, and burrow into the substance
of the plant attacked. But, more commonly, the contents of the spore
divide into six or eight separate portions. The coat of the spore gives
way, and each portion then emerges as an independent organism, which has
the shape of a bean, rather narrower at one end than the other, convex on
one side, and depressed or concave on the opposite. From the depression,
two long and delicate cilia proceed, one shorter than the other, and
directed forwards. Close to the origin of these cilia, in the substance
of the body, is a regularly pulsating, contractile vacuole. The shorter
cilium vibrates actively, and effects the locomotion of the organism,
while the other trails behind; the whole body rolling on its axis with
its pointed end forwards.
The eminent botanist, De Bary, who was not thinking of our problem, tells
us, in describing the movements of these "Zoospores," that, as they swim
about, "Foreign bodies are carefully avoided, and the whole movement has
a deceptive likeness to the voluntary changes of place which are observed
in microscopic animals."
After swarming about in this way in the moisture on the surface of a leaf
or stem (which, film though it may be, is an ocean to such a fish) for
half an hour, more or less, the movement of the zoospore becomes slower,
and is limited to a slow turning upon its axis, without change of place.
It then becomes quite quiet, the cilia disappear, it assumes a spherical
form, and surrounds itself with a distinct, though delicate, membranous
coat. A protuberance then grows out from one side of the sphere, and
rapidly increasing in length, assumes the character of a hypha. The
latter penetrates into the substance of the potato plant, either by
entering a stomate, or by boring through the wall of an epidermic cell,
and ramifies, as a mycelium, in the substance of the plant, destroying
the tissues with which it comes in contact. As these processes of
multiplication take place very rapidly, millions of spores are soon set
free from a single infested plant; and, from their minuteness, they are
readily transported by the gentlest breeze. Since, again, the zoospores
set free from each spore, in virtue of their powers of locomotion,
swiftly disperse themselves over the surface, it is no wonder that the
infection, once started, soon spreads from field to field, and extends
its ravages over a whole country.
However, it does not enter into my present plan to treat of the potato
disease, instructively as its history bears upon that of other epidemics;
and I have selected the case of the _Peroganspora_ simply because it
affords an example of an organism, which, in one stage of its existence,
is truly a "Monad," indistinguishable by any important character from our
_Heteromita_, and extraordinarily like it in some respects. And yet this
"Monad" can be traced, step by step, through the series of metamorphoses
which I have described, until it assumes the features of an organism,
which is as much a plant as is an oak or an elm.
Moreover, it would be possible to pursue the analogy farther. Under
certain circumstances, a process of conjugation takes place in the
_Peronospora_. Two separate portions of its protoplasm become fused
together, surround themselves with a thick coat and give rise to a sort
of vegetable egg called an _oospore_. After a period of rest, the
contents of the oospore break up into a number of zoospores like those
already described, each of which, after a period of activity, germinates
in the ordinary way. This process obviously corresponds with the
conjugation and subsequent setting free of germs in the _Heteromita_.
But it may be said that the _Peronospora_ is, after all, a questionable
sort of plant; that it seems to be wanting in the manufacturing power,
selected as the main distinctive character of vegetable life; or, at any
rate, that there is no proof that it does not get its protein matter
ready made from the potato plant.
Let us, therefore, take a case which is not open to these objections.
There are some small plants known to botanists as members of the genus
_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.