1
THE reasons for some animals being long-lived and others
short-lived, and, in a word, causes of the length and brevity of
life call for investigation.
The necessary beginning to our inquiry is a statement of the
difficulties about these points. For it is not clear whether in
animals and plants universally it is a single or diverse cause that
makes some to be long-lived, others short-lived. Plants too have in
some cases a long life, while in others it lasts but for a year.
Further, in a natural structure are longevity and a sound
constitution coincident, or is shortness of life independent of
unhealthiness? Perhaps in the case of certain maladies a diseased
state of the body and shortness of life are interchangeable, while
in the case of others ill-health is perfectly compatible with long
life.
Of sleep and waking we have already treated; about life and death we
shall speak later on, and likewise about health and disease, in so far
as it belongs to the science of nature to do so. But at present we
have to investigate the causes of some creatures being long-lived, and
others short-lived. We find this distinction affecting not only entire
genera opposed as wholes to one another, but applying also to
contrasted sets of individuals within the same species. As an instance
of the difference applying to the genus I give man and horse (for
mankind has a longer life than the horse), while within the species
there is the difference between man and man; for of men also some
are long-lived, others short-lived, differing from each other in
respect of the different regions in which they dwell. Races inhabiting
warm countries have longer life, those living in a cold climate live a
shorter time. Likewise there are similar differences among individuals
occupying the same locality.
2
In order to find premisses for our argument, we must answer the
question, What is that which, in natural objects, makes them easily
destroyed, or the reverse? Since fire and water, and whatsoever is
akin thereto, do not possess identical powers they are reciprocal
causes of generation and decay. Hence it is natural to infer that
everything else arising from them and composed of them should share in
the same nature, in all cases where things are not, like a house, a
composite unity formed by the synthesis of many things.
In other matters a different account must be given; for in many
things their mode of dissolution is something peculiar to
themselves, e.g. in knowledge and health and disease. These pass
away even though the medium in which they are found is not destroyed
but continues to exist; for example, take the termination of
ignorance, which is recollection or learning, while knowledge passes
away into forgetfulness, or error. But accidentally the disintegration
of a natural object is accompanied by the destruction of the
non-physical reality; for, when the animal dies, the health or
knowledge resident in it passes away too. Hence from these
considerations we may draw a conclusion about the soul too; for, if
the inherence of soul in body is not a matter of nature but like
that of knowledge in the soul, there would be another mode of
dissolution pertaining to it besides that which occurs when the body
is destroyed. But since evidently it does not admit of this dual
dissolution, the soul must stand in a different case in respect of its
union with the body.
3
Perhaps one might reasonably raise the question whether there is any
place where what is corruptible becomes incorruptible, as fire does in
the upper regions where it meets with no opposite. Opposites destroy
each other, and hence accidentally, by their destruction, whatsoever
is attributed to them is destroyed. But no opposite in a real
substance is accidentally destroyed, because real substance is not
predicated of any subject. Hence a thing which has no opposite, or
which is situated where it has no opposite, cannot be destroyed. For
what will that be which can destroy it, if destruction comes only
through contraries, but no contrary to it exists either absolutely
or in the particular place where it is? But perhaps this is in one
sense true, in another sense not true, for it is impossible that
anything containing matter should not have in any sense an opposite.
Heat and straightness can be present in every part of a thing, but
it is impossible that the thing should be nothing but hot or white
or straight; for, if that were so, attributes would have an
independent existence. Hence if, in all cases, whenever the active and
the passive exist together, the one acts and the other is acted on, it
is impossible that no change should occur. Further, this is so if a
waste product is an opposite, and waste must always be produced; for
opposition is always the source of change, and refuse is what
remains of the previous opposite. But, after expelling everything of a
nature actually opposed, would an object in this case also be
imperishable? No, it would be destroyed by the environment.
If then that is so, what we have said sufficiently accounts for
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
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.
6
It is amongst the plants that we find the longest life-more than
among the animals, for, in the first place, they are less watery and
hence less easily frozen. Further they have an oiliness and a
viscosity which makes them retain their moisture in a form not
easily dried up, even though they are dry and earthy.
But we must discover the reason why trees are of an enduring
constitution, for it is peculiar to them and is not found in any
animals except the insects.
Plants continually renew themselves and hence last for a long
time. New shoots continually come and the others grow old, and with
the roots the same thing happens. But both processes do not occur
together. Rather it happens that at one time the trunk and the
branches alone die and new ones grow up beside them, and it is only
when this has taken place that the fresh roots spring from the
surviving part. Thus it continues, one part dying and the other
growing, and hence also it lives a long time.
There is a similarity, as has been already said, between plants
and insects, for they live, though divided, and two or more may be
derived from a single one. Insects, however, though managing to
live, are not able to do so long, for they do not possess organs;
nor can the principle resident in each of the separated parts create
organs. In the case of a plant, however, it can do so; every part of a
plant contains potentially both root and stem. Hence it is from this
source that issues that continued growth when one part is renewed
and the other grows old; it is practically a case of longevity. The
taking of slips furnishes a similar instance, for we might say that,
in a way, when we take a slip the same thing happens; the shoot cut
off is part of the plant. Thus in taking slips this perpetuation of
life occurs though their connexion with the plant is severed, but in
the former case it is the continuity that is operative. The reason
is that the life principle potentially belonging to them is present in
every part.
Identical phenomena are found both in plants and in animals. For
in animals the males are, in general, the longer-lived. They have
their upper parts larger than the lower (the male is more of the dwarf
type of build than the female), and it is in the upper part that
warmth resides, in the lower cold. In plants also those with great
heads are longer-lived, and such are those that are not annual but
of the tree-type, for the roots are the head and upper part of a
plant, and among the annuals growth occurs in the direction of their
lower parts and the fruit.
These matters however will be specially investigated in the work
On Plants. But this is our account of the reasons for the duration
of life and for short life in animals. It remains for us to discuss
youth and age, and life and death. To come to a definite understanding
about these matters would complete our course of study on animals.
-THE END-
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