We may infer, then, that if in the
primary conformation of the embryo an infinitesimally minute but
absolutely essential organ sustain a change of magnitude one way or
the other, the animal will in one case turn to male and in the other
to female; and also that, if the said organ be obliterated altogether,
the animal will be of neither one sex nor the other. And so by the
occurrence of modification in minute organs it comes to pass that
one animal is terrestrial and another aquatic, in both senses of these
terms. And, again, some animals are amphibious whilst other animals
are not amphibious, owing to the circumstance that in their
conformation while in the embryonic condition there got intermixed
into them some portion of the matter of which their subsequent food is
constituted; for, as was said above, what is in conformity with nature
is to every single animal pleasant and agreeable.
Animals then have been categorized into terrestrial and
aquatic in three ways, according to their assumption of air or of
water, the temperament of their bodies, or the character of their
food; and the mode of life of an animal corresponds to the category in
which it is found.
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