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 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, 647^a).

(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, 646^b).

(7) We meet in Aristotle an idea which later acquired considerable
vogue, that of the _Echelle des etres_(or "scale of beings"), that
o