CUVIER
Cuvier was perhaps the greatest of comparative anatomists; his work
is, in the best sense of the word, classical.
Like all his predecessors, like Aristotle, like the Italian
anatomists, Cuvier studied structure and function together, even gave
function the primacy.
Some functions, he says,[41] are common to all organised bodies--origin
by generation, growth by nutrition, end by death. There are also
secondary functions. Of these the most important, in animals at least,
are the faculties of feeling and moving. These two faculties are
necessarily bound up together; if Nature has given animals sensation
she must also have given them the power of movement, the power to flee
from what is harmful and draw near to what is good. These two
faculties determine all the others. A creature that feels and moves
requires a stomach to carry food in. Food requires instruments to
divide it, liquids to digest it. Plants, which do not feel and do not
move, have no need of a stomach, but have roots instead. Thus the
"Animal Functions" of feeling and moving determine the character of
the organs of the second order, the organs of digestion. These in
their turn are prior to the organs of circulation, which are a means
to the end of distributing the nutrient fluid or blood to all parts of
the body. These organs of the third order are not only dependent on
those of the second order, but are also not even necessary, for many
animals are without them. Only animals with a circulatory system can
have definite breathing organs--lungs or gills. Plants, and animals
without a circulation, breathe by their whole surface.
There is accordingly a rational order of functions, and therefore of
the systems of organs which perform them. The most important are the
Animal Functions, with their great organ-system, the neuro-muscular
mechanism. Then come the digestive functions, and after them, and in a
sense accessory to them, the functions and organs of circulation and
respiration. The last three may be grouped as the Vital Functions.
The Animal Functions not only determine the character of the Vital
Functions, but influence also the primary faculty of generation, for
animals' power of movement has rendered their mode of fecundation more
simple, has therefore had an effect on their organs of generation.
This division into "Animal" and "Vital" functions recalls Buffon's and
Bichat's distinction of the "animal" and the "vegetative" lives.
Cuvier apparently took this idea from Buffon, for he says that a plant
is an animal that sleeps.[42] But the idea is as old as Aristotle, who
discusses the "sleep" of embryos and of plants in the last book of the
_De Generatione animalium_. The distinction between animal and
vegetative life is, of course, based for Aristotle in the difference
between the [Greek: psyche aisthetike] and the [Greek: psyche
threptike]. Cuvier, like Aristotle, Buffon, and Bichat, makes the
heart the centre of the "vegetative" organs.
It is important to note that Cuvier puts function before structure,
and infers from function what the organ will be. "Plants," he writes,
"having few faculties, have a very simple organisation."[43] It is only
after having discussed and classified functions that Cuvier goes on to
examine organs.
First his views on the composition of the animal body. Aristotle
distinguished three degrees of composition--the "elements," the
homogeneous parts, and the heterogeneous parts or organs. Cuvier does
the same. Some small advance has been made in the two thousand years'
interval, due in the first place to the progress of chemistry, and in
the second to the invention of the microscope. To the first
circumstance Cuvier owes his knowledge that the inorganic substances
forming the first degree of composition are principally C, N, H, O,
and P, combined to form albumen, fibrine, and the like, which are in
their turn combined to form the solids and fluids of the body. To the
latter circumstance Cuvier owes the statement that the finest
fragments into which mechanical division can resolve the organism are
little flakes and filaments, which, joined up loosely together, form a
"cellulosity." The discovery of the true cellular nature of animal
tissues did not come till much later, till some years after Cuvier's
death in 1832. Knowledge of histological detail was, however,
considerable by the beginning of the 19th century. Cuvier knew, for
example, that each muscle fibre has its own nerve fibre. But he gives
no elaborate account of the homogeneous parts, no detailed histology.
On the other hand his treatment of the heterogeneous parts or organs
is detailed and masterly.[44]
The main systems of organs are, in order of importance, the nervous
and muscular, the digestive, the circulatory, and the respiratory.
Each organ or system of organs may have many forms. If any form of any
organ could exist in combination with any form of all the others there
would be an enormous number of combinations theoretically possible.
But these combinations do not all exist in Nature, for organs are not
merely assembled (_rapproche's_), but act upon one another, and act
all together for a common end. Accordingly only the combinations that
fulfil these conditions exist in Nature. Cuvier thus dismisses the
question of a science of possible organic forms and considers only the
forms or combinations actually existing. This question of the
possibility of a "theoretical" morphology of living things, after the
fashion of the morphology of crystals with their sixteen possible
types, was raised in later years by K. G. Carus, Bronn, and Haeckel.
Organisms, then, are harmonious combinations of organs, and the
harmony is primarily a harmony of functions. Every function depends
upon every other, and all are necessary. The harmony of organs and
their mutual dependence are the results of the interdependence of
function. This thought, the recognition of the functional unity of the
organism, is the fundamental one at the base of all Cuvier's work.
Before him men had recognised more or less clearly the harmony of
structure and function, and had based much of their work upon this
unanalysed assumption. Cuvier was the first naturalist to raise this
thought to the level of a principle peculiar to natural history. "It
is on this mutual dependence of the functions and the assistance which
they lend one to another that are founded the laws that determine the
relations of their organs; these laws are as inevitable as the laws of
metaphysics and mathematics, for it is evident that a proper harmony
between organs that act one upon another is a necessary condition of
the existence of the being to which they belong."[45]
This rational principle, peculiar to natural history, Cuvier calls the
principle of the conditions of existence, for the following
reason:--"Since nothing can exist that does not fulfil the conditions
which render its existence possible, the different parts of each being
must be co-ordinated in such a way as to render possible the existence
of the being as a whole, not only in itself, but also in its relations
with other beings, and the analysis of these conditions often leads to
general laws which are as certain as those which are derived from
calculation or from experiment."[46]
By "conditions of existence" he means something quite different from
what is now commonly understood. The idea of the external conditions
of existence, the environment, enters very little into his thought. He
is intent on the adaptations of function and organ within the living
creature--a point of view rather neglected nowadays, but essential for
the understanding of living things. The very condition of existence of
a living thing, and part of the essential definition of it, is that
its parts work together for the good of the whole.
The principle of the adaptedness of parts may be used as an
explanatory principle, enabling the naturalist to trace out in detail
the interdependence of functions and their organs. When you have
discovered how one organ is adapted to another and to the whole, you
have gone a certain way towards understanding it. That is using
teleology as a regulative principle, in Kant's sense of the word.
Cuvier was indeed a teleologist after the fashion of Kant, and there
can be no doubt that he was influenced, at least in the exposition of
his ideas, by Kant's _Kritik der Urtheilskraft_, which appeared ten
years before the publication of the _Lecons d'Anatomie Comparee_.
Teleology in Kant's sense is and will always be a necessary postulate
of biology. It does not supply an explanation of organic forms and
activities, but without it one cannot even begin to understand living
things. Adaptedness is the most general fact of life, and innumerable
lesser facts can be grouped as particular cases of it, can be, so far,
understood.
Cuvier's famous principle of correlation, the corner-stone of his
work, is simply the practical application to the facts of structure of
the principle of functional adaptedness. By the principle of
correlation, from one part of an animal, given sufficient knowledge of
the structure of its like, you can in a general way construct the
whole. "This must necessarily be so: for all the organs of an animal
form a single system, the parts of which hang together, and act and
re-act upon one another; and no modifications can appear in one part
without bringing about corresponding modifications in all the
rest."[47] The logical basis of the principle is sound. The functions
of the parts are all intimately bound up with one another, and one
function cannot vary without bringing in its train corresponding
modifications in the others. Structure and function are bound up
together; every modification of a function entails therefore the
modification of an organ. Hence from the shape of one organ you can
infer the shape of the other organs--if you have sufficiently
extensive empirical knowledge of functions, and of the relation of
structure to function in each kind of organ. Given an alimentary canal
capable of digesting only flesh, and possessing therefore a certain
form, you know that the other functions must be adapted to this
particular function of the alimentary canal. The animal must have keen
sight, fine smell, speed, agility, and strength in paws and jaws.
These particular functions must have correspondingly modified organs,
well-developed eyes and ears, claws and teeth. Further, you know from
experience that such and such definitely modified organs are
invariably found with the carnivorous habit, carnassial teeth, for
example, and reduced clavicles. From a "carnivorous" alimentary canal,
then, you can infer with certainty that the animal possessed
carnassial teeth and the other structural peculiarities of carnivorous
animals, _e.g._, the peculiar coronoid process of the mandible. From
the carnassial tooth you can infer the reduced clavicle, and so on.
"In a word, the form of the tooth implies the form of the condyle;
that of the shoulder blade that of the claws, just as the equation of
a curve implies all its properties."[48]
Similarly the great respiratory power of birds is correlated with
their great muscular strength, and renders necessary great digestive
powers. Hence the correlated structure of lungs, muscles and their
attachments, and alimentary canal, in birds.
Not only do systems of organs, by being adjusted to special
modifications of function, influence one another, but so also do parts
of the same organ. This is noticeably the case with the skeleton,
where hardly a facet can vary without the others varying
proportionately, so that from one bone you can up to a certain point
deduce all the rest.
We deduce the necessity, the constancy, of these co-existences of
organs from the observed reciprocal influence of their functions. That
being established, we can argue from observed constancy of relation
between two organs an action of one upon the other, and so be led to a
discovery of their functions. But even if we do not discover the
functional interdependencies of the parts, we can use the established
fact of the constant co-existence of two parts as proof of a
functional correlation between them.
Correlation is either a rational or an empirical principle, according
as we know or do not know the interdependence of function of which it
is the expression. Even when we apply the rational principle of
correlation it would be useless in our hands if we had not extensive
empirical knowledge; when we use an empirical rule of correlation we
depend entirely upon observation. "There are a great many cases,"
writes Cuvier,[49] "where our theoretical knowledge of the relations of
forms would not suffice, if it were not filled out by observation,"
that is to say, there are many cases of correlation not yet explicable
in terms of function. From a hoof you can deduce the main characters
of herbivores (with a certain amount of assistance from your empirical
knowledge of herbivores), but could you from a cloven hoof deduce that
the animal is a ruminant, unless you had observed the constancy of
relation, not directly explicable in terms of function, between cloven
hoofs and chewing the cud? Or could you deduce from the existence of
frontal horns that the animal ruminates? "Nevertheless, since these
relations are constant, they must necessarily have a sufficient cause;
but as we are ignorant of this cause, observation must supplement
theory; observation establishes empirical laws which become almost as
certain as the rational laws, when they are based upon a sufficient
number of observations.... But that there exist all the same hidden
reasons for all these relations is partly revealed by observation
itself, independently of general philosophy."[50] That is to say, even
correlations for which no explanation in terms of function can be
supplied are probably in reality functional correlations. This may, in
some cases, be inferred from the graded correspondence of two sets of
organs. For example, ungulates which do not ruminate, and have not a
cloven hoof, have a more perfect dentition and more bones in the foot
than the true cloven-hoofed ruminants. There is a correlation between
the state of development of the teeth and of the foot. This
correlation is a graded one, for camels, which have a more perfect
dentition than other ruminants, have also a bone more in their tarsus.
It seems probable, therefore, that there is some reason, that is, some
explanation in terms of function, for this case of correlation.
Nevertheless, the fact remains that many correlations are not
explicable in terms of function, and the substitution of correlation
as an empirical principle for correlation as a rational principle
marks for Cuvier a step away from his functional comparative anatomy
towards a pure morphology. It is significant that in later times the
term correlation has come to be applied more especially to the purely
empirical constancies of relation, and has lost most of its functional
significance. But the correlation of the parts of an organism is no
mere mathematical concept, to be expressed by a coefficient, but
something deeper and more vital.
Cuvier interpreted the functional dependence of the parts in terms of
what we now call the general metabolism. He had a clear vision of the
constant movement of molecules in the living tissue, combining and
recombining, of the organism taking in and intercalating molecules
from outside from the food and rejecting molecules in the excretions,
a ceaseless _tourbillon vital_. "This general movement, universal in
every part, is so unmistakably the very essence of life that parts
separated from a living body straightway die."[51] The organisation of
the body, the arrangement of its solids and liquids, is adapted to
further the _tourbillon vital_. "Each part contributes to this general
movement its own particular action and is affected by it in particular
ways, with the result that, in every being, life is a unity which
results from the mutual action and reaction of all its parts."[52]
Cuvier, however, did not resolve life into metabolism, nor reduce
vital happenings to the chemical level. The form of organised bodies
is more essential than the matter of which they are composed, for the
matter changes ceaselessly while the form remains unchanged. It is in
form that we must seek the differences between species, and not in the
combinations of matter, which are almost the same in all.[53] The
differences are to be sought at the level of the second and third
degrees of composition.
The existence of differences of form introduces a new problem, the
problem of diversity. There are only a few possible combinations of
the principal organs, but as you get down to less important parts the
possible scope of variation is greatly increased, and most of the
possible variations do exist. Nature seems prodigal of form, of form
which needs not to be useful in order to exist. "It needs only to be
possible, _i.e._, of such a character that it does not, destroy the
harmony of the whole."[54] We seize here the relation of the principle
of the adaptedness of parts to the problem of the variety of form. The
former is in a sense a regulative and conservative principle which
lays down limits beyond which variation may not stray. In itself it is
not a fountain of change; there must be another cause of change. This
thought is of great importance for theories of descent.
Cuvier has no theory to account for the variety of form: he contents
himself with a classification. There are two main ways of classifying
forms; you may classify according to single organs or according to the
totality of organs. By the first method you can have as many
classifications as you have organs, and the classifications will not
necessarily coincide. Thus you can divide animals according to their
organs of digestion into two classes, those in which the alimentary
canal is a sac with one opening (zoophytes) and those in which the
canal has two openings,[55] a curious forestalment, in the rough, of
the modern division of Metazoa into Coelentera and Coelomata.
It is only by taking single organs that you can arrange animals into
long series, and you will have as many series as you take organs. Only
in this way can you form any _Echelle des etres_ or graded series; and
you can get even this kind of gradation only within each of the big
groups formed on a common plan of structure; you can never grade, for
example, from Invertebrates to Vertebrates through intermediate
forms[56] (which is perfectly true, in spite of Amphioxus and
Balanoglossus!).
In the _Regne Animal_ Cuvier restricts the application of the idea of
the _Echelle_ within even narrower limits, refusing to admit its
validity within the bounds of the vertebrate phylum, or even within
the vertebrate classes. This seems, however, to refer to a seriation
of whole organisms and not of organs, so that the possibility of a
seriation of organs within a class is not denied. Cuvier was, above
all, a positive spirit, and he looked askance at all speculation which
went beyond the facts. "The pretended scale of beings," he wrote, "is
only an erroneous application to the totality of creation of partial
observations, which have validity only when confined to the sphere
within which they were made."[57] This remark, which is after all only
just, perfectly expresses Cuvier's attitude to the transcendental
theories, and was probably a protest against the sweeping
generalisations of his colleague, Etienne Geoffroy St Hilaire.
A true classification should be based upon the comparison of all
organs, but all organs are not of equal value for classification, nor
are all the variations of each organ equally important. In estimating
the value of variations more stress should be laid on function than on
form, for only those variations are important which affect the mode of
functioning. These are the principles on which Cuvier bases the
classification of animals given in the _Lecons_, Article V., "Division
des animaux d'apres l'ensemble de leur organisation." The scheme of
classification actually given in the _Lecons_ recalls curiously that
of Aristotle, for there is the same broad division into Vertebrates,
with red blood, and Invertebrates, almost all with white blood. Nine
classes altogether are distinguished--Mammals, Birds, Reptiles,
Fishes, Molluscs, Crustacea, Insects, Worms, Zoophytes (including
Echinoderms and Coelenterates).
A maturer theory and practice of classification is given in the _Regne
Animal_ of seventeen years later. Here the principle of the
subordination of characters (which seems to have been first explicitly
stated by the younger de Jussieu in his _Genera Plantarum_, 1789,[58])
is more clearly recognised. The properties or peculiarities of
structure which have the greatest number of relations of
incompatibility and coexistence, and therefore influence the whole in
the greatest degree, are the important or dominating characters, to
which the others must be subordinated in classification. These
dominant characters are also the most constant.[59] In deciding which
characters are the most important Cuvier makes use of his fundamental
classification of functions and organs into two main sets. "The heart
and the organs of circulation are a kind of centre for the vegetative
functions, as the brain and the spinal cord are for the animal
functions."[60] These two organ-systems vary in harmony, and their
characters must form the basis for the delimitation of the great
groups. Judged by this standard there are four principal types of
form,[61] of which all the others are but modifications. These four
types are Vertebrates, Molluscs, Articulates, and Radiates. The first
three have bilateral, the last has radial symmetry. Vertebrates and
Molluscs have blood-vessels, but Articulates show a functional
transition from the blood-vessel to the tracheal system. Radiates
approach the homogeneity of plants; they appear to lack a distinct
nervous system and sense organs, and the lowest of them show only a
homogeneous pulp which is mobile and sensitive. All four classes are
principally distinguished from one another by the broad structural
relations of their neuromuscular system, of the organs of the animal
functions. Vertebrates have a spinal cord and brain, an internal
skeleton built on a definite plan, with an axis and appendages; in
Molluscs the muscles are attached to the skin and the shell, and the
nervous system consists of separate masses; Articulates have a hard
external skeleton and jointed limbs, and their nervous system consists
of two long ventral cords; Radiates have ill-defined nervous and
muscular systems, and in their lowest forms possess the animal
functions without the animal organs.
This well-rounded classification of animal forms is in a sense the
crown of Cuvier's work, for the principle of the subordination of
characters, in the interpretation which he gives to it, is a direct
application of his principle of functional correlation. Each of the
great groups is built upon one plan. The idea of the unity of plan has
become for Cuvier a commonplace of his thought, and it is tacitly
recognised in all his anatomical work. But he never takes it as a
hard-and-fast principle which must at all costs be imposed upon the
facts.
Cuvier has become known as the greatest champion of the fixity of
species, but it is not often recognised that his attitude to this
problem is at least as scientific as that of the evolutionists of his
own and later times. No doubt he became dogmatic in his rejection of
evolution-theory, but he was on sure ground in maintaining that the
evolutionists of his day went beyond their facts. He considered that
certain forms (species) have reproduced themselves from the origin of
things without exceeding the limits of variation. His definition of a
species was, "the individuals descended from one another or from
common parents, together with those that resemble them as much as they
resemble one another."[62] "These forms are neither produced nor do
they change of themselves; life presupposes their existence, for it
cannot arise save in organisations ready prepared for it."[63]
He based his rejection of all theories of descent upon the absence of
definite evidence for evolution. If species have gradually changed, he
argued, one ought to find traces of these gradual modifications.[64]
Palaeontology does not furnish such traces. Again, the limits of
variation, even under domestication, are narrow, and the most extreme
variation does not fundamentally alter the specific type. Thus the dog
has varied perhaps most of all, in size, in shape, in colour. "But
throughout all these variations the relations of the bones remain the
same, and the form of the teeth never changes to an appreciable
extent; at most there are some individuals in which an additional
false molar develops on one side or the other."[65] This second
objection is the objection of the morphologist. It would be an
interesting study to compare Cuvier's views on variation with those of
Darwin, who was essentially a systematist.
Cuvier's first objection was of course determined to some extent by
the imperfection of the palaeontological knowledge of his time. But
even at the present day the objection has a certain force, for
although we have definite evidence of many serial transformations of
one species into another along a single line, for example, Neumayr's
_Paludina_ series, yet at any one geological level the species, the
lines of descent, are all distinct from one another.[66]
Cuvier recognised very clearly that there is a succession of forms in
time, and that on the whole the most primitive forms are the earliest
to appear. Mammals are later than reptiles, and fishes appear earlier
than either. As Deperet puts it, "Cuvier not only demonstrated the
presence in the sedimentary strata of a series of terrestrial faunas
superimposed and distinct, but he was the first to express, and that
very clearly, the idea of the gradual increase in complexity of these
faunas from the oldest to the most recent" (p. 10).
He did not believe that the fauna of one epoch was transformed into
the fauna of the next. He explained the disappearance of the one by
the hypothesis of sudden catastrophes, and the appearance of the next
by the hypothesis of immigration. He nowhere advanced the hypothesis
of successive new creations. "For the rest, when I maintain that the
stony layers contain the bones of several genera and the earthy layers
those of several species which no longer exist, I do not mean that a
new creation has been necessary to produce the existing species, I
merely say that they did not exist in the same localities and must
have come thither from elsewhere."[67] It was left to d'Orbigny to
teach the doctrine of successive creations, of which he distinguished
twenty-seven (_Cours elementaire de palaeontologie stratigraphique_,
1849).
Cuvier, however, can hardly have believed that all species were
present at the beginning, since he does admit a progression of forms.
Probably he had no theory on the subject, for theories without facts
had little interest for him. At any rate it is a mistake to think that
Cuvier was a supporter of the theological doctrine of special
creation. His philosophy of Nature was mechanistic, and he dedicated
his _Recherches sur les Ossemens Fossiles_ to his friend Laplace. He
admitted the idea of evolution at least so far as to conceive of a
development of man from a savage to a civilised state.[68] He refused
to accept the extravagant evolutionary theory of Demaillet and the
somewhat confused theory of Lamarck (whom he joins with Demaillet),[69]
just as he rejected the transcendental theories of Geoffroy St
Hilaire, because they seemed to him not based upon facts.
[41] _Lecons d'Anatomie Comparee_, tome i., pp. 10 _et
scq._, 1800.
[42] _Lecons d'Anatomie Comparee_, i., p. 18.
[43] _Loc. cit._, i., p. 13.
[44] _Lecons d'Anatomie Comparee_, tome i., Articles
iii.-iv., 1800.
[45] _Lecons d'Anatomie Comparee_, i., p. 47.
[46] _Le Regne Animal_, i., p. 6, 1817.
[47] _Histoire des Progres des Sciences naturelles depuis
1789_, i., p. 310, 1826.
[48] _Recherches sur les Ossemens Fossiles_, i., p. 60,
1812.
[49] _Ossemens fossiles_, i., p. 60.
[50] _Loc. cit._, i., p. 63.
[51] _Lecons d'Anatomie Comparee_, i., p. 6.
[52] _Le Regne Animal_, i., p. 16.
[53] _Hist. Prog. Sci. Nat._, i., p. 187, 1826.
[54] _Lecons_, i., p. 58.
[55] _Loc. cit._, i., Article iii.
[56] _Loc. cit._, i., p. 60.
[57] _Regne Animal_, i., p. xx.
[58] Cuvier, _Hist. Prog. Sci. Nat._, i., p. 288, 1826.
[59] _Regne Animal_, i., p. 10.
[60] _Regne Animal_, p. 55.
[61] First propounded by Cuvier in 1812, _Ann. Mus.
d'Hist. Nat._, xix.
[62] _Regne Animal_, i., p. 19.
[63] _Loc. cit._, p. 20.
[64] _Recherches sur les Ossemens Fossiles_, i., p. 74,
1812.
[65] _Loc. cit._, p. 79.
[66] See C. Deperet, _Les transformations du Monde
animal_, Paris, 1907, and G. Steinmann, _Die
geologischen Grundlagen der Abstammungslehre_, Leipzig,
1908.
[67] _Recherches_, i., p. 81.
[68] _Regne Animal_, i., p. 91.
[69] _Ossemens Fossiles_, i., p. 26.