Form and Function | Page 8

(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 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.,
I) 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 2. Externally
viviparous Sanguinea } animal. 3. Oviparous Sanguinea--producing a
perfect egg. 4. Animals producing an imperfect egg (one which
increases in size after being laid). 5. Insects, producing a scolex (or
grub).
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