On The Parts Of Animals   

animal nature, but only some part or parts of it. Moreover, it is
impossible that any abstraction can form a subject of natural science,
seeing that everything that Nature makes is means to an end. For just
as human creations are the products of art, so living objects are
manifest in the products of an analogous cause or principle, not
external but internal, derived like the hot and the cold from the
environing universe. And that the heaven, if it had an origin, was
evolved and is maintained by such a cause, there is therefore even
more reason to believe, than that mortal animals so originated. For
order and definiteness are much more plainly manifest in the celestial
bodies than in our own frame; while change and chance are
characteristic of the perishable things of earth. Yet there are some
who, while they allow that every animal exists and was generated by
nature, nevertheless hold that the heaven was constructed to be what
it is by chance and spontaneity; the heaven, in which not the faintest
sign of haphazard or of disorder is discernible! Again, whenever there
is plainly some final end, to which a motion tends should nothing
stand in the way, we always say that such final end is the aim or
purpose of the motion; and from this it is evident that there must be
a something or other really existing, corresponding to what we call by
the name of Nature. For a given germ does not give rise to any chance
living being, nor spring from any chance one; but each germ springs
from a definite parent and gives rise to a definite progeny. And thus
it is the germ that is the ruling influence and fabricator of the
offspring. For these it is by nature, the offspring being at any rate
that which in nature will spring from it. At the same time the
offspring is anterior to the germ; for germ and perfected progeny are
related as the developmental process and the result. Anterior,
however, to both germ and product is the organism from which the germ
was derived. For every germ implies two organisms, the parent and the
progeny. For germ or seed is both the seed of the organism from which
it came, of the horse, for instance, from which it was derived, and
the seed of the organism that will eventually arise from it, of the
mule, for example, which is developed from the seed of the horse. The
same seed then is the seed both of the horse and of the mule, though
in different ways as here set forth. Moreover, the seed is potentially
that which will spring from it, and the relation of potentiality to
actuality we know.
There are then two causes, namely, necessity and the final end. For
many things are produced, simply as the results of necessity. It may,
however, be asked, of what mode of necessity are we speaking when we
say this. For it can be of neither of those two modes which are set
forth in the philosophical treatises. There is, however, the third
mode, in such things at any rate as are generated. For instance, we
say that food is necessary; because an animal cannot possibly do
without it. This third mode is what may be called hypothetical
necessity. Here is another example of it. If a piece of wood is to be
split with an axe, the axe must of necessity be hard; and, if hard,
must of necessity be made of bronze or iron. Now exactly in the same
way the body, which like the axe is an instrument-for both the body as
a whole and its several parts individually have definite operations
for which they are made-just in the same way, I say, the body, if it
is to do its work, must of necessity be of such and such a character,
and made of such and such materials.
It is plain then that there are two modes of causation, and that both
of these must, so far as possible, be taken into account in explaining
the works of nature, or that at any rate an attempt must be made to
include them both; and that those who fail in this tell us in reality
nothing about nature. For primary cause constitutes the nature of an
animal much more than does its matter. There are indeed passages in
which even Empedocles hits upon this, and following the guidance of
fact, finds himself constrained to speak of the ratio (olugos) as
constituting the essence and real nature of things. Such, for
instance, is the case when he explains what is a bone. For he does not