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On The Gait Of Animals   

backwards, nor has a moving animal any division whereby it may make

a change of position towards its front or back; but right and left,

superior and inferior are so distinguished. 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.


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. Moreover, since

this property belongs almost peculiarly to Sanguineous animals, we see

that no Sanguineous animal can progress at more points than four,

and that if it is the nature of anything so to progress at four points

it must of necessity be Sanguineous.

What we observe in the animal world is in agreement with the above

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