Part 4

This explanation is based on Aristotle’s fundamental doctrine of the opposite _qualities_, heat, cold, wetness, and dryness, that are found combined in pairs in the four _elements_, earth, air, fire, and water. The theory was of the utmost importance for the whole subsequent development of science and was not displaced until quite modern times. It was not an original conception of Aristotle, for something resembling it had been set forth long before his time in figurative language by Empedocles (_c._ 500-_c._ 430 B. C.), as Aristotle himself tells us.[44] The same view had been foreshadowed by Pythagoras (_c._ 580-_c._ 490 B. C.) at an even earlier date and was perhaps of much greater antiquity. But Aristotle developed the doctrine and was the main channel for its conveyance to later ages, so that his name will always be associated with it. Matter in general and living matter in particular was held by him to be composed of these four essential so-called _elements_ (στοιχεῑ), each of which is in turn compounded from two of the primary _qualities_ (δυνάμεις) which Aristotle brought into relation with the elements. Thus earth was cold and dry, water cold and wet, air hot and wet, and fire hot and dry (Fig. 7b).

[44] _Metaphysics_, i. 4. _De generatione et corruptione_, ii. 1.

The theory of the elements and qualities is applicable to all matter and not specially to living things. The distinction between the living and not-living is to be sought not so much in its material constitution, but in the presence or absence of ‘soul’, and his teaching on that topic is to be found in his great work περὶ ψυχῆς, _On Soul_. He does not think of matter as organic or inorganic—that is a distinction of the seventeenth century physiologists—nor does he think of things as divided into animal, vegetable, and mineral—that is a distinction of the mediaeval alchemists,—but he thinks of things as either with soul or without soul (ἔμψυχα or ἄαψυχα).

His belief as to the relationship of this soul to material things is a difficult and complicated subject which would take us far beyond the topics included in biological writings to-day, but he tells us that ‘there is a class of existent things which we call substance, including under that term, firstly, matter, which in itself is not this nor that; secondly, shape or form, in virtue of which the term this or that is at once applied; thirdly, the whole made up of matter and form. Matter is identical with potentiality, form with actuality,’ the soul being, in living things, that which gives the form or actuality. ‘Of natural bodies’, he continues, ‘some possess life and some do not: where by life we mean the power of self-nourishment and of independent growth and decay’.[45] It should here be noted that in the Aristotelian sense the ovum is not at first a living thing, for in its earliest stage and before fertilization it does not possess soul even in its most elementary form.

[45] _De anima_, ii. 1, ii.

‘The term life is used in various senses, and, if life is present in 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.