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On The Generation Of Animals   

simultaneous with the whole embryo if considered as a part of the end.
Therefore all the organic parts whose nature is to bring others into
being must always themselves exist before them, for they are for the
sake of something else, as the beginning for the sake of the end;
all those parts which are for the sake of something else but are not
of the nature of beginnings must come into being later. So it is not
easy to distinguish which of the parts are prior, those which are
for the sake of another or that for the sake of which are the
former. For the parts which cause the movement, being prior to the end
in order of development, come in to cause confusion, and it is not
easy to distinguish these as compared with the organic parts. And
yet it is in accordance with this method that we must inquire what
comes into being after what; for the end is later than some parts
and earlier than others. And for this reason that part which
contains the first principle comes into being first, next to this
the upper half of the body. This is why the parts about the head,
and particularly the eyes, appear largest in the embryo at an early
stage, while the parts below the umbilicus, as the legs, are small;
for the lower parts are for the sake of the upper, and are neither
parts of the end nor able to form it.

But they do not say well nor do they assign a necessary cause who
say simply that 'it always happens so', and imagine that this is a
first principle in these cases. Thus Democritus of Abdera says that
'there is no beginning of the infinite; now the cause is a
beginning, and the eternal is infinite; in consequence, to ask the
cause of anything of this kind is to seek for a beginning of the
infinite'. Yet according to this argument, which forbids us to seek
the cause, there will be no proof of any eternal truth whatever; but
we see that there is a proof of many such, whether by 'eternal' we
mean what always happens or what exists eternally; it is an eternal
truth that the angles of a triangle are always equal to two right
angles, or that the diagonal of a square is incommensurable with the
side, and nevertheless a cause and a proof can be given for these
truths. While, then, it is well said that we must not take on us to
seek a beginning (or first principle) of all things, yet this is not
well said of all things whatever that always are or always happen, but
only of those which really are first principles of the eternal things;
for it is by another method, not by proof, that we acquire knowledge
of the first principle. Now in that which is immovable and
unchanging the first principle is simply the essence of the thing, but
when we come to those things which come into being the principles
are more than one, varying in kind and not all of the same kind; one
of this number is the principle of movement, and therefore in all
the sanguinea the heart is formed first, as was said at the beginning,
and in the other animals that which is analogous to the heart.

From the heart the blood-vessels extend throughout the body as in
the anatomical diagrams which are represented on the wall, for the
parts lie round these because they are formed out of them. The
homogeneous parts are formed by heat and cold, for some are put
together and solidified by the one and some by the other. The
difference between these has already been discussed elsewhere, and
it has been stated what kinds of things are soluble by liquid and
fire, and what are not soluble by liquid and cannot be melted by fire.
The nutriment then oozes through the blood-vessels and the passages in
each of the parts, like water in unbaked pottery, and thus is formed
the flesh or its analogues, being solidified by cold, which is why
it is also dissolved by fire. But all the particles given off which
are too earthy, having but little moisture and heat, cool as the
moisture evaporates along with the heat; so they become hard and
earthy in character, as nails, horns, hoofs, and beaks, and
therefore they are softened by fire but none of them is melted by
it, while some of them, as egg-shells, are soluble in liquids. The

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