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, 647a).
(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, 646b).
(7) We meet in Aristotle an idea which later acquired considerable vogue, that of the Échelle des êtres(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., 1) 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 animal |
2. Externally viviparous Sanguinea | ||
3. OviparousSanguinea—producing a perfect egg. | ||
4. Animals producing an imperfectegg (one which increases in size after being laid). | ||
5. Insects, producing a scolex (orgrub). | ||
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, 656a, 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."