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 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, wit