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On The Generation Of Animals   

own form; in this latter division the sanguinea lay eggs, the
bloodless animals either lay an egg or give birth to a scolex. The
difference between egg and scolex is this: an egg is that from a
part of which the young comes into being, the rest being nutriment for
it; but the whole of a scolex is developed into the whole of the young
animal. Of the vivipara, which bring into the world an animal like
themselves, some are internally viviparous (as men, horses, cattle,
and of marine animals dolphins and the other cetacea); others first
lay eggs within themselves, and only after this are externally
viviparous (as the cartilaginous fishes). Among the ovipara some
produce the egg in a perfect condition (as birds and all oviparous
quadrupeds and footless animals, e.g. lizards and tortoises and most
snakes; for the eggs of all these do not increase when once laid).
The eggs of others are imperfect; such are those of fishes,
crustaceans, and cephalopods, for their eggs increase after being

All the vivipara are sanguineous, and the sanguinea are either
viviparous or oviparous, except those which are altogether
infertile. Among bloodless animals the insects produce a scolex, alike
those that are generated by copulation and those that copulate
themselves though not so generated. For there are some insects of this
sort, which though they come into being by spontaneous generation
are yet male and female; from their union something is produced,
only it is imperfect; the reason of this has been previously stated.

These classes admit of much cross-division. Not all bipeds are
viviparous (for birds are oviparous), nor are they all oviparous
(for man is viviparous), nor are all quadrupeds oviparous (for
horses, cattle, and countless others are viviparous), nor are they
all viviparous (for lizards, crocodiles, and many others lay eggs).

Nor does the presence or absence of feet make the difference
between them, for not only are some footless animals viviparous, as
vipers and the cartilaginous fishes, while others are oviparous, as
the other fishes and serpents, but also among those which have feet
many are oviparous and many viviparous, as the quadrupeds above
mentioned. And some which have feet, as man, and some which have
not, as the whale and dolphin, are internally viviparous. By this
character then it is not possible to divide them, nor is any of the
locomotive organs the cause of this difference, but it is those
animals which are more perfect in their nature and participate in a
purer element which are viviparous, for nothing is internally
viviparous unless it receive and breathe out air. But the more perfect
are those which are hotter in their nature and have more moisture
and are not earthy in their composition. And the measure of natural
heat is the lung when it has blood in it, for generally those
animals which have a lung are hotter than those which have not, and in
the former class again those whose lung is not spongy nor solid nor
containing only a little blood, but soft and full of blood. And as the
animal is perfect but the egg and the scolex are imperfect, so the
perfect is naturally produced from the more perfect. If animals are
hotter as shown by their possessing a lung but drier in their
nature, or are colder but have more moisture, then they either lay a
perfect egg or are viviparous after laying an egg within themselves.
For birds and scaly reptiles because of their heat produce a perfect
egg, but because of their dryness it is only an egg; the cartilaginous
fishes have less heat than these but more moisture, so that they are
intermediate, for they are both oviparous and viviparous within
themselves, the former because they are cold, the latter because of
their moisture; for moisture is vivifying, whereas dryness is furthest
removed from what has life. Since they have neither feathers nor
scales such as either reptiles or other fishes have, all which are
signs rather of a dry and earthy nature, the egg they produce is soft;

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