Form and Function | Page 7

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, fæces 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"