The following considerations may enable us to understand the reasons
for all these facts. We must remember that an animal is by nature
humid and warm, and to live is to be of such a constitution, while old
age is dry and cold, and so is a corpse. This is plain to observation.
But the material constituting the bodies of all things consists of the
following-the hot and the cold, the dry and the moist. Hence when they
age they must become dry, and therefore the fluid in them requires
to be not easily dried up. Thus we explain why fat things are not
liable to decay. The reason is that they contain air; now air
relatively to the other elements is fire, and fire never becomes
corrupted.

    Again the humid element in animals must not be small in quantity,
for a small quantity is easily dried up. This is why both plants and
animals that are large are, as a general rule, longer-lived than the
rest, as was said before; it is to be expected that the larger
should contain more moisture. But it is not merely this that makes
them longer lived; for the cause is twofold, to wit, the quality as
well as the quantity of the fluid. Hence the moisture must be not only
great in amount but also warm, in order to be neither easily congealed
nor easily dried up.

    It is for this reason also that man lives longer than some animals
which are larger; for animals live longer though there is a deficiency
in the amount of their moisture, if the ratio of its qualitative
superiority exceeds that of its quantitative deficiency.

    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. But in cold countries the humidity in animals is
more of a watery nature, and hence is readily congealed.
Consequently it happens that animals with little or no blood are in
northerly regions either entirely absent (both the land animals with
feet and the water creatures whose home is the sea) or, when they do
occur, they are smaller and have shorter life; for the frost
prevents growth.

    Both plants and animals perish if not fed, for in that case they
consume themselves; just as a large flame consumes and burns up a
small one by using up its nutriment, so the natural warmth which is
the primary cause of digestion consumes the material in which it is
located.

    Water animals have a shorter life than terrestrial creatures, not
strictly because they are humid, but because they are watery, and
watery moisture is easily destroyed, since it is cold and readily
congealed. For the same reason bloodless animals perish readily unless
protected by great size, for there is neither fatness nor sweetness
about them. In animals fat is sweet, and hence bees are longer-lived
than other animals of larger size.