For the
upper part of the upright creature is long so that its leg has to be
correspondingly long; in consequence there must be flexion. For
since a stationary position is perpendicular, if that which moves
cannot bend it will either fall forward as the right angle becomes
acute or will not be able to progress. For if one leg is at right
angles to the ground and the other is advanced, the latter will be
at once equal and greater. For it will be equal to the stationary
leg and also equivalent to the hypotenuse of a right-angled
triangle. That which goes forward therefore must bend, and while
bending one, extend the other leg simultaneously, so as to incline
forward and make a stride and still remain above the perpendicular;
for the legs form an isosceles triangle, and the head sinks lower when
it is perpendicularly above the base on which it stands.
Of limbless animals, some progress by undulations (and this
happens in two ways, either they undulate on the ground, like
snakes, or up and down, like caterpillars), and undulation is a
flexion; others by a telescopic action, like what are called
earthworms and leeches. These go forward, first one part leading and
then drawing the whole of the rest of the body up to this, and so they
change from place to place. It is plain too that if the two curves
were not greater than the one line which subtends them undulating
animals could not move themselves; when the flexure is extended they
would not have moved forward at all if the flexure or arc were equal
to the chord subtended; as it is, it reaches further when it is
straightened out, and then this part stays still and it draws up
what is left behind.
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