On The Parts Of Animals   

reason and explanation of each subsequent step that he takes, and of
his acting in this or that way as the case may be. Now in the works of
nature the good end and the final cause is still more dominant than in
works of art such as these, nor is necessity a factor with the same
significance in them all; though almost all writers, while they try to
refer their origin to this cause, do so without distinguishing the
various senses in which the term necessity is used. For there is
absolute necessity, manifested in eternal phenomena; and there is
hypothetical necessity, manifested in everything that is generated by
nature as in everything that is produced by art, be it a house or what
it may. For if a house or other such final object is to be realized,
it is necessary that such and such material shall exist; and it is
necessary that first this then that shall be produced, and first this
and then that set in motion, and so on in continuous succession, until
the end and final result is reached, for the sake of which each prior
thing is produced and exists. As with these productions of art, so
also is it with the productions of nature. The mode of necessity,
however, and the mode of ratiocination are different in natural
science from what they are in the theoretical sciences; of which we
have spoken elsewhere. For in the latter the starting-point is that
which is; in the former that which is to be. For it is that which is
yet to be-health, let us say, or a man-that, owing to its being of
such and such characters, necessitates the pre-existence or previous
production of this and that antecedent; and not this or that
antecedent which, because it exists or has been generated, makes it
necessary that health or a man is in, or shall come into, existence.
Nor is it possible to track back the series of necessary antecedents
to a starting-point, of which you can say that, existing itself from
eternity, it has determined their existence as its consequent. These
however again, are matters that have been dealt with in another
treatise. There too it was stated in what cases absolute and
hypothetical necessity exist; in what cases also the proposition
expressing hypothetical necessity is simply convertible, and what
cause it is that determines this convertibility.
Another matter which must not be passed over without consideration is,
whether the proper subject of our exposition is that with which the
ancient writers concerned themselves, namely, what is the process of
formation of each animal; or whether it is not rather, what are the
characters of a given creature when formed. For there is no small
difference between these two views. The best course appears to be that
we should follow the method already mentioned, and begin with the
phenomena presented by each group of animals, and, when this is done,
proceed afterwards to state the causes of those phenomena, and to deal
with their evolution. For elsewhere, as for instance in house
building, this is the true sequence. The plan of the house, or the
house, has this and that form; and because it has this and that form,
therefore is its construction carried out in this or that manner. For
the process of evolution is for the sake of the thing Anally evolved,
and not this for the sake of the process. Empedocles, then, was in
error when he said that many of the characters presented by animals
were merely the results of incidental occurrences during their
development; for instance, that the backbone was divided as it is into
vertebrae, because it happened to be broken owing to the contorted
position of the foetus in the womb. In so saying he overlooked the
fact that propagation implies a creative seed endowed with certain
formative properties. Secondly, he neglected another fact, namely,
that the parent animal pre-exists, not only in idea, but actually in
time. For man is generated from man; and thus it is the possession of
certain characters by the parent that determines the development of
like characters in the child. The same statement holds good also for
the operations of art, and even for those which are apparently
spontaneous. For the same result as is produced by art may occur
spontaneously. Spontaneity, for instance, may bring about the
restoration of health. The products of art, however, require the