On The Generation Of Animals
motions of the sun and moon that fix the limit both of the beginning
and of the end of these processes. Just as we see the sea and all
bodies of water settling and changing according to the movement or
rest of the winds, and the air and winds again according to the course
of the sun and moon, so also the things which grow out of these or are
in these must needs follow suit. For it is reasonable that the periods
of the less important should follow those of the more important. For
in a sense a wind, too, has a life and birth and death.
As for the revolutions of the sun and moon, they may perhaps
depend on other principles. It is the aim, then, of Nature to
measure the coming into being and the end of animals by the measure of
these higher periods, but she does not bring this to pass accurately
because matter cannot be easily brought under rule and because there
are many principles which hinder generation and decay from being
according to Nature, and often cause things to fall out contrary to
We have now spoken of the nourishment of animals within the mother
and of their birth into the world, both of each kind separately and of
all in common.
WE must now investigate the qualities by which the parts of
animals differ. I mean such qualities of the parts as blueness and
blackness in the eyes, height and depth of pitch in the voice, and
differences in colour whether of the skin or of hair and feathers.
Some such qualities are found to characterize the whole of a kind of
animals sometimes, while in other kinds they occur at random, as is
especially the case in man. Further, in connexion with the changes
in the time of life, all animals are alike in some points, but are
opposed in others as in the case of the voice and the colour of the
hair, for some do not grow grey visibly in old age, while man is
subject to this more than any other animal. And some of these
affections appear immediately after birth, while others become plain
as age advances or in old age.
Now we must no longer suppose that the cause of these and all such
phenomena is the same. For whenever things are not the product of
Nature working upon the animal kingdom as a whole, nor yet
characteristic of each separate kind, then none of these things is
such as it is or is so developed for any final cause. The eye for
instance exists for a final cause, but it is not blue for a final
cause unless this condition be characteristic of the kind of animal.
In fact in some cases this condition has no connexion with the essence
of the animal's being, but we must refer the causes to the material
and the motive principle or efficient cause, on the view that these
things come into being by Necessity. For, as was said originally in
the outset of our discussion, when we are dealing with definite and
ordered products of Nature, we must not say that each is of a
certain quality because it becomes so, but rather that they become
so and so because they are so and so, for the process of Becoming or
development attends upon Being and is for the sake of Being, not
The ancient Nature-philosophers however took the opposite view.
The reason of this is that they did not see that the causes were
numerous, but only saw the material and efficient and did not
distinguish even these, while they made no inquiry at all into the
formal and final causes.