excellent illustration of Cuvier’s sagacity, and he evidently takes
some pride in telling his story about it. A split slab of stone arrived
from the quarries of Montmartre, the two halves of which contained the
greater part of the skeleton of a small animal. On careful examinations
of the characters of the teeth and of the lower jaw, which happened
to be exposed, Cuvier assured himself that they presented such a very
close resemblance to the corresponding parts in the living opossums
that he at once assigned the fossil to that genus.
Now the opossums are unlike most mammals in that they possess two
bones attached to the fore part of the pelvis, which are commonly
called “marsupial bones.” The name is a misnomer, originally conferred
because it was thought that these bones have something to do with the
support of the pouch, or marsupium, with which some, but not all, of
the opossums are provided. As a matter of fact, they have nothing to do
with the support of the pouch, and they exist as much in those opossums
which have no pouches as in those which possess them. In truth, no one
knows what the use of these bones may be, nor has any valid theory
of their physiological import yet been suggested. And if we have no
knowledge of the physiological importance of the bones themselves, it
is obviously absurd to pretend that we are able to give physiological
reasons why the presence of these bones is associated with certain
peculiarities of the teeth and of the jaws. If any one knows why four
molar teeth and an inflected angle of the jaw are very generally found
along with marsupial bones, he has not yet communicated that knowledge
to the world.
If, however, Zadig was right in concluding from the likeness of the
hoof-prints which he observed to a horse’s that the creature which made
them had a tail like that of a horse, Cuvier, seeing that the teeth
and jaw of his fossil were just like those of an opossum, had the same
right to conclude that the pelvis would also be like an opossum’s; and
so strong was his conviction that this retrospective prophecy, about
an animal which he had never seen before, and which had been dead and
buried for millions of years, would be verified, that he went to work
upon the slab which contained the pelvis in confident expectation of
finding and laying bare the “marsupial bones,” to the satisfaction of
some persons whom he had invited to witness their disinterment. As
he says:—“Cette opération se fit en présence de quelques personnes à
qui j’en avais annoncé d’avance le résultat, dans l’intention de leur
prouver par le fait la justice de nos théories zoologiques; puisque le
vrai cachet d’une théorie est sans contredit la faculté qu’elle donne
de prévoir les phénomènes.”
In the “Ossemens fossiles” Cuvier leaves his paper just as it first
appeared in the “Annales du Muséum,” as “a curious monument of the
force of zoological laws and of the use which may be made of them.”
Zoological laws truly, but not physiological laws. If one sees a
live dog’s head, it is extremely probable that a dog’s tail is not
far off, though nobody can say why that sort of head and that sort
of tail go together; what physiological connection there is between
the two. So, in the case of the Montmartre fossil, Cuvier, finding a
thorough opossum’s head, concluded that the pelvis also would be like
an opossum’s. But, most assuredly, the most advanced physiologist of
the present day could throw no light on the question why these are
associated, nor could pretend to affirm that the existence of the one
is necessarily connected with that of the other. In fact, had it so
happened that the pelvis of the fossil had been originally exposed,
while the head lay hidden, the presence of the “marsupial bones,”
however like they might have been to an opossum’s, would by no means
have warranted the prediction that the skull would turn out to be that
of the opossum. It might just as well have been like that of some
other Marsupial; or even like that of the totally different group of
Monotremes, of which the only living representatives are the _Echidna_
and the _Ornithorhynchus_.
For all practical purposes, however, the empirical laws of
co-ordination of structures, which are embodied in the generalisations
of morphology, may be confidently trusted, if employed with due
caution, to lead to a just interpretation of fossil remains; or, in
other words, we may look for the verification of the retrospective
prophecies which are based upon them.
And if this be the case, the late advances which have been made in
palæontological discovery open out a new field for such prophecies.
For it has been ascertained with respect to many groups of animals,
that, as we trace them back in time, their ancestors gradually cease
to exhibit those special modifications which at present characterise
the type, and more nearly embody the general plan of the group to which
they belong.
Thus, in the well-known case of the horse, the toes which are
suppressed in the living horse are found to be more and more complete
in the older members of the group, until, at the bottom of the Tertiary
series of America, we find an equine animal which has four toes in
front and three behind. No remains of the horse tribe are at present
known from any Mesozoic deposit. Yet who can doubt that, whenever a
sufficiently extensive series of lacustrine and fluviatile beds of that
age becomes known, the lineage which has been traced thus far will be
continued by equine quadrupeds with an increasing number of digits,
until the horse type merges in the five-toed form towards which these
gradations point?
But the argument which holds good for the horse, holds good, not only
for all mammals, but for the whole animal world. And as the study of
the pedigrees, or lines of evolution, to which, at present, we have
access, brings to light, as it assuredly will do, the laws of that
process, we shall be able to reason from the facts with which the
geological record furnishes us to those which have hitherto remained,
and many of which, perhaps, may for ever remain, hidden. The same
method of reasoning which enables us, when furnished with a fragment of
an extinct animal, to prophesy the character which the whole organism
exhibited, will, sooner or later, enable us, when we know a few of the
later terms of a genealogical series, to predict the nature of the
earlier terms.
In no very distant future, the method of Zadig, applied to a greater
body of facts than the present generation is fortunate enough to
handle, will enable the biologist to reconstruct the scheme of life
from its beginning, and to speak as confidently of the character of
long extinct living beings, no trace of which has been preserved, as
Zadig did of the queen’s spaniel and the king’s horse. Let us hope that
they may be better rewarded for their toil and their sagacity than was
the Babylonian philosopher; for perhaps, by that time, the Magi also
may be reckoned among the members of a forgotten Fauna, extinguished in
the struggle for existence against their great rival, common sense.
VII.
ON THE BORDER TERRITORY BETWEEN THE ANIMAL AND THE
VEGETABLE KINGDOMS.
In the whole history of science there is nothing more remarkable than
the rapidity of the growth of biological knowledge within the last
half-century, and the extent of the modification which has thereby been
effected in some of the fundamental conceptions of the naturalist.
In the second edition of the “Règne Animal,” published in 1828,
Cuvier devotes a special section to the “Division of Organised Beings
into Animals and Vegetables,” in which the question is treated with
that comprehensiveness of knowledge and clear critical judgment
which characterise his writings, and justify us in regarding them
as representative expressions of the most extensive, if not the
profoundest, knowledge of his time. He tells us that living beings have
been subdivided from the earliest times into _animated beings_, which
possess sense and motion, and _inanimated beings_, which are devoid of
these functions, and simply vegetate.
Although the roots of plants direct themselves towards moisture, and
their leaves towards air and light,—although the parts of some plants
exhibit oscillating movements without any perceptible cause, and the
leaves of others retract when touched,—yet none of these movements
justify the ascription to plants of perception or of will. From the
mobility of animals, Cuvier, with his characteristic partiality for
teleological reasoning, deduces the necessity of the existence in them
of an alimentary cavity, or reservoir of food, whence their nutrition
may be drawn by the vessels, which are a sort of internal roots; and,
in the presence of this alimentary cavity, he naturally sees the
primary and the most important distinction between animals and plants.
Following out his teleological argument, Cuvier remarks that the
organisation of this cavity and its appurtenances must needs vary
according to the nature of the aliment, and the operations which it
has to undergo, before it can be converted into substances fitted for
absorption; while the atmosphere and the earth supply plants with
juices ready prepared, and which can be absorbed immediately. As the
animal body required to be independent of heat and of the atmosphere,
there were no means by which the motion of its fluids could be produced
by internal causes. Hence arose the second great distinctive character
of animals, or the circulatory system, which is less important than the
digestive, since it was unnecessary, and therefore is absent, in the
more simple animals.
Animals further needed muscles for locomotion and nerves for
sensibility. Hence, says Cuvier, it was necessary that the chemical
composition of the animal body should be more complicated than that of
the plant; and it is so, inasmuch as an additional substance, nitrogen,
enters into it as an essential element; while, in plants, nitrogen is
only accidentally joined with the three other fundamental constituents
of organic beings—carbon, hydrogen, and oxygen. Indeed, he afterwards
affirms that nitrogen is peculiar to animals; and herein he places the
third distinction between the animal and the plant. The soil and the
atmosphere supply plants with water, composed of hydrogen and oxygen;
air, consisting of nitrogen and oxygen; and carbonic acid, containing
carbon and oxygen. They retain the hydrogen and the carbon, exhale the
superfluous oxygen, and absorb little or no nitrogen. The essential
character of vegetable life is the exhalation of oxygen, which is
effected through the agency of light. Animals, on the contrary,
derive their nourishment either directly or indirectly from plants.
They get rid of the superfluous hydrogen and carbon, and accumulate
nitrogen. The relations of plants and animals to the atmosphere are
therefore inverse. The plant withdraws water and carbonic acid from
the atmosphere, the animal contributes both to it. Respiration—that
is, the absorption of oxygen and the exhalation of carbonic acid—is
the specially animal function of animals, and constitutes their fourth
distinctive character.
Thus wrote Cuvier in 1828. But, in the fourth and fifth decades of
this century, the greatest and most rapid revolution which biological
science has ever undergone was effected by the application of the
modern microscope to the investigation of organic structure; by the
introduction of exact and easily manageable methods of conducting the
chemical analysis of organic compounds; and finally, by the employment
of instruments of precision for the measurement of the physical forces
which are at work in the living economy.
That the semi-fluid contents (which we now term protoplasm) of the
cells of certain plants, such as the _Charæ_, are in constant and
regular motion, was made out by Bonaventura Corti a century ago; but
the fact, important as it was, fell into oblivion, and had to be
rediscovered by Treviranus in 1807. Robert Brown noted the more complex
motions of the protoplasm in the cells of _Tradescantia_ in 1831; and
now such movements of the living substance of plants are well known to
be some of the most widely-prevalent phenomena of vegetable life.
Agardh, and other of the botanists of Cuvier’s generation, who occupied
themselves with the lower plants, had observed that, under particular
circumstances, the contents of the cells of certain water-weeds were
set free, and moved about with considerable velocity, and with all the
appearances of spontaneity, as locomotive bodies, which, from their
similarity to animals of simple organisation, were called “zoospores.”
Even as late as 1845, however, a botanist of Schleiden’s eminence dealt
very sceptically with these statements; and his scepticism was the more
justified, since Ehrenberg, in his elaborate and comprehensive work
on the _Infusoria_, had declared the greater number of what are now
recognised as locomotive plants to be animals.
At the present day, innumerable plants and free plant cells are known
to pass the whole or part of their lives in an actively locomotive
condition, in no wise distinguishable from that of one of the simpler
animals; and, while in this condition, their movements are, to all
appearance, as spontaneous—as much the product of volition—as those of
such animals.
Hence the teleological argument for Cuvier’s first diagnostic
character—the presence in animals of an alimentary cavity, or internal
pocket, in which they can carry about their nutriment—has broken
down, so far, at least, as his mode of stating it goes. And, with the
advance of microscopic anatomy, the universality of the fact itself
among animals has ceased to be predicable. Many animals of even complex
structure, which live parasitically within others, are wholly devoid
of an alimentary cavity. Their food is provided for them, not only
ready cooked, but ready digested, and the alimentary canal, become
superfluous, has disappeared. Again, the males of most Rotifers have no
digestive apparatus; as a German naturalist has remarked, they devote
themselves entirely to the “Minnedienst,” and are to be reckoned among
the few realisations of the Byronic ideal of a lover. Finally, amidst
the lowest forms of animal life, the speck of gelatinous protoplasm,
which constitutes the whole body, has no permanent digestive cavity or
mouth, but takes in its food anywhere; and digests, so to speak, all
over its body.
But although Cuvier’s leading diagnosis of the animal from the plant
will not stand a strict test, it remains one of the most constant of
the distinctive characters of animals. And, if we substitute for the
possession of an alimentary cavity, the power of taking solid nutriment
into the body and there digesting it, the definition so changed
will cover all animals, except certain parasites, and the few and
exceptional cases of non-parasitic animals which do not feed at all. On
the other hand, the definition thus amended will exclude all ordinary
vegetable organisms.
Cuvier himself practically gives up his second distinctive mark when he
admits that it is wanting in the simpler animals.
The third distinction is based on a completely erroneous conception of
the chemical differences and resemblances between the constituents of
animal and vegetable organisms, for which Cuvier is not responsible,
as it was current among contemporary chemists. It is now established
that nitrogen is as essential a constituent of vegetable as of animal
living matter; and that the latter is, chemically speaking, just as
complicated as the former. Starchy substances, cellulose and sugar,
once supposed to be exclusively confined to plants, are now known to
be regular and normal products of animals. Amylaceous and saccharine
substances are largely manufactured, even by the highest animals;
cellulose is widespread as a constituent of the skeletons of the lower
animals; and it is probable that amyloid substances are universally
present in the animal organism, though not in the precise form of
starch.
Moreover, although it remains true that there is an inverse relation
between the green plant in sunshine and the animal, in so far as, under
these circumstances, the green plant decomposes carbonic acid and
exhales oxygen, while the animal absorbs oxygen and exhales carbonic
acid; yet, the exact researches of the modern chemical investigators
of the physiological processes of plants have clearly demonstrated the
fallacy of attempting to draw any general distinction between animals
and vegetables on this ground. In fact, the difference vanishes with
the sunshine, even in the case of the green plant; which, in the dark,
absorbs oxygen and gives out carbonic acid like any animal.[30] On
the other hand, those plants, such as the fungi, which contain no
chlorophyll and are not green, are always, so far as respiration is
concerned, in the exact position of animals. They absorb oxygen and
give out carbonic acid.
Thus, by the progress of knowledge, Cuvier’s fourth distinction
between the animal and the plant has been as completely invalidated as
the third and second; and even the first can be retained only in a
modified form and subject to exceptions.
But has the advance of biology simply tended to break down old
distinctions, without establishing new ones?
With a qualification, to be considered presently, the answer to this
question is undoubtedly in the affirmative. The famous researches of
Schwann and Schleiden in 1837 and the following years, founded the
modern science of histology, or that branch of anatomy which deals
with the ultimate visible structure of organisms, as revealed by
the microscope; and, from that day to this, the rapid improvement
of methods of investigation, and the energy of a host of accurate
observers, have given greater and greater breadth and firmness to
Schwann’s great generalisation, that a fundamental unity of structure
obtains in animals and plants; and that, however diverse may be the
fabrics, or _tissues_, of which their bodies are composed, all these
varied structures result from the metamorphosis of morphological units
(termed _cells_, in a more general sense than that in which the word
“cells” was at first employed), which are not only similar in animals
and in plants respectively, but present a close resemblance, when those
of animals and those of plants are compared together.
The contractility which is the fundamental condition of locomotion,
has not only been discovered to exist far more widely among plants
than was formerly imagined; but, in plants, the act of contraction has
been found to be accompanied, as Dr. Burdon Sanderson’s interesting
investigations have shown, by a disturbance of the electrical state of
the contractile substance, comparable to that which was found by Du
Bois Reymond to be a concomitant of the activity of ordinary muscle in
animals.
Again, I know of no test by which the reaction of the leaves of the
Sundew and of other plants to stimuli, so fully and carefully studied
by Mr. Darwin, can be distinguished from those acts of contraction
following upon stimuli, which are called “reflex” in animals.
On each lobe of the bilobed leaf of Venus’s fly trap (_Dionæa
muscipula_) are three delicate filaments which stand out at right angle
from the surface of the leaf. Touch one of them with the end of a fine
human hair and the lobes of the leaf instantly close together[31] in
virtue of an act of contraction of part of their substance, just as the
body of a snail contracts into its shell when one of its “horns” is
irritated.
The reflex action of the snail is the result of the presence of a
nervous system in the animal. A molecular change takes place in the
nerve of the tentacle, is propagated to the muscles by which the body
is retracted, and causing them to contract, the act of retraction
is brought about. Of course the similarity of the acts does not
necessarily involve the conclusion that the mechanism by which they are
effected is the same; but it suggests a suspicion of their identity
which needs careful testing.
The results of recent inquiries into the structure of the nervous
system of animals converge towards the conclusion that the nerve
fibres, which we have hitherto regarded as ultimate elements of nervous
tissue, are not such, but are simply the visible aggregations of vastly
more attenuated filaments, the diameter of which dwindles down to the
limits of our present microscopic vision, greatly as these have been
extended by modern improvements of the microscope; and that a nerve
is, in its essence, nothing but a linear tract of specially modified
protoplasm between two points of an organism—one of which is able to
affect the other by means of the communication so established. Hence,
it is conceivable that even the simplest living being may possess a
nervous system. And the question whether plants are provided with a
nervous system or not, thus acquires a new aspect, and presents the
histologist and physiologist with a problem of extreme difficulty,
which must be attacked from a new point of view and by the aid of
methods which have yet to be invented.
Thus it must be admitted that plants may be contractile and locomotive;
that, while locomotive, their movements may have as much appearance
of spontaneity as those of the lowest animals; and that many exhibit
actions, comparable to those which are brought about by the agency of a
nervous system in animals. And it must be allowed to be possible that
further research may reveal the existence of something comparable to a
nervous system in plants. So that I know not where we can hope to find
any absolute distinction between animals and plants, unless we return
to their mode of nutrition, and inquire whether certain differences of
a more occult character than those imagined to exist by Cuvier, and
which certainly hold good for the vast majority of animals and plants,
are of universal application.
A bean may be supplied with water in which salts of ammonia and
certain other mineral salts are dissolved in due proportion; with
atmospheric air containing its ordinary minute dose of carbonic acid;
and with nothing else but sunlight and heat. Under these circumstances,
unnatural as they are, with proper management, the bean will thrust
forth its radicle and its plumule; the former will grow down into
roots, the latter grow up into the stem and leaves of a vigorous bean
plant; and this plant will, in due time, flower and produce its crop of
beans, just as if it were grown in the garden or in the field.
The weight of the nitrogenous protein compounds, of the oily, starchy,
saccharine and woody substances contained in the full-grown plant
and its seeds, will be vastly greater than the weight of the same
substances contained in the bean from which it sprang. But nothing has
been supplied to the bean save water, carbonic acid, ammonia, potash,
lime, iron, and the like, in combination with phosphoric, sulphuric,
and other acids. Neither protein, nor fat, nor starch, nor sugar, nor
any substance in the slightest degree resembling them, has formed part
of the food of the bean. But the weights of the carbon, hydrogen,
oxygen, nitrogen, phosphorus, sulphur, and other elementary bodies
contained in the bean-plant, and in the seeds which it produces, are
exactly equivalent to the weights of the same elements which have
disappeared from the materials supplied to the bean during its growth.
Whence it follows that the bean has taken in only the raw materials of
its fabric, and has manufactured them into bean stuffs.