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.
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