the change; but, if not, we must assume that something of actually
opposite character is in the changing object, and refuse is produced.

  Hence accidentally a lesser flame is consumed by a greater one,
for the nutriment, to wit the smoke, which the former takes a long
period to expend, is used up by the big flame quickly.

  Hence [too] all things are at all times in a state of transition and
are coming into being and passing away. The environment acts on them
either favourably or antagonistically, and, owing to this, things that
change their situation become more or less enduring than their
nature warrants, but never are they eternal when they contain contrary
qualities; for their matter is an immediate source of contrariety,
so that if it involves locality they show change of situation, if
quantity, increase and diminution, while if it involves qualitative
affection we find alteration of character.

                                 4

  We find that a superior immunity from decay attaches neither to
the largest animals (the horse has shorter life than man) nor to those
that are small (for most insects live but for a year). Nor are
plants as a whole less liable to perish than animals (many plants
are annuals), nor have sanguineous animals the pre-eminence (for the
bee is longer-lived than certain sanguineous animals). Neither is it
the bloodless animals that live longest (for molluscs live only a
year, though bloodless), nor terrestrial organisms (there are both
plants and terrestrial animals of which a single year is the
period), nor the occupants of the sea (for there we find the
crustaceans and the molluscs, which are short-lived).

  Speaking generally, the longest-lived things occur among the plants,
e.g. the date-palm. Next in order we find them among the sanguineous
animals rather than among the bloodless, and among those with feet
rather than among the denizens of the water. Hence, taking these two
characters together, the longest-lived animals fall among
sanguineous animals which have feet, e.g. man and elephant. As a
matter of fact also it is a general rule that the larger live longer
than the smaller, for the other long-lived animals too happen to be of
a large size, as are also those I have mentioned.

                                 5

  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