Form and Function | Page 6

all animals, or to
assemblages, like the class of Birds, of closely allied groups
differentiated by gradation, or to groups like Man not differentiated
into subordinate groups. In the first case the common attributes may be
called analogous, in the second generic, in the third specific" (i, 5,
645^b, trans. Ogle). The alimentary canal is a good example of a part
which is "analogous" throughout the animal kingdom, for "all animals
possess in common those parts by which they take in food, and into
which they receive it" (Cresswell, loc. cit., p. 6).
The De Partibus Animalium becomes in form a comparative
organography, but the emphasis is always on function and community
of function. Thus he treats of bone, "fish-spine," and cartilage together
(De Partibus, ii., 9, 655^a), because they have the same function,
though he says elsewhere that they are only analogous structures (ii., 8,
653^b). In the same connection he describes also the supporting tissues
of Invertebrates--the hard exoskeleton of Crustacea and Insects, the

shell of Testacea, the "bone" of Sepia (ii., 8, 654^a). Aristotle took
much more interest in analogies, in organs of similar function, than in
homologies. He did recognise the existence of homologies, but rather
malgré lui, because the facts forced it upon him.
His only excursion into the realm of "transcendental anatomy" is his
comparison of a Cephalopod to a doubled-up Vertebrate whose legs
have become adherent to its head, whose alimentary canal has doubled
upon itself in such a way as to bring the anus near the mouth (De
Partibus, iv., 9, 684^b). It is clear, however, that Aristotle did not seek
to establish by this comparison any true homologies of parts, but
merely analogies, thus avoiding the error into which Meyranx and
Laurencet fell more than two thousand years later in their paper
communicated to the Académie des Sciences, which formed the
starting-point of the famous controversy between Cuvier and E.
Geoffroy St Hilaire (see Chap. V., below).
Moreover, Aristotle did not so much compare a Cephalopod with a
doubled-up Vertebrate as contrast Cephalopods (and also Testacea)
with all other animals. Other animals have their organs in a straight line;
Cephalopods and Testacea alone show this peculiar doubling up of the
body.
(4) Aristotle was much struck with certain facts of correlation, of the
interdependence of two organs which are not apparently in functional
dependence on one another. Such correlation may be positive or
negative; the presence of one organ may either entail the presence of
the other, or it may entail its absence. Aristotle has various ways of
explaining facts of correlation. He observed that no animal has both
tusks and horns, but this fact could easily be explained on the principle
that Nature never makes anything superfluous or in vain. If an animal is
protected by the possession of tusks it does not require horns, and vice
versa. The correlation of a multiple stomach with deficient
development of the teeth (as in Ruminants) is accounted for by saying
that the animal needs its complex stomach to make up for the
shortcomings of its teeth! (De Partibus, iii., 14, 674^b.) Other
examples of correlation were not susceptible of this explanation in

terms of final causes. He lays stress on the fact, in the main true, of the
inverse development of horns and front teeth in the upper jaw,
exemplified in Ruminants. He explains the fact in this way. Teeth and
horns are formed from earthy matter in the body and there is not
enough to form both teeth and horns, so "Nature by subtracting from
the teeth adds to the horns; the nutriment which in most animals goes to
the former being here spent on the augmentation of the latter" (De
Partibus, iii., 2, 664^a, trans. Ogle). A similar kind of explanation is
offered of the fact that Selachia have cartilage instead of bone, "in these
Selachia Nature has used all the earthy matter on the skin [i.e., on the
placoid scales]; and she is unable to allot to many different parts one
and the same superfluity of material" (De Partibus, ii., 9, 655^a, trans.
Ogle). Speaking generally, "Nature invariably gives to one part what
she subtracts from another" (loc. cit., ii., 14, 658^a).
This thought reappears again in the 19th century in E. Geoffroy St
Hilaire's loi de balancement and also in Goethe's writings on
morphology. For Aristotle it meant that Nature was limited by the
nature of her means, that finality was limited by necessity. Thus in the
larger animals there is an excess of earthy matter, as a necessary result
of the material nature of the animal; this excess is turned by Nature to
good account, but there is not enough to serve both for teeth and for
horns (loc. cit., iii., 2, 663^b).
But there are other instances of