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 Academie 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];