but a single one of these senses, we speak of a thing as living. Thus
there is intellect, sensation, motion from place to place and rest, the
motion concerned with nutrition, and, further, [there are the processes
of] decay and growth,’ all various meanings or at least exhibitions
of some form of life. Hence even ‘plants are supposed to have life,
for they have within themselves a faculty and principle whereby they
grow and decay.... They grow and continue to live so long as they are
capable of absorbing nutriment. This form of life can be separated
from the others ... and plants have no other faculty of soul at all,’
but only this lowest vegetative soul. ‘It is then in virtue of this
principle that all living things live, whether animals or plants. But
it is sensation which primarily constitutes the animal. For, provided
they have sensation, even those creatures that are devoid of movement
and do not change their place are called animals.... As the nutritive
faculty may exist without touch or any form of sensation, so also touch
may exist apart from other senses.’[46] Apart from these two lower
forms of soul, the _vegetative_ or nutritive and reproductive and the
_animal_ or sensitive, stands the _rational_ or intellectual soul
peculiar to man, a form of soul with which we shall here hardly concern
ourselves.[47]
[46] _De anima_, ii. 2, ii; 413ᵅ 22.
[47] The question of Aristotle’s meaning in connexion with this topic,
of primary importance for all thought, has a vast literature. An
authoritative work is R. D. Hicks, _Aristotle, De anima_, Cambridge,
1907.
The possession of one or more of the three types of soul, vegetative,
sensitive, and rational, provides in itself a basis for an elementary
form of arrangement of living things in an ascending scale. We
have already seen that Aristotle certainly describes something
resembling a ‘Scala Naturae’ and that such a scheme can easily be
drawn up from passages in his works. It may, however, be doubted
whether his phraseology is capable of extension so as to include a
true _classification_ of animals in any modern sense. It is true
that he repeatedly divides animals into classes, _Sanguineous_ and
_Non-sanguineous_, _Oviparous_ and _Viviparous_, _Terrestrial_
and _Aquatic_, &c., but his divisions are for the most part simply
dichotomic. He certainly defines a few groups of animals as the Lophura
(_Equidae_), the Cete (_Cetacea_), and the Selache (_Elasmobranchiae_
together with the _Lophiidae_) in a way that fairly corresponds
to similar groups in later systems. In most cases, however, his
definitions are not exact enough for modern needs, for the same
animal may fall into more than one of his classes and widely
different animals into the same class. Thus he invents a category
_Carcharodonta_ for animals with sharp interlocking teeth and includes
in it carnivores, reptiles, and fish; again, the horse kind must be
included both among his _Anepallacta_ or animals having flat crowned
teeth as well as among the _Amphodonta_ or animals with front teeth in
both jaws. Such words as these are really terms of _description_, not
of classification in the modern biological sense of that word.
There are, however, scattered through the biological works, certain
terms which are applied to animal groups and organs and are defined in
such a way as to suggest that they might ultimately have been developed
for classificatory purposes. Thus his lowest group is the _species_.
‘The individuals comprised within a single _species_ (εîδος) ... are
the real existences; but inasmuch as these individuals possess one
common specific form, it will suffice to state the universal attributes
of the species, that is, the attributes common to all its individuals,
once and for all.’[48] This is surely not very far removed from the
modern biological conception of a species.
[48] _De partibus animalium_, i. 4; 644ᵅ 22.
‘But as regards the larger groups—such as birds—which comprehend many
species, there may be a question. For on the one hand it may be urged
that as the ultimate species represent the real existences, it will be
well, if practicable, to examine these ultimate species separately,
just as we examine the species Man separately; to examine, that is, not
the whole class Birds collectively, but the Ostrich, the Crane, and the
other indivisible groups or species belonging to the class.
‘On the other hand, this course would involve repeated mention of the
same attribute, as the same attribute is common to many species, and so
far would be somewhat irrational and tedious. Perhaps, then, it will be
best to treat generically the universal attributes of the groups that
have a common nature and contain closely allied subordinate forms,
whether they are groups recognized by a true instinct of mankind,
such as Birds and Fishes, or groups not popularly known by a common
appellation, but withal composed of closely allied subordinate groups;
and only to deal individually with the attributes of a single species,
when such species—man, for instance, and any other such, if such there
be—stands apart from others, and does not constitute with them a larger
natural group.
‘It is generally similarity in the shape of particular organs, or of
the whole body, that has determined the formation of the larger groups.
It is in virtue of such a similarity that Birds, Fishes, Cephalopoda,
and Testacea have been made to form each a separate _genus_ (γένος).
For within the limits of each such _genus_, the parts do not differ
in that they have no nearer resemblance than that of analogy—such as
exists between the bone of man and the spine of fish—but they differ
merely in respect of such corporeal conditions as largeness smallness,
softness hardness, smoothness roughness, and other similar oppositions,
or, in one word, in respect of degree.’[49]
[49] _De partibus animalium_, i. 4; 644ᵅ 27.
The Aristotelian _genus_ thus differs widely from the term as used in
modern biology. In another passage he comes nearer to defining it and
the analogy of parts which extends from genus to genus.
‘Groups that differ only in the degree, and in the more or less of
an identical element that they possess are aggregated together under
a single _genus_; groups whose attributes are not identical but
_analogous_ are separated. For instance, bird differs from bird by
gradation, or by excess and defect; some birds have long feathers,
others short ones, but all are feathered. Bird and Fish are more remote
and only agree in having analogous organs; for what in the bird is
feather, in the fish is scale. Such _analogies_ can scarcely, however,
serve universally as indications for the formation of groups, for
almost all animals present analogies in their corresponding parts.’[50]
Aristotle nowhere gives to his term _genus_ a rigid application that
can be applied throughout the animal kingdom. He uses the word in fact
much as we should use the conveniently flexible term _group_, now
for a larger and less definite, now for a smaller and more definite
collection of species. This varying use of a technical word makes it
impossible to draw up a classification based on his _genera_ or indeed
with any consistent use of the terms which he actually employs.
The difficulty or impossibility of drawing up a satisfactory
classificatory system from the Aristotelian writings has not, however,
deterred numerous naturalists and scholars from making the attempt, and
the subject has in itself a considerable history and literature[51]
extending from the days of Edward Wotton (1492-1555) downward.[52] The
more recent efforts at drawing up an Aristotelian classificatory system
have been based on the methods of reproduction to which he certainly
attached very great importance.[53] Provided that it be remembered
that Aristotle does not himself detail any such system there can be
no harm in constructing one from his works. At worst it will serve as
a _memoria technica_ for the extent and character of his knowledge of
natural history, and at best it may represent a scheme to which he was
tending.
[50] _De partibus animalium_, i. 4; 644ᵅ 16.
[51] The classificatory system of Aristotle and its history are
discussed in great detail by J. B. Meyer, _Aristoteles’ Thierkunde:
ein Beitrag zur Geschichte der Zoologie, Physiologie und alten
Philosophie_, Berlin, 1855.
[52] The work by which Wotton is known is his _De differentiis
animalium_, Paris, 1552.
[53] There is a valuable chapter on the subject of the Aristotelian
classificatory system as based on the method of reproduction in W.
Ogle, _Aristotle on the Parts of Animals_, London, 1882.
_ENAIMA_ (_Sanguineous and either viviparous or oviparous_)
= _vertebrates_.
{ 1. ἅνθρωπος. Man.
{ 2. κήτη. Cetaceans.
{ 3. ζῷα τετράποδα ζωοτόκα ὲν αὑτοῖς.
{ Viviparous quadrupeds.
{ (a) μὴ ἀμφώδοντα. Non-amphodonts
Viviparous in the { = Ruminants with incisor in
internal sense. { lower jaw only and with cloven
{ hoofs.
{ (b) μώνυχα. Solid-hoofed animals.
{ i. λόφουρα. Equidae.
{ ii. μώνυχα ἔτερα. Other
solid-hoofed animals.
{ 4. ὄρνιθες. Birds.
{ (a) γαμψώνυχα. Birds of prey with
{ talons.
{ (b) στεγανόποδες. Swimmers with
{ With { webbed feet.
{ perfect { (c) περιστεροειδῆ. Pigeons, doves, &c.
{ ovum. { (d) ἄποδες. Swifts, martins, &c.
{ { (e) ὄρνιθες ἕτεροι. Other birds.
Oviparous { { 5. ζὌῷα τετράποδα ῷοτόκα. Oviparous
though { { quadrupeds = Amphibians and most
sometimes { { reptiles.
_externally_ { { 6. ὀφιώδη. Serpents.
viviparous. {
{ { 7. ἰχθύες. Fishes.
{ With { (a) σελάχη. Selachians. Cartilaginous
{ imperfect { fishes and, doubtfully, the
{ { fishing-frog.
{ ovum. { (b) ιχθύες ἕτεροι. Other fishes.
_ANAIMA_ (_Non-sanguineous and either viviparous, vermiparous or
budding_) = _Invertebrates_.
With perfect ovum. { 8. μαλάκια. Cephalopods.
{ 9. μαλακόστρακα. Crustaceans.
With ‘scolex’. 10. ἔντομα. Insects, spiders,
scorpions, &c.
With generative }11. ὀστρακόδερμα. Molluscs (except
slime, buds or } Cephalopods), Echinoderms, &c.
spontaneous generation. }
With spontaneous }12. ζωόφυτα. Sponges, Coelenterates,
generation only. } &c.
Some of the elements in this classification are fundamentally
unsatisfactory in that they are based on negative characters. Such
is the group of _Anaima_ which is parallelled by our own equally
convenient and negative though morphologically meaningless equivalent
_Invertebrata_. Others, such as the subdivisions of the viviparous
quadrupeds, can only be forcibly extracted out of Aristotle’s text.
But there are yet others, such as the separation of the cartilaginous
from the bony fishes, that exhibit true genius and betray a knowledge
that can only have been reached by careful investigation. Remarkably
brilliant too is his treatment of Molluscs. There can be no doubt that
he dissected the bodies and carefully watched the habits of octopuses
and squids, _Malacia_ as he calls them. He separates them too far from
the other Molluscs, grouped by him as _Ostracoderma_, but his actual
descriptions of the structure and sexual process of the cephalopods
are exceedingly remarkable, and after being long disregarded or
misunderstood were verified and repeated in the course of the
nineteenth century.[54]
[54] The rediscovery and verification of this and other Aristotelian
observations is detailed by C. Singer, ‘Greek Biology and the Rise of
Modern Biology,’ _Studies in the History and Method of Science_, vol.
ii, Oxford, 1921.
Passing from his general ideas on the nature and division of living
creatures we may turn to some of the most noteworthy of his actual
observations. In the realm of comparative anatomy proper we may
instance that of the stomach of ruminants. He must have dissected
these animals, for he gives a clear and correct account of the four
chambers. ‘Animals’, he says, ‘present diversities in the structure
of their stomachs. Of the viviparous quadrupeds, such of the horned
animals as are not equally furnished with teeth in both jaws are
furnished with four such chambers. These animals are those that are
said to chew the cud. In these animals the oesophagus extends from
the mouth downwards along the lung, from the midriff to the _big
stomach_ [_rumen_, or paunch], and this stomach is rough inside and
semi-partitional. And connected with it near to the entry of the
oesophagus is what is called the _kekryphalos_ [_reticulum_, or
honeycomb bag]; for outside it is like the stomach, but inside it
resembles a netted cap; and the kekryphalos is a good deal smaller
than the _big stomach_.’ The term _kekryphalos_ was applied to the net
that women wore over their hair to keep it in order. ‘Connected with
this kekryphalos,’ he continues, ‘is the _echinos_ [_psalterium_, or
_manyplies_], rough inside and laminated, and of about the same size as
the kekryphalos. Next after this comes what is called the _enystron_
[_abomasum_], larger and longer than the echinos, furnished inside with
numerous folds or ridges, large and smooth. After all this comes the
gut....’[55] ‘All animals that have horns, the sheep for instance, the
ox, the goat, the deer and the like, have these several stomachs....
The several cavities receive the food one from the other in succession:
the first taking the unreduced substances, the second the same when
somewhat reduced, the third when reduction is complete, and the fourth
when the whole has become a smooth pulp....’[56] ‘Such is the stomach
of those quadrupeds that are horned and have an unsymmetrical dentition
(μὴ ἀμφώδοντα); and these animals differ one from another in the shape
and size of the parts, and in the fact of the oesophagus reaching the
stomach central-wise in some cases and sideways in others. Animals that
are furnished equally with teeth in both jaws (ἀμφώδοντα) have one
stomach; as man, the pig, the dog, the bear, the lion, the wolf.’[57]
[55] _Historia animalium_, ii. 17; 507ᵅ 33.
[56] _De partibus animalium_, ii. 17; 507ᵇ 12.
[57] _Historia animalium_, ii. 17; 507ᵇ 12.
A very famous example in the Aristotelian works anticipating modern
biological knowledge is afforded by his reference to the mode of
reproduction of the cephalopods. ‘The Malacia such as the octopus,
the sepia, and the calamary, have sexual intercourse all in the same
way; that is to say, they unite at the mouth by an interlacing of
their tentacles. When, then, the octopus rests its so-called head
against the ground and spreads abroad its tentacles, the other sex
fits into the outspreading of these tentacles, and the two sexes then
bring their suckers into mutual connexion. Some assert that the male
has a kind of penis in one of his tentacles, the one in which are the
largest suckers; and they further assert that the organ is tendinous
in character growing attached right up to the middle of the tentacle,
and that the latter enables it to enter the nostril or funnel of the
female.’[58]
[58] _Historia animalium_, v. 6; 541ᵇ 1. The hectocotylization of the
cephalopod arm which is here recorded as an element in the reproductive
process of these animals is denied in the _De generatione animalium_,
i. 15; 720ᵇ 32, where we read that ‘the insertion of the arm of
the male into the funnel of the female ... is only for the sake of
attachment, and it is not an organ useful for generation, for it is
outside the passage in the male and indeed outside the body of the male
altogether.‘ Yet even here Aristotle knows of the physical relationship
of the arm. See note on this point in the translation of the passage by
A. Platt, Oxford, 1910.
The reproductive processes of the Cephalopods were unknown to modern
naturalists until the middle of the nineteenth century. Before that
time several observers had noted the occasional presence of a peculiar
parasite in the mantle cavity of female cephalopods and had described
its supposed structure without tracing any relationship to the process
of generation. In 1851 it was first shown that this supposed parasite
was the arm of the male animal specially modified for reproductive
purposes and broken off on insertion into the mantle cavity of the
female[59]. The actual process of reproduction does not seem to have
been observed until 1894[60].
[59] J. B. Verany, _Mollusques méditerranéens_, Genoa, 1851.
[60] E. Racovitza. _Archives de zoologie experimentale_, Paris, 1894.
Aristotle is perhaps at his best and happiest when describing the
habits of living animals that he has himself observed. Among his most
pleasing accounts are those of the fishing-frog and torpedo. In these
creatures he did not fail to notice the displacement of the fins
associated with the depressed form of the body.
‘In marine creatures,’ he says, ‘one may observe many ingenious devices
adapted to the circumstances of their lives. For the account commonly
given of the frog-fish or angler is quite true; as is also that of the
torpedo....
‘In the Torpedo and the Fishing-frog the breadth of the anterior part
of the body is not so great as to render locomotion by fins impossible,
but in consequence of it the upper pair [_pectorals_] are placed
further back and the under pair [_ventrals_] are placed close to the
head, while to compensate for this advancement they are reduced in size
so as to be smaller than the upper ones.
‘In the Torpedo the two upper fins [pectorals] are placed in the tail,
and the fish uses the broad expansion of its body to supply their
place, each lateral half of its circumference serving the office of a
fin.... The torpedo narcotizes the creatures that it wants to catch,
overpowering them by the force of shock that is resident in its body,
and feeds upon them; it also hides in the sand and mud, and catches
all the creatures that swim in its way and come under its narcotizing
influence. This phenomenon has been actually observed in operation....
The torpedo-fish is known to cause a numbness even in human beings.
‘The frog-fish has a set of filaments that project in front of its
eyes; they are long and thin, like hairs, and are round at the tips;
they lie on either side, and are used as baits.... The little creatures
on which this fish feeds swim up to the filaments, taking them for bits
of seaweed such as they feed upon. Accordingly, when the frog-fish
stirs himself up a place where there is plenty of sand and mud and
conceals himself therein, it raises the filaments, and when the little
fish strike against them the frog-fish draws them in underneath into
its mouth.... That the creatures get their living by this means is
obvious from the fact that, whereas they are peculiarly inactive,
they are often caught with mullets, the swiftest of fishes, in their
interior. Furthermore, the frog-fish is usually thin when he is caught
after losing the tips of his filaments.’[61]
[61] The paragraphs concerning the fishing-frog and torpedo are made up
of sentences rearranged from the _De partibus animalium_, iv. 13; 696ᵅ
26, and the _Historia animalium_, ix. 37; 620ᵇ 15.
The modification of the musculature of the torpedo-fish for electric
purposes and the fishing habits of the fishing-frog or _Lophius_ are
now well known, but it was many centuries before naturalists had
confirmed the observations of the father of biology.
When we turn from Aristotle’s observations in the department of natural
history to his discussion of the actual mechanism of the living body,
the subject now contained under the heading _Experimental Physiology_,
we are in the presence of much less satisfactory material. Aristotle
here exhibits his weakness in physics and not being endowed with any
experimental knowledge of that subject his physiological development is
very greatly handicapped. He seems often to accept fancies of his own
in place of generalizations from collated observations. This tendency
of his was conveyed to his successors and delayed physiological
advance for many centuries. It forms a striking contrast to the
method of certain of the Hippocratic works such as the _Epidemics_
and the _Aphorisms_ which exhibit an investigator intent on recording
actual observations and on deducing general laws therefrom. Had the
Hippocratic method been extended by Aristotle beyond the field of
natural history, where he freely follows it, to that of physiology, the
succeeding generations might have established medicine far more firmly
as a science.
An important factor in Aristotle’s physical and physiological
teaching is the doctrine that matter is continuous and not made
up of indivisible parts. He thus rejected the atomic views of his
predecessors Leucippus and Democritus which have been preserved for us
by the poem of Lucretius. The different kinds of matter existing merely
in their state of simple mixture formed various uniform or homogeneous
substances, _homoeomeria_, of which the _tissues_ of living bodies
provided one type. We now consider tissues as having structure made up
of living cells or their products, but to Aristotle their structure was
an essential fact following on their particular elemental constitution.
The structure of muscle or flesh was perhaps comparable to that of
a crystalline substance, for, as we have seen, Aristotle made no
fundamental distinction between organic and inorganic _substances_,
which are in his view alike subject to the processes of generation and
corruption. The difference between them lies not in their structure
but in their potential relation to the various degrees of soul, the
vegetative, the animal, and the rational.
‘There are’, says Aristotle, ‘three degrees of composition, and of
these the first in order is composition out of what some call the
_elements_, earth, air, water, and fire....
‘The second degree of composition is that by which the _homogeneous_
parts of animals (ὁμοιομερῆ), such as bone, flesh, and the like, are
constituted out of [these] primary substances.
‘The third and last stage is the composition which forms the
_heterogeneous_ parts (ἀνομοιομερῆ) such as face, hand, and the
rest.’[62]
[62] _De partibus animalium_, ii. 1; 646ᵅ 12.
The distinctions are not altogether clear but may perhaps be explained
along such lines as the following. The division into homogeneous and
heterogeneous corresponds in a general way to the later division
into Tissues and Organs, the former, however, including much that we
should not call tissue. The homogeneous parts were again of two kinds:
(_a_) simple tissues or stuffs without any notion of size or shape,
that is, mere substance capable of endowment with life or soul, e.g.
cartilaginous or osseous tissues; and (_b_) simple structure, that is
actual structure made of such a single tissue but with definite form
and size, matter to which form had been added and which either was
actually or had been endowed with soul, e.g. _a_ cartilage or _a_ bone.
As a physiologist Aristotle is, in fact, in much the same position as
he is as a physicist. He never dissected the human body, he had only
the roughest idea of the course of the vessels, and his description of
the vascular system is so difficult and confused that a considerable
literature has been written on its interpretation. He regarded the
heart as the central organ of the body and the seat of sensation and he
probably believed that the arteries contained air as well as blood. He
made no adequate distinction between veins and arteries. He tells us
that two great vessels arise from the heart and that the heart is, as
it were, a part of these vessels. The two vessels are apparently the
aorta and the vena cava, and a very elementary and not very accurate
description is given of the branches of these vessels. He believed that
the heart had three chambers or cavities and that it took in air direct
from the lung.
The brain was for him mainly an organ by which were secreted certain
cold humours which prevented any overheating of the body by the furnace
of the heart under the action of the bellows of the lung. He formally
rejected the older views of Diogenes of Apollonia, of Alcmaeon of
Croton, and of the Hippocratic writings, that placed the seat of
sensation in the brain.[63] He failed to trace any adequate relation of
sense organs and nerves to brain. He considered that the spinal marrow
served to hold the vertebrae together.
[63] _De partibus animalium_, ii. 10.