On The Gait Of Animals   

At the beginning of the inquiry we must postulate the principles

we are accustomed constantly to use for our scientific investigation

of nature, that is we must take for granted principles of this

universal character which appear in all Nature's work. Of these one is

that Nature creates nothing without a purpose, but always the best

possible in each kind of living creature by reference to its essential

constitution. Accordingly if one way is better than another that is

the way of Nature. Next we must take for granted the different species

of dimensions which inhere in various things; of these there are three

pairs of two each, superior and inferior, before and behind, to the

right and to the left. Further we must assume that the originals of

movements in place are thrusts and pulls. (These are the essential

place-movements, it is only accidentally that what is carried by

another is moved; it is not thought to move itself, but to be moved by

something else.)

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After these preliminaries, we go on to the next questions in order.

Now of animals which change their position some move with the

whole body at once, for example jumping animals, others move one

part first and then the other, for example walking (and running)

animals. In both these changes the moving creature always changes

its position by pressing against what lies below it. Accordingly if

what is below gives way too quickly for that which is moving upon it

to lean against it, or if it affords no resistance at all to what is

moving, the latter can of itself effect no movement upon it. For an

animal which jumps makes its jump both by leaning against its own

upper part and also against what is beneath its feet; for at the

joints the parts do in a sense lean upon one another, and in general

that which pushes down leans upon what is pushed down. That is why

athletes jump further with weights in their hands than without, and