In some creatures the warm element is their fatty substance, which
prevents at once desiccation and congelation; but in others it assumes
a different flavour. Further, that which is designed to be not
easily destroyed should not yield waste products. Anything of such a
nature causes death either by disease or naturally, for the potency of
the waste product works adversely and destroys now the entire
constitution, now a particular member.
This is why salacious animals and those abounding in seed age
quickly; the seed is a residue, and further, by being lost, it
produces dryness. Hence the mule lives longer than either the horse or
the ass from which it sprang, and females live longer than males if
the males are salacious. Accordingly cock-sparrows have a shorter life
than the females. Again males subject to great toil are short-lived
and age more quickly owing to the labour; toil produces dryness and
old age is dry. But by natural constitution and as a general rule
males live longer than females, and the reason is that the male is
an animal with more warmth than the female.
The same kind of animals are longer-lived in warm than in cold
climates for the same reason, on account of which they are of larger
size. The size of animals of cold constitution illustrates this
particularly well, and hence snakes and lizards and scaly reptiles are
of great size in warm localities, as also are testacea in the Red Sea:
the warm humidity there is the cause equally of their augmented size
and of their life.
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