Form and Function

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The _De Partibus Animalium_ becomes in form a comparative
organography, but the emphasis is always on function and community of
function. Thus he treats of bone, "fish-spine," and cartilage together
(_De Partibus_, ii., 9, 655^a), because they have the same function,
though he says elsewhere that they are only analogous structures (ii.,
8, 653^b). In the same connection he describes also the supporting
tissues of Invertebrates--the hard exoskeleton of Crustacea and
Insects, the shell of Testacea, the "bone" of _Sepia_ (ii., 8,
654^a). Aristotle took much more interest in analogies, in organs of
similar function, than in homologies. He did recognise the existence
of homologies, but rather _malgre lui_, because the facts forced it
upon him.

His only excursion into the realm of "transcendental anatomy" is his
comparison of a Cephalopod to a doubled-up Vertebrate whose legs have
become adherent to its head, whose alimentary canal has doubled upon
itself in such a way as to bring the anus near the mouth (_De
Partibus_, iv., 9, 684^b). It is clear, however, that Aristotle did
not seek to establish by this comparison any true homologies of parts,
but merely analogies, thus avoiding the error into which Meyranx and
Laurencet fell more than two thousand years later in their paper
communicated to the Academie des Sciences, which formed the
starting-point of the famous controversy between Cuvier and E.
Geoffroy St Hilaire (see Chap. V., below).

Moreover, Aristotle did not so much compare a Cephalopod with a
doubled-up Vertebrate as contrast Cephalopods (and also Testacea) with
all other animals. Other animals have their organs in a straight line;
Cephalopods and Testacea alone show this peculiar doubling up of the
body.

(4) Aristotle was much struck with certain facts of correlation, of
the interdependence of two organs which are not apparently in
functional dependence on one another. Such correlation may be positive
or negative; the presence of one organ may either entail the presence
of the other, or it may entail its absence. Aristotle has various ways
of explaining facts of correlation. He observed that no animal has
both tusks and horns, but this fact could easily be explained on the
principle that Nature never makes anything superfluous or in vain. If
an animal is protected by the possession of tusks it does not require
horns, and _vice versa_. The correlation of a multiple stomach with
deficient development of the teeth (as in Ruminants) is accounted for
by saying that the animal needs its complex stomach to make up for the
shortcomings of its teeth! (_De Partibus_, iii., 14, 674^b.) Other
examples of correlation were not susceptible of this explanation in
terms of final causes. He lays stress on the fact, in the main true,
of the inverse development of horns and front teeth in the upper jaw,
exemplified in Ruminants. He explains the fact in this way. Teeth and
horns are formed from earthy matter in the body and there is not
enough to form both teeth and horns, so "Nature by subtracting from
the teeth adds to the horns; the nutriment which in most animals goes
to the former being here spent on the augmentation of the latter" (_De
Partibus_, iii., 2, 664^a, trans. Ogle). A similar kind of explanation
is offered of the fact that Selachia have cartilage instead of bone,
"in these Selachia Nature has used all the earthy matter on the skin
[_i.e._, on the placoid scales]; and she is unable to allot to many
different parts one and the same superfluity of material" (_De
Partibus_, ii., 9, 655^a, trans. Ogle). Speaking generally, "Nature
invariably gives to one part what she subtracts from another" (_loc.
cit._, ii., 14, 658^a).

This thought reappears again in the 19th century in E. Geoffroy St
Hilaire's _loi de balancement_ and also in Goethe's writings on
morphology. For Aristotle it meant that Nature was limited by the
nature of her means, that finality was limited by necessity. Thus in
the larger animals there is an excess of earthy matter, as a necessary
result of the material nature of the animal; this excess is turned by
Nature to good account, but there is not enough to serve both for
teeth and for horns (_loc. cit._, iii., 2, 663^b).

But there are other instances of correlation which seem to have taxed
even Aristotle's ingenuity beyond its powers. Thus he knew that all
animals (meaning viviparous quadrupeds) with no front teeth in the
upper jaw have cotyledons on their foetal membranes, and that most
animals which have front teeth in both jaws and no horns have no
cotyledons (_De Generatione_, ii., 7). He offers no explanation of
this, but accepts it as a fact.

We may conveniently refer here to one or two other ideas of Aristotle
regarding the causes of form. He makes the profound remark that the
possible range of form of an organ is limited to some extent by its
existing differentiation. Thus he explains the absence of external
(projecting) ears in birds and reptiles by the fact that their skin is
hard and does not easily take on the form of an external ear (_De
Partibus_, ii, 12). The fact of the inverse correlation is certain;
the explanation is, though very vague, probably correct.

In one passage of the _De Partibus_ Aristotle clearly enunciates the
principle of the division of labour, afterwards emphasised by H.
Milne-Edwards. In some insects, he says, the proboscis combines the
functions of a tongue and a sting, in others the tongue and the sting
are quite separate. "Now it is better," he goes on, "that one and the
same instrument shall not be made to serve several dissimilar ends;
but that there shall be one organ to serve as a weapon, which can then
be very sharp, and a distinct one to serve as a tongue, which can then
be of spongy texture and fit to absorb nutriment. Whenever, therefore,
Nature is able to provide two separate instruments for two separate
uses, without the one hampering the other, she does so, instead of
acting like a coppersmith who for cheapness makes a spit and
lampholder in one" (iv., 6, 683^a).

(5) The first sentence of the _Historia Animalium_ formulates, with
that simplicity and directness which is so characteristic of
Aristotle, the distinction between homogeneous and heterogeneous
parts, in the mass the distinction between tissues and organs. "Some
parts of animals are simple, and these can be divided into like parts,
as flesh into pieces of flesh; others are compound, and cannot be
divided into like parts, as the hand cannot be divided into hands, nor
the face into faces. All the compound parts also are made up of simple
parts--the hand, for example, of flesh and sinew and bone" (Cresswell,
_loc. cit_., p. I).

In the _De Partibus Animalium_ he broadens the conception by adding
another form of composition. "Now there are," he says, "three degrees
of composition; and of these the first in order, as all will allow, is
composition out of what some call the elements, such as earth, air,
water, fire.... The second degree of composition is that by which the
homogeneous parts of animals, such as bone, flesh, and the like, are
constituted out of the primary substances. The third and last stage is
the composition which forms the heterogeneous parts, such as face,
hand, and the rest" (ii., 1, 646^a, trans. Ogle).

In the _Historia Animalium_ the homogeneous parts are divided into (1)
the soft and moist (or fluid), such as blood, serum, flesh, fat, suet,
marrow, semen, gall, milk, phlegm, faeces and urine, and (2) the hard
and dry (or solid), such as sinew, vein, hair, bone, cartilage, nail,
and horn. It would appear from this enumeration that Aristotle's
distinction of simple and complex parts does not altogether coincide
with our distinction of tissues and organs. We should not call vein a
tissue, nor do we include under this heading non-living secretions.
But in the _De Partibus Animalium_ Aristotle, while still holding to
the distinction set forth above, is alive to the fact that his simple
parts include several different sorts of substances. He distinguishes
among the homogeneous parts three sets. The first of these comprises
the tissues out of which the heterogeneous parts are constructed,
_e.g._, flesh and bone; the second set form the nutriment of the
parts, and are invariably fluid; while the third set are the residue
of the second and constitute the residual excretions of the body (ii.,
2, 647^b). He sees clearly the difficulty of calling vein or
blood-vessel a simple part, for while a bloodvessel and a part of it
are both blood-vessel, as we should say vascular tissue, yet a part of
a blood-vessel is not a bloodvessel. There is form superadded to
homogeneity of structure (ii., 2, 647^b). Similarly for the heart and
the other viscera. "The heart, like the other viscera, is one of the
homogeneous parts; for, if cut up, its pieces are homogeneous in
substance with each other. But it is at the same time heterogeneous in
virtue of its definite configuration" (ii., 1, 647^a, trans. Ogle).

Aristotle, therefore, came very near our conception of tissue. He was
of course not a histologist; he describes not the structure of
tissues, which he could not know, but rather their distribution within
the organism; his section on the homogeneous parts of Sanguinea
(_Historia Animalium_, iii., second half) is largely a comparative
topographical anatomy; in it, for instance, he describes the venous
and skeletal systems.

This distinction which Aristotle drew plays an important part in all
his writings on animals, particularly in his theory of development. It
was a distinction of immense value, and is full of meaning even at the
present day. No one has ever given a better definition of organ than
is implied in Aristotle's description of the heterogeneous parts--"The
capacity of action resides in the compound parts" (Cresswell, _loc.
cit._, p. 7). The heterogeneous parts were distinguished by the
faculty of doing something, they were the active or executive parts.
The homogeneous parts were distinguished mainly by physical characters
(_De Generatione_, i., 18), but certain of them had other than purely
physical properties, they were the organs of touch (_De Partibus_,
ii., 1, 647^a).

(6) In a passage in the _De Generatione_ (ii, 3) Aristotle says that
the embryo is an animal before it is a particular animal, that the
general characters appear before the special. This is a foreshadowing
of the essential point in von Baer's law (see Chap. IX. below).

He considers also that tissues arise before organs. The homogeneous
parts are anterior genetically to the heterogeneous parts and
posterior to the elementary material (_De Partibus_, ii., 1, 646^b).

(7) We meet in Aristotle an idea which later acquired considerable
vogue, that of the _Echelle des etres_(or "scale of beings"), that
organisms, or even all objects organic or inorganic, can be arranged
in a single ascending series. The idea is a common one; its first
literary expression is found perhaps in primitive creation-myths, in
which inorganic things are created before organic, and plants before
animals. It may be recognised also in Anaximander's theory that land
animals arose from aquatic animals, more clearly still in Anaxagoras'
theory that life took its origin on this globe from vegetable germs
which fell to earth with the rain. Anaxagoras considered animals
higher in the scale than plants, for while the latter participated in
pleasure (when they grew) and pain (when they lost their leaves),
animals had in addition "Nous." In Empedocles' theory of evolution,
the vegetable world preceded the animal. Plato, in the _Timaeus_,
describes the whole organic world as being formed by degradation from
man, who is created first. Man sinks first into woman, then into brute
form, traversing all the stages from the higher to the lower animals,
and becoming finally a plant. This is a reversal of the more usual
notion, but the idea of gradation is equally present.

Aristotle seems not to have believed in any transformation of species,
but he saw that Nature passes gradually from inanimate to animate
things without a clear dividing line. "The race of plants succeeds
immediately that of inanimate objects" (Cresswell, _loc. cit._, p.
94). Within the organic realm the passage from plants to animals is
gradual. Some creatures, for example, the sea-anemones and sponges,
might belong to either class.

Aristotle recognised also a natural series among the groups of
animals, a series of increasing complexity of structure. He begins his
study of structure with man, who is the most intricate, and then takes
up in turn viviparous and oviparous quadrupeds, then birds, then
fishes. After the Sanguinea he considers the Exsanguinea, and of the
latter first the most highly organised, the Cephalopods, and last the
simplest, the lower members of his class of the Testacea. In treating
of generation (in _Hist. Animalium_, v.) he reverses this order. In
the _De Generatione_ (Book ii., I) there is given another serial
arrangement of animals, this time in relation to their manner of
reproduction. There is a gradation, he says, of the following kind:--

1. Internally viviparous Sanguinea } producing a perfect
2. Externally viviparous Sanguinea } animal.
3. Oviparous Sanguinea--producing a perfect egg.
4. Animals producing an imperfect egg (one which
increases in size after being laid).
5. Insects, producing a scolex (or grub).

In Aristotle's view the gradation of organic forms is the consequence,
not the cause, of the gradation observable in their activities. Plants
have no work to do beside nutrition, growth, and reproduction; they
possess only the nutritive soul. Animals possess in addition sensation
and the sensitive or perceptive soul--"their manner of life differs in
their having pleasure in sexual intercourse, in their mode of
parturition and rearing their young" (_Hist. Anim._, viii., trans.
Cresswell, p. 195). Man alone has the rational soul in addition to the
two lower kinds.

As it is put in the _De Partibus_ (ii., 10, 656^a, trans. Ogle),
"Plants, again, inasmuch as they are without locomotion, present no
great variety in their heterogeneous parts. For, where the functions
are but few, few also are the organs required to effect them....
Animals, however, that not only live but feel, present a greater
multiformity of parts, and this diversity is greater in some animals
than in others, being most varied in those to whose share has fallen
not mere life but life of high degree. Now such an animal is man."

With the great exception of Aristotle, the philosophers of Greece and
Rome made little contribution to morphological theory. Passing mention
may be made of the Atomists--Leucippus, Democritus, and their great
disciple Lucretius, who in his magnificent poem "De Natura Rerum" gave
impassioned expression to the materialistic conception of the
universe. But the full effect of materialism upon morphology does not
become apparent till the rise of physiology in the 17th and 18th
centuries, and reaches its culmination in the 19th century. The
evolutionary ideas of Lucretius exercised no immediate influence upon
the development of morphology.

[1] E. Zeller, _Greek Philosophy_, Eng. trans., i., 522
f.n., London 1881. Other particulars as to Alcmaeon in
T. Gomperz, _Greek Thinkers_, Eng. trans., i., London,
1901.

[2] Zeller, _loc. cit._, i., p. 297.

[3] Gomperz, _loc. cit._, i., p. 244.

[4] R. Burckhardt, _Biologie u. Humanismus_, p. 85,
Jena, 1907.

[5] See the interesting account of Aristotle's
biological work in Prof. D'Arcy W. Thompson's Herbert
Spencer lecture (1913) and his translation of the
_Historia Animalium_ in the Oxford series.

[6] On Aristotle's forerunners, see R. Burckhardt, "Das
koische Tiersystem, eine Vorstufe des zoologischen
Systematik des Aristoteles." _Verh. Naturf. Ges. Basel_,
xx., 1904.

[7] T.E. Lones, _Aristotle's Researches in Natural
Science_, pp. 82-3, London, 1912.

[8] _De Partibus Animalium_, i., 4, 644^a trans. W.
Ogle, Oxford, 1911.

CHAPTER II

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.

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