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ON THE GAIT OF ANIMALS
by Aristotle
translated by A。 S。 L。 Farquharson
1
WE have now to consider the parts which are useful to animals for
movement in place （locomotion）； first； why each part is such as it
is and to what end they possess them； and second； the differences
between these parts both in one and the same creature； and again by
comparison of the parts of creatures of different species with one
another。 First then let us lay down how many questions we have to
consider。
The first is what are the fewest points of motion necessary to
animal progression； the second why sanguineous animals have four
points and not more； but bloodless animals more than four； and
generally why some animals are footless； others bipeds； others
quadrupeds； others polypods； and why all have an even number of
feet； if they have feet at all； why in fine the points on which
progression depends are even in number。
Next； why are man and bird bipeds； but fish footless； and why do man
and bird； though both bipeds； have an opposite curvature of the
legs。 For man bends his legs convexly； a bird has his bent
concavely； again； man bends his arms and legs in opposite
directions； for he has his arms bent convexly； but his legs concavely。
And a viviparous quadruped bends his limbs in opposite directions to a
man's； and in opposite directions to one another； for he has his
forelegs bent convexly； his hind legs concavely。 Again； quadrupeds
which are not viviparous but oviparous have a peculiar curvature of
the limbs laterally away from the body。 Again； why do quadrupeds
move their legs criss…cross？
We have to examine the reasons for all these facts； and others
cognate to them； that the facts are such is clear from our Natural
History； we have now to ask reasons for the facts。
2
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。）
3
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
runners run faster if they swing their arms； there is in extending the
arms a kind of leaning against the hands and wrists。 In all cases then
that which moves makes its change of position by the use of at least
two parts of the body； one part so to speak squeezes； the other is
squeezed； for the part that is still is squeezed as it has to carry
the weight； the part that is lifted strains against that which carries
the weight。 It follows then that nothing without parts can move itself
in this way； for it has not in it the distinction of the part which is
passive and that which is active。
4
Again； the boundaries by which living beings are naturally
determined are six in number； superior and inferior； before and
behind； right and left。 Of these all living beings have a superior and
an inferior part； for superior and inferior is in plants too； not only
in animals。 And this distinction is one of function； not merely of
position relatively to our earth and the sky above our heads。 The
superior is that from which flows in each kind the distribution of
nutriment and the process of growth； the inferior is that to which the
process flows and in which it ends。 One is a starting…point； the other
an end； and the starting…point is the superior。 And yet it might be
thought that in the case of plants at least the inferior is rather the
appropriate starting…point； for in them the superior and inferior
are in position other than in animals。 Still they are similarly
situated from the point of view of function； though not in their
position relatively to the universe。 The roots are the superior part
of a plant； for from them the nutriment is distributed to the
growing members； and a plant takes it with its roots as an animal does
with its mouth。
Things that are not only alive but are animals have both a front and
a back； because they all have sense； and front and back are
distinguished by reference to sense。 The front is the part in which
sense is innate； and whence each thing gets its sensations； the
opposite parts are the back。
All animals which partake not only in sense； but are able of
themselves to make a change of place； have a further distinction of
left and right besides those already enumerated； like the former these
are distinctions of function and not of position。 The right is that
from which change of position naturally begins； the opposite which
naturally depends upon this is the left。
This distinction （of right and left） is more articulate and detailed
in some than in others。 For animals which make the aforesaid change
（of place） by the help of organized parts （I mean feet for example； or
wings or similar organs） have the left and right distinguished in
greater detail； while those which are not differentiated into such
parts； but make the differentiation in the body itself and so
progress； like some footless animals （for example snakes and
caterpillars after their kind； and besides what men call earth…worms）；
all these have the distinction spoken of； although it is not made so
manifest to us。 That the beginning of movement is on the right is
indicated by the fact that all men carry burdens on the left shoulder；
in this way they set free the side which initiates movement and enable
the side which bears the weight to be moved。 And so men hop easier
on the left leg； for the nature of the right is to initiate
movement； that of the left to be moved。 The burden then must rest on
the side which is to be moved； not on that which is going to cause
movement； and if it be set on the moving side； which is the original
of movement； it will either not be moved at all or with more labour。
Another indication that the right is the source of movement is the way
we put our feet forward； all men lead off with the left； and after
standing still prefer to put the left foot forward； unless something
happens to prevent it。 The reason is that their movement comes from
the leg they step off； not from the one put forward。 Again； men
guard themselves with their right。 And this is the reason why the
right is the same in all； for that from which motion begins is the
same for all； and has its natural position in the same place； and
for this reason the spiral…shaped Testaceans have their shells on
the right； for they do not move in the direction of the spire； but all
go forward in the direction opposite to the spire。 Examples are the
murex and the ceryx。 As all animals then start movement from the
right； and the right moves in the same direction as the whole； it is
necessary for all to be alike right…handed。 And man has the left limbs
detached more than any other animal because he is natural in a
higher degree than the other animals； now the r