Form and Function | Page 7

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, f?ces 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