for the right becomes the inner. (Let the right front point be A,

the left B, the right hind C, the left D.)

Among land animals this is the character of the movement of

snakes, and among water animals of eels, and conger-eels and also

lampreys, in fact of all that have their form snakelike. However, some

marine animals of this shape have no fin, lampreys for example, but

put the sea to the same use as snakes do both land and water (for

snakes swim precisely as they move on the ground). Others have two

fins only, for example conger-eels and eels and a kind of cestreus

which breeds in the lake of Siphae. 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