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On The Generation Of Animals   


After this, as said already, the internal parts come into being
before the external. The greater become visible before the less,
even if some of them do not come into being before them. First the
parts above the hypozoma are differentiated and are superior in
size; the part below is both smaller and less differentiated. This
happens in all animals in which exists the distinction of upper and
lower, except in the insects; the growth of those that produce a
scolex is towards the upper part, for this is smaller in the
beginning. The cephalopoda are the only locomotive animals in which
the distinction of upper and lower does not exist.

What has been said applies to plants also, that the upper portion is
earlier in development than the lower, for the roots push out from the
seed before the shoots.

The agency by which the parts of animals are differentiated is
air, not however that of the mother nor yet of the embryo itself, as
some of the physicists say. This is manifest in birds, fishes, and
insects. For some of these are separated from the mother and
produced from an egg, within which the differentiation takes place;
other animals do not breathe at all, but are produced as a scolex or
an egg; those which do breathe and whose parts are differentiated
within the mother's uterus yet do not breathe until the lung is
perfected, and the lung and the preceding parts are differentiated
before they breathe. Moreover, all polydactylous quadrupeds, as dog,
lion, wolf, fox, jackal, produce their young blind, and the eyelids do
not separate till after birth. Manifestly the same holds also in all
the other parts; as the qualitative, so also the quantitative
differentia comes into being, pre-existing potentially but being
actualized later by the same causes by which the qualitative
distinction is produced, and so the eyelids become two instead of one.
Of course air must be present, because heat and moisture are
present, the former acting and the latter being acted upon.

Some of the ancient nature-philosolphers made an attempt to state
which part comes into being after which, but were not sufficiently
acquainted with the facts. It is with the parts as with other
things; one naturally exists prior to another. But the word 'prior' is
used in more senses than one. For there is a difference between the
end or final cause and that which exists for the sake of it; the
latter is prior in order of development, the former is prior in
reality. Again, that which exists for the sake of the end admits of
division into two classes, (1) the origin of the movement, (2) that
which is used by the end; I mean, for instance, (1) that which can
generate, (2) that which serves as an instrument to what is generated,
for the one of these, that which makes, must exist first, as the
teacher before the learner, and the other later, as the pipes are
later than he who learns to play upon them, for it is superfluous that
men who do not know how to play should have pipes. Thus there are
three things: first, the end, by which we mean that for the sake of
which something else exists; secondly, the principle of movement and
of generation, existing for the sake of the end (for that which can
make and generate, considered simply as such, exists only in
relation to what is made and generated); thirdly, the useful, that is
to say what the end uses. Accordingly, there must first exist some
part in which is the principle of movement (I say a part because this
is from the first one part of the end and the most important part
too); next after this the whole and the end; thirdly and lastly,
the organic parts serving these for certain uses. Hence if there is
anything of this sort which must exist in animals, containing the
principle and end of all their nature, this must be the first to
come into being- first, that is, considered as the moving power, but

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