Accordingly, all animals
which progress by the use of distinct members have these members
distinguished not by the differences of before and behind, but only of
the remaining two pairs; the prior difference dividing these members
into right and left (a difference which must appear as soon as you
have division into two), and the other difference appearing of
necessity where there is division into four.
Since then these two pairs, the superior and inferior and the
right and left, are linked to one another by the same common
original (by which I mean that which controls their movement), and
further, everything which is intended to make a movement in each
such part properly must have the original cause of all the said
movements arranged in a certain definite position relatively to the
distances from it of the originals of the movements of the
individual members (and these centres of the individual parts are in
pairs arranged coordinately or diagonally, and the common centre is
the original from which the animal's movements of right and left,
and similarly of superior and inferior, start); each animal must
have this original at a point where it is equally or nearly equally
related to each of the centres in the four parts described.
7
It is clear then how locomotion belongs to those animals only
which make their changes of place by means of two or four points in
their structure, or to such animals par excellence.
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