Their food depends chiefly on the
substance of which they are severally constituted; for the source of
their growth in all cases will be this substance. And whatsoever is in
conformity with nature is pleasant, and all animals pursue pleasure in
keeping with their nature.
2
Animals are also differentiated locally: that is to say, some
live upon dry land, while others live in the water. And this
differentiation may be interpreted in two different ways. Thus, some
animals are termed terrestrial as inhaling air, and others aquatic
as taking in water; and there are others which do not actually take in
these elements, but nevertheless are constitutionally adapted to the
cooling influence, so far as is needful to them, of one element or the
other, and hence are called terrestrial or aquatic though they neither
breathe air nor take in water. Again, other animals are so called from
their finding their food and fixing their habitat on land or in water:
for many animals, although they inhale air and breed on land, yet
derive their food from the water, and live in water for the greater
part of their lives; and these are the only animals to which as living
in and on two elements the term 'amphibious' is applicable.
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