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On The Generation Of Animals   

sinews and bones are formed by the internal heat as the moisture
dries, and hence the bones are insoluble by fire like pottery, for
like it they have been as it were baked in an oven by the heat in
the process of development. But it is not anything whatever that is
made into flesh or bone by the heat, but only something naturally
fitted for the purpose; nor is it made in any place or time
whatever, but only in a place and time naturally so fitted. For
neither will that which exists potentially be made except by that
moving agent which possesses the actuality, nor will that which
possesses the actuality make anything whatever; the carpenter would
not make a box except out of wood, nor will a box be made out of the
wood without the carpenter. The heat exists in the seminal
secretion, and the movement and activity in it is sufficient in kind
and in quantity to correspond to each of the parts. In so far as there
is any deficiency or excess, the resulting product is in worse
condition or physically defective, in like manner as in the case of
external substances which are thickened by boiling that they may be
more palatable or for any other purpose. But in the latter case it
is we who apply the heat in due measure for the motion required; in
the former it is the nature of the male parent that gives it, or
with animals spontaneously generated it is the movement and heat
imparted by the right season of the year that it is the cause.

Cooling, again, is mere deprivation of heat. Nature makes use of
both; they have of necessity the power of bringing about different
results, but in the development of the embryo we find that the one
cools and the other heats for some definite purpose, and so each of
the parts is formed; thus it is in one sense by necessity, in
another for a final cause, that they make the flesh soft, the sinews
solid and elastic, the bones solid and brittle. The skin, again, is
formed by the drying of the flesh, like the scum upon boiled
substances; it is so formed not only because it is on the outside, but
also because what is glutinous, being unable to evaporate, remains
on the surface. While in other animals the glutinous is dry, for which
reason the covering of the invertebrates is testaceous or crustaceous,
in the vertebrates it is rather of the nature of fat. In all of
these which are not of too earthy a nature the fat is collected
under the covering of the skin, a fact which points to the skin
being formed out of such a glutinous substance, for fat is somewhat
glutinous. As we said, all these things must be understood to be
formed in one sense of necessity, but in another sense not of
necessity but for a final cause.

The upper half of the body, then, is first marked out in the order
of development; as time goes on the lower also reaches its full size
in the sanguinea. All the parts are first marked out in their outlines
and acquire later on their colour and softness or hardness, exactly as
if Nature were a painter producing a work of art, for painters, too,
first sketch in the animal with lines and only after that put in the

Because the source of the sensations is in the heart, therefore this
is the part first formed in the whole animal, and because of the
heat of this organ the cold forms the brain, where the blood-vessels
terminate above, corresponding to the heat of the heart. Hence the
parts about the head begin to form next in order after the heart,
and surpass the other parts in size, for the brain is from the first
large and fluid.

There is a difficulty about what happens with the eyes of animals.
Though from the beginning they appear very large in all creatures,
whether they walk or swim or fly, yet they are the last of the parts
to be formed completely, for in the intervening time they collapse.
The reason is this. The sense-organ of the eyes is set upon certain

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