On this account too those that are
accustomed to live on land, for example all the eels, move with
fewer flexions in a fluid than on land, while the kind of cestreus
which has two fins, by its flexion in a fluid makes up the remaining
points.
8
The reason why snakes are limbless is first that nature makes
nothing without purpose, but always regards what is the best
possible for each individual, preserving the peculiar essence of
each and its intended character, and secondly the principle we laid
down above that no Sanguineous creature can move itself at more than
four points. Granting this it is evident that Sanguineous animals like
snakes, whose length is out of proportion to the rest of their
dimensions, cannot possibly have limbs; for they cannot have more than
four (or they would be bloodless), and if they had two or four they
would be practically stationary; so slow and unprofitable would
their movement necessarily be.
But every limbed animal has necessarily an even number of such
limbs. For those which only jump and so move from place to place do
not need limbs for this movement at least, but those which not only
jump but also need to walk, finding that movement not sufficient for
their purposes, evidently either are better able to progress with even
limbs or cannot otherwise progress at all every animal which has limbs
must have an even us for as this kind of movement is effected by
part of the body at a time, and not by the whole at once as in the
movement of leaping, some of the limbs must in turn remain at rest,
and others be moved, and the animal must act in each of these cases
with opposite limbs, shifting the weight from the limbs that are being
moved to those at rest.
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