COMPARATIVE ANATOMY BEFORE CUVIER
For two thousand years after Aristotle little advance was made upon
his comparative anatomy. Knowledge of the human body was increased not
long after his death by Herophilus and Erasistratus, but not even
Galen more than four centuries later made any essential additions to
Aristotle's anatomy.
During the Middle Ages, particularly after the introduction to Europe
in the 13th century of the Arab texts and commentaries, Aristotle
dominated men's thoughts of Nature. The commentary of Albertus Magnus,
based upon that of Avicenna, did much to impose Aristotle upon the
learned world. Albertus seems to have contented himself with following
closely in the footsteps of his master. There are noted, however, by
Bonnier certain improvements made by Albertus on Aristotle's view of
the seriation of living things. "He is the first," writes Bonnier, "to
take the correct view that fungi are lower plants allied to the most
lowly organised animals. From this point there start, for Albertus
Magnus, two series of living creatures, and he regards the plant
series as culminating in the trees which have well-developed
flowers."[9]
Aristotle's influence is predominant also in the work of Edward Wotton
(1492-1555), who in his book _De differentiis animalium_ adopted a
classification similar to that proposed by Aristotle. He too laid
stress upon the gradation shown from the lower to the higher forms.
In the 16th century, two groups of men helped to lay foundations for a
future science of comparative anatomy--the great Italian anatomists
Vesalius, Fallopius and Fabricius, and the first systematists (though
their "systems" were little more than catalogues) Rondeletius,
Aldrovandus and Gesner.
The anatomists, however, took little interest in problems of pure
morphology; the anatomy of the human body was for them simply the
necessary preliminary of the discovery of the functions of the
parts--they were quite as much physiologists as anatomists.
One of them, Fabricius, made observations on the development of the
chick (1615). Harvey, who was a pupil of Fabricius, likewise published
an account of the embryology of the chick.[10] In his philosophy and
habit of thought Harvey was a follower of Aristotle. It is worth
noting that in his _Exercitationes anatomicae de motu cordis_ (1628)
there is a passage which dimly foreshadows the law of recapitulation
in development which later had so much vogue.[11]
A stimulating contribution to comparative anatomy was made by
Belon,[12] who published in 1555 a _Histoire de la nature des Oyseaux_,
in which he showed opposite one another a skeleton of a bird and of a
mammal, giving the same names to homologous bones. The anatomy of
animals other than man was indeed not altogether neglected at this
time. Coiter (1535-1600) studied the anatomy of Vertebrates,
discovering among other things the fibrous structure of the brain.
Carlo Ruini of Bologna wrote in 1598 a book on the anatomy of the
horse.[13] Somewhat later Severino, professor at Naples, dissected many
animals and came to the conclusion that they were built upon the same
plan as man.[14] Willis, of Oxford and London, in his _Cerebri Anatome_
(1659) recognised the necessity for comparative study of the structure
of the brain. He found out that the brain of man is very like that of
other mammals, the brain of birds, on the contrary, like that of
fishes![15] He described the anatomy of the oyster and the crayfish. He
had, however, not much feeling for morphology.
The foundation of the Jardin des Plantes at Paris in 1626 and the
subsequent addition to it of a Museum of Natural History and a
menagerie gave a great impulse to the study of comparative anatomy by
supplying a rich material for dissection. Advantage was taken of these
facilities, particularly by Claude Perrault and Duverney.[16] In a
volume entitled _De la Mecanique des Animaux_, Perrault recognises
clearly the idea of unity of type, and even pushes it too far, seeking
to prove that in plants there exists an arterial system and veins
provided with valves.[17]
The beginning of the 17th century saw the invention of the microscope,
which was to have such an enormous influence upon the development of
biological studies. It did not come into scientific use until well on
in the middle of the century. Just before it came into use Francis
Glisson (1597-1677), an Englishman, gave in the introduction to his
treatise on the liver an account of the notions then current on the
structure of organic bodies. He classifies the parts as "similar" and
"organic," the former determined by their material, the latter by the
form which they assume. The similar parts are divided into the
sanguineous or rich in blood and the spermatic. Both sets are further
subdivided according to their physical characters,[18] the latter, for
instance, into the hard, soft, and tensile tissues. The classification
resembles greatly that propounded by Aristotle, though it is notably
inferior in the details of its working out.
For Aristotle, as for all anatomists before the days of the
microscope, the tissues were not much more than inorganic substances,
differing from one another in texture, in hardness, and other physical
properties. They possessed indeed properties, such as contractility,
which were not inorganic, but as far as their visible structure was
concerned there was little to raise them above the inorganic level.
The application of the microscope changed all that, for it revealed in
the tissues an organic structure as complex in its grade as the gross
and visible structure of the whole organism. Of the four men who first
made adequate use of the new aid, Malpighi, Hooke, Leeuenhoek, and
Swammerdam, the first-named contributed the most to make current the
new conceptions of organic structure. He studied in some detail the
development of the chick. He described the minute structure of the
lungs (1661), demonstrating for the first time, by his discovery of
the capillaries, the connection of the arteries with the veins. In his
work, _De viscerum structura_ (1666), he describes the histology of
the spleen, the kidney, the liver, and the cortex of the brain,
establishing among other things the fact that the liver was really a
conglomerate gland, and discovering the Malpighian bodies in the
kidney. This work was done on a broad comparative basis. "Since in the
higher, more perfect, red-blooded animals, the simplicity of their
structure is wont to be involved by many obscurities, it is necessary
that we should approach the subject by the observation of the lower,
imperfect animals."[19] So he wrote in the _De viscerum structura_, and
accordingly he studied the liver first in the snail, then in fishes,
reptiles, mammals, and finally man. In the introduction to his
_Anatome plantarum_ (1675), in which he laid the foundations of plant
histology, he vindicates the comparative method in the following
words:--"In the enthusiasm of youth I applied myself to Anatomy, and
although I was interested in particular problems, yet I dared to pry
into them in the higher animals. But since these matters enveloped in
peculiar mystery still lie in obscurity, they require the comparison
of simpler conditions, and so the investigation of insects[20] at once
attracted me; finally, since this also has its own difficulties I
applied my mind to the study of plants, intending after prolonged
occupation with this domain, to retrace my steps by way of the
vegetable kingdom, and get back to my former studies. But perhaps not
even this will be sufficient; since the simpler world of minerals and
the elements should have been taken first. In this case, however, the
undertaking becomes enormous and far beyond my powers."[21] There is
something fine in this life of broad outlines, devoted whole-heartedly
to an idea, to a plan of research, which required a lifetime to carry
out.
An important histological discovery dating from this time is that of
the finer structure of muscle, made by Stensen (or Steno) in 1664. He
described the structure of muscle-fibres, resolving them into their
constituent fibrils.
To the microscope we owe not only histology but the comparative
anatomy of the lower animals. Throughout the 17th and 18th centuries
the discovery of structure in the lower animals went on continuously,
as may be read in any history of Zoology.[22] We content ourselves here
with mentioning only some representative names.
In the 17th century Leeuenhoek, applying the microscope almost at
random, discovered fact after fact, his most famous, discovery being
that of the "spermatic animalcules."
Swammerdam studied the metamorphoses of insects and made wonderfully
minute dissections of all sorts of animals, snails and insects
particularly. He described also the development of the frog. It is
curious to see what a grip his conception of metamorphosis had upon
him when he homologises the stages of the frog's development with the
Egg, the Worm, and the Nymph of insects (_Book of Nature_, p. 104,
Eng. trans., 1785). He even speaks of the human embryo as being at a
certain stage a Man-Vermicle.
In the 18th century, Reaumur and Bonnet continued the minute study of
insects, laying more stress, however, on their habits and physiology
than upon their anatomy. Lyonnet made a most laborious investigation
of the anatomy of the willow-caterpillar (1762). John Hunter (1728-93)
dissected all kinds of animals, from holothurians to whales. His
interest was, however, that of the physiologist, and he was not
specially interested in problems of form. It is interesting to note a
formulation in somewhat confused language of the recapitulation
theory. The passage occurs in his description of the drawings he made
to illustrate the development of the chick. It is quoted in full by
Owen (J. Hunter, _Observations on certain Parts of the Animal
OEconomy_, with Notes by Richard Owen. London, 1837. Preface, p.
xxvi). We give here the last and clearest sentence--"If we were to
take a series of animals from the more imperfect to the perfect, we
should probably find an imperfect animal corresponding with some stage
of the most perfect."
The tendency of the time was not towards morphology, but rather to
general natural history and to systematics, the latter under the
powerful influence of Linnaeus (1707-1778). The former tendency is
well represented by Reaumur (1683-1757) with his observations on
insects, the digestion of birds, the regeneration of the crayfish's
legs, and a hundred other matters. To this tendency belong also
Trembley's famous experiments on Hydra (1744), and Roesel von
Rosenhof's _Insektenbelustigungen_ (1746-1761).
Bonnet (1720-1793) deserves special mention here, since in his _Traite
d'Insectologie_ (1745), and more fully in his _Contemplation de la
Nature_ (1764), he gives the most complete expression to the idea of
the _Echelle des etres_.
This idea seems to have taken complete possession of his imagination.
He extends it to the universe. Every world has its own scale of
beings, and all the scales when joined together form but one, which
then contains all the possible orders of perfection. At the end of the
Preface to his _Traite_ _d'Insectologie_ (OEuvres, i., 1779) he
gives a long table, headed "Idee d'une Echelle des etres naturels,"
and rather resembling a ladder, on the rungs of which the following
names appear:--
MAN.
Orang-utan.
Ape.
QUADRUPEDS.
Flying squirrel.
Bat.
Ostrich.
BIRDS.
Aquatic birds.
Amphibious birds.
Flying Fish.
FISH.
Creeping fish.
Eels.
Water serpents.
SERPENTS.
Slugs.
Snails.
SHELL FISH.
Tube-worms.
Clothes-moths.
INSECTS.
Gall insects.
Taenia.
Polyps.
Sea Nettles.
Sensitive plant.
PLANTS.
Lichens.
Moulds.
Fungi, Agarics.
Truffles.
Corals, and Coralloids.
Lithophytes.
Asbestos.
Talcs, Gypsums.
Selenites, Slates.
STONES.
Figured stones.
Crystals.
SALTS.
Vitriols.
METALS.
HALF-METALS.
SULPHURS.
Bitumens.
EARTHS.
Pure earth.
WATER.
AIR.
FIRE.
More subtile matter.
The nature of the transitional forms which he inserts between his
principal classes show very clearly his entire lack of morphological
insight--the transitions are functional. The positions assigned to
clothes-moths and corals are very curious! The whole scheme, so
fantastic in its details, was largely influenced by Leibniz's
continuity philosophy, and is in no way an improvement on the older
and saner Aristotelian scheme.
Robinet, in the fifth volume of his book _De la nature_ (1761-6),
foreshadows the somewhat similar views of the German
transcendentalists. "All beings," he writes, "have been conceived and
formed on one single plan, of which they are the endlessly graduated
variations: this prototype is the human form, the metamorphoses of
which are to be considered as so many steps towards the most excellent
form of being."[23]
The idea of a gradation of beings appears also in Buffon (1707-1788),
but here it takes more definitely its true character as a functional
gradation.[24] "Since everything in Nature shades into everything
else," he says, "it is possible to establish a scale for judging of
the degrees of the intrinsic qualities of every animal."[25]
He is quite well aware that the groups of Invertebrates are different
in structural plan from the Vertebrates--"The animal kingdom includes
various animated beings, whose organisation is very different from our
own and from that of the animals whose body is similarly constructed
to ours."[26]
He limits himself to a consideration of the Vertebrates, deeming that
the economy of an oyster ought not to form part of his subject matter!
He has a clear perception of the unity of plan which reigns throughout
the vertebrate series.[27] What is new in Buffon is his interpretation
of the unity of plan. For the first time we find clearly expressed the
thought that unity of plan is to be explained by community of origin.
Buffon's utterances on this point are, as is well known, somewhat
vacillating. The famous passage, however, which occurs in his account
of the Ass shows pretty clearly that Buffon saw no theoretical
objection to the descent of all the varied species of animals from one
single form. Once admit, he argues, that within the bounds of a single
family one species may originate from the type species by
"degeneration," then one might reasonably suppose that from a single
being Nature could in time produce all the other organised beings.[28]
Elsewhere, _e.g._, in the discourse _De la Degeneration des
Animaux_,[29] Buffon expresses himself with more caution. He finds that
it is possible to reduce the two hundred species of quadrupeds which
he has described to quite a small number of families "from which it is
not impossible that all the rest are derived."[30] Within each of the
families the species branch off from a parent or type species. This we
may note is a great advance on the linear arrangement implied in the
idea of an _Echelle des etres_.[31]
It is a mistake to suppose that Buffon was par excellence a maker of
hypotheses. On the contrary he saw things very sanely and with a very
open mind. He expressly mentions the great difficulties which one
encounters in supposing that one species may arise from another by
"degeneration." How does it happen that two individuals "degenerate"
just in the right direction and to the right stage so as to be capable
of breeding together? How is it that one does not find intermediate
links between species? One is reminded of the objections, not
altogether without validity, which were made to the Darwinian theory
in its early days. I cannot agree with those who think that Buffon was
an out-and-out evolutionist, who concealed his opinions for fear of
the Church. No doubt he did trim his sails--the palpably insincere
"Mais non, il est certain, par la revelation, que tous les animaux ont
egalement participe a la grace de la creation,"[32] following hard upon
the too bold hypothesis of the origin of all species from a single
one, is proof of it. But he was too sane and matter-of-fact a thinker
to go much beyond his facts, and his evolution doctrine remained
always tentative. One thing, however, he was sure of, that evolution
would give a rational foundation to the classification which, almost
in spite of himself, he recognised in Nature. If, and only if, the
species of one family originated from a single type species, could
families, be founded rationally, _avec raison_.
Buffon was, curiously enough, rather unwilling to recognise any
systematic unit higher than the species. Strictly; speaking there are
only individuals in Nature; but there are also groups of individuals
which resemble one another from generation to generation and are able
to breed together. These are species--Buffon adheres to the genetic
definition of species--and the species is a much more definite unit
than the genus, the order, the class, which are not divisions imposed
by us upon Nature. Species are definitely discontinuous,[33] and this
is the only discontinuity which Nature shows us. Buffon put his views
into practice in his _Histoire Naturelle_, where he describes species
after species, never uniting them into larger groups. We have seen,
however, how the facts forced upon him the conception of the "family."
Buffon was no morphologist. He left to Daubenton what one might call
the "dirty work" of his book, the dissection and minute description of
the animals treated.
But Buffon was a man of genius, and accordingly his ideas on
morphology are fresh and illuminating. Few naturalists have been so
free from the prejudices and traditions of their trade. He makes in
the _Discours sur la Nature des Animaux_[34] a distinction, which
Bichat and Cuvier later developed with much profit, between the
"animal" and the "vegetative" part of animals.[35] The vegetative or
organic functions go on continuously, even in sleep, and are performed
by the internal organs, of which the heart is the central one. The
active waking life of the animal, that part of its life which
distinguishes it from the plant, involves the external parts--the
sense-organs and the extremities. An animal is, as it were, made up of
a complex of organs performing the vegetative functions, assimilation,
growth, and reproduction, surrounded by an envelope formed by the
limbs, the sense-organs, the nerves and the brain, which is the centre
of this "envelope."[36] Animals may differ from one another enormously
in the external parts, particularly in the appendicular skeleton,
without showing any great difference in the plan and arrangement of
their internal organs. Quadrupeds, Cetacea, birds, amphibians and fish
are as unlike as possible in external form and in the shape of their
limbs; but they all resemble one another in their internal organs. Let
the internal organs change, however--the external parts will change
infinitely more, and you will get another animal, an animal of a
totally different nature. Thus an insect has a most singular internal
economy, and, in consequence, you find it is in every point different
from any vertebrate animal.
In this contrast, on the whole justified, between the importance of
variations in the "vegetative" and variations in the "animal" parts,
one may see without doing violence to Buffon's thought, an indication
of the difference between homology and analogy. It is usually in the
external parts, in the organs by which the animal adapts itself to its
environment, that one meets with the greatest number of analogical
resemblances. This contrast of vegetative and animal parts and their
relative importance for the discovery of affinities was at any rate a
considerable step towards an analysis of the concept of unity of plan.
To Xavier Bichat (1771-1802) belongs the credit of working out in
detail the distinction drawn by Aristotle and Buffon between the
animal and the vegetative functions. Bichat was not a comparative
anatomist; his interest lay in human anatomy, normal and pathological.
So his views are drawn chiefly from the consideration of human
structure.
He classifies functions into those relating to the individual and
those relating to the species. The functions pertaining to the
individual may be divided into those of the animal and those of the
organic life.[37] "I call _animal life_ that order of functions which
connects us with surrounding bodies; signifying thereby that this
order belongs only to animals" (p. lxxviii.). Its organs are the
afferent and efferent nerves, the brain, the sense-organs and the
voluntary muscles; the brain is its central organ. "Digestion,
circulation, respiration, exhalation, absorption, secretion,
nutrition, calorification, or production of animal heat, compose
organic life, whose principal and central organ is the heart" (p.
lxxix.).
The contrast of the animal and the organic life runs through all
Bichat's work; it receives classical expression in his _Recherches
Physiologiques sur la Vie et la Mort_ (1800). The plant and the animal
stand for two different modes of living. The plant lives within
itself, and has with the external world only relations of nutrition;
the animal adds to this organic life a life of active relation with
surrounding things (3rd ed., 1805, p. 2). "One might almost say that
the plant is the framework, the foundation of the animal, and that to
form the animal it sufficed to cover this foundation with a system of
organs fitted to establish relations with the world outside. It
follows that the functions of the animal form two quite distinct
classes. One class consists in a continual succession of assimilation
and excretion; through these functions the animal incessantly
transforms into its own substance the molecules of surrounding bodies,
later to reject these molecules when they have become heterogeneous to
it. Through this first class of functions the animal exists only
within itself; through the other class it exists outside; it is an
inhabitant of the world, and not, like the plant, of the place which
saw its birth. The animal feels and perceives its surroundings,
reflects its sensations, moves of its own will under their influence,
and, as a rule, can communicate by its voice its desires and its
fears, its pleasures or its pains. I call organic life the sum of the
functions of the former class, for all organised creatures, plants or
animals, possess them to a more or less marked degree, and organised
structure is the sole condition necessary to their exercise. The
combined functions of the second class form the 'animal' life, so
named because it is the exclusive attribute of the animal kingdom"
(pp. 2-3).
In both lives there is a double movement, in the animal life from the
periphery to the centre and from the centre to the periphery, in the
organic life also from the exterior to the interior and back again,
but here a movement of composition and decomposition. As the brain
mediates between sensation and motion, so the vascular system is the
go-between of the organs of assimilation and the organs of
dissimilation.
The most essential structural difference between the organs of animal
life and the organs of organic life is in man and the higher animals
at least, the symmetry of the one set and the irregularity of the
other--compare the symmetry of the nerves and muscles of the animal
life with the asymmetrical disposition of the visceral muscles and the
sympathetic nerves, which belong to the organic life.
Noteworthy differences exist between the two lives with respect to the
influence of habit. Everything in the animal life is under the
dominion of habit. Habit dulls sensation, habit strengthens the
judgment. In the organic life, on the contrary, habit exercises no
influence. The difference comes out clearly in the development of the
individual. The organs of the organic life attain their full
perfection independently of use; the organs of the animal life require
an education, and without education they do not reach perfection
(_Loc. cit._, p. 127).
Bichat was the founder of what was known for a time as General
Anatomy--the study of the constituent tissues of the body in health
and disease. His classification of tissues was macroscopical and
physiological; he relied upon texture and function in distinguishing
them rather than upon microscopical structure. The tissues he
distinguished are as follows:--[38]
1. The cellular membrane.
2. Nerves of animal life.
3. Nerves of organic life.
4. Arteries.
5. Veins.
6. Exhalants.
7. Absorbents and glands.
8. Bones.
9. Medulla.
10. Cartilage.
11. Fibrous tissue.
12. Fibro-cartilage.
13. Muscles of organic life.
14. Muscles of animal life.
15. Mucous membrane.
16. Serous membrane.
17. Synovial membrane.
18. The Glands.
19. The Dermis.
20. Epidermis.
21. Cutis.
The "cellular membrane" seems to mean undifferentiated connective
tissue; "exhalants" are imperceptible tubes arising from the
capillaries and secreting fat, serum, marrow, etc.; the "absorbents
and glands" are the lymphatics and the lymphatic glands.
In Bichat's eyes this resolution of the organism into tissues had a
deeper significance than any separation into organs, for to each
tissue must be attributed a _vie propre_, an individual and peculiar
life. "When we study a function we must consider the complicated organ
which performs it in a general way; but if we would be instructed in
the properties and life of that organ we must absolutely resolve it
into its constituent parts."[39] The tissues have, too, a great
importance for pathology, for diseases are often diseases of tissues
rather than of organs.[40]
[9] _Le Monde vegetal_, p. 41, Paris, 1907.
[10] _Exercitationes de generatione animalium_,1651. For
an account of Harvey's work on generation and
development, see Em. Radl's masterly _Geschichte der
biologischen Theorien_, i., pp. 31-8, Leipzig, 1905.
[11] The passage runs:--"Sic natura perfecta et divina
nihil faciens frustra, nec quipiam animali cor addidit,
ubi non erat opus, neque priusquam esset ejus usus,
fecit; sed iisdem gradibus in formatione cujuscumque
animalis, transiens per omnium animalium constitutiones
(ut ita dicam) ovum, vermem, foetum, perfectionem in
singulis acquirit."
[12] See I. Geoffroy St Hilaire, _Essais de Zoologie
generale_, p. 71, Paris, 1841.
[13] M. Foster, _Lectures on the History of Physiology_,
Cambridge, p. 53, 1901.
[14] _Zootomia democritea_, Nuremberg, 1645;
_Antiperipatias, seu de respiratione piscium_,
Amsterdam, 1661.
[15] Radl, _loc. cit._, i., p. 50.
[16] Perrault et Duverney, _Memoires pour servir a
l'histoire des Animaux_, Paris, 1699.
[17] F. Houssay, _Nature et Sciences naturelles_, Paris,
p. 76, n.d.
[18] Foster, _loc. cit._, p. 85.
[19] Trans, by Foster, _loc. cit._, p. 113.
[20] He made a careful study of the silkworm.
[21] "Etenim, ferventi aetatis calore, Anatomica
aggressus, licet circa peculiaria fuerim solicitus, in
_perfectioribus_ tamen haec rimari sum ausus. Verum, cum
haec propriis tenebris obscura jaceant, simplicium
analogismo egent; inde _insectorum_ indago illico
arrisit; quae cum et ipsa suas habeat difficultates ad
Plantarum perquisitionem animum _postremo_ adjeci, ut
diu hoc lustrato mundo gressu retroacto Vegetantis
Naturae gradu, ad prima studia iter mihi aperirem. Sed
nec forte hoc ipsum sufficiet cum simplicior _Mineralium
Elementorumque_ mundus praeire debeat. At in immensum
excrescit opus, et meis viribus omnino impar," _Opera
Omnia_, i., p. 1, London, 1686.
[22] See particularly E. Radl, _loc. cit._. I Teil. J. V.
Carus, _Geschichte der Zoologie_, Muenchen, 1872.
[23] For a good historical account of the gradation
theories see Thienemann's paper in the _Zoologische
Annalen_(Wuerzburg) iii., pp. 185-274, 1910, from which
the quotation from Robinet is taken.
[24] _Histoire naturelle_, i., p. 13; ii, p. 9; iv., p.
101; and xiv., pp. 28-9, 1749 and later.
[25] No translation can render the beauty of the
original--"Comme tout se fait et que tout est par nuance
dans la Nature ..." (iv., p. 101).
[26] _Hist. nat._, iv., p. 5.
[27] See particularly his comparison of the skeleton of
the horse with that of man. _Hist. Nat._, iv., p. 381,
also p. 13.
[28] _Loc. cit._, p. 382.
[29] Tome xiv., pp. 311-374.
[30] Tome xiv., p. 358.
[31] See also "Oiseaux," Tome i., pp. 394, 395. Pallas in
1766 adopted for the whole animal kingdom this branching
arrangement.
[32] "But this cannot be, for it is certain by revelation
that all animals have equally participated in the grace
of creation."
[33] iv., p. 385.
[34] iv., pp. 3-110.
[35] It has been revived in our own days by Bergson,
_Matiere et Memoire_, p. 57.
[36] iv., pp. 7-15.
[37] _Anatomie Generale_, Paris, 1801, Eng. trans. 1824.
[38] _Anatomie Generale_, Eng. trans., i., p. lii.
[39] _Anatomie Generale_, Eng. trans., i., p. lviii.
[40] _Loc cit._, i., sect. vii.