We must now inquire into the causes why some animals live long and others only a short time, and into the general subject of longevity and shortness of life. The initial point in our inquiry must be the stating of certain preliminary problems touching these phenomena. For it is not clear whether or not the cause of longevity and shortness of life is the same in animals and plants. Some plants are annuals, and others attain great age. Further, one may ask whether amongst the creatures of nature the long-lived and the naturally healthy are identical. Or is shortness of life to be kept distinct from questions of disease? Or is it true that some diseases cause the body whose nature is affected, to be short lived, while other diseases in no wise prevent long life?
Regarding sleeping and waking we have already spoken, and we must later on treat the subjects of life and death, and likewise disease and health, so far as they fall within the province of the philosophy of nature. At present we have to consider, as already said, the causes of longevity and shortness of life. This distinction of longevity marks entire genera in comparison with others, and again it marks certain members of one species in comparison with other members. By this I mean there is a generic difference in longevity applicable e.g. to man and horse (for the genus man is longer lived than the genus horse), and again within the species one man is longer lived than another. For some men are long lived and others short lived, according to the districts they inhabit. Nations that inhabit warm countries live longer; the inhabitants of cold countries are less long lived. And amongst those that inhabit the the same locality there are also between individuals differences in this respect.
We must understand what is easily destroyed in natural structures and what is destroyed with difficulty. Fire and water and other elements akin to them, without having their power, are, in their reciprocal action, the causes of generation and decay. Consequently, everything else, one may reasonably suppose, that is derived from or composed of these elements, shares in their nature, excepting such things as are artificially composed of a great many parts, like a house. The discussion of these other elements does not belong here. Many things are subject to destruction from causes peculiar to themselves, as e.g. knowledge, health, and disease. For these are destroyed when the things of which they are qualities are not destroyed but survive, e.g. the agency which destroys ignorance is recollection and learning; the agency which destroys knowledge is forgetfulness and error. Accidentally, the destruction of other properties goes hand in hand with the destruction of the natural body. For when animals are destroyed the knowledge and health that are in them are also destroyed. From this one might draw a conclusion regarding the soul. For if the soul is not in the body by natural growth, but is there just as knowledge is in the soul, then it would be exposed to another destroying agency in addition to that to which it is liable in the destruction of the body. But this does not appear to be the case; the relation between soul and body must be differently understood.
Perhaps one might reasonably ask the question: Is there any place where the perishable is imperishable, as in the case of fire in the Empyrean, which is subject to no opposing influence; for the properties that attach to opposites are incidentally destroyed by the destruction of the thing itself. For opposites destroy one another. No opposite, however, which belongs to substance is incidentally destroyed, for substance is not predicated of any subject. Consequently, in whatsoever thing there is no principle of opposition, and where there is no such principle, there can be no destruction. For what is there to work destruction, if destruction is effected exclusively by opposites? But there is no opposition present, either absolutely or in any particular part. Or is this true in one sense and in another false? For whatever has matter cannot avoid being in some sense subject to opposition. It can be everywhere hot or straight, but it cannot be in its entirety hot or straight or white. For these qualities would then be separate entities. Whenever the active and passive come together, if the one always acts and the other is always acted upon, change must take place. Further, if change necessarily produces a residue, then residue involves opposition. For change is always the result of opposition, and residue is that which remains from a state prior to change. But if the actually opposed were entirely excluded, a thing would, in this case, be imperishable. Or is this untrue, but a thing in this event would be destroyed by its environment? If this happens, then the above explanation is adequate. If the destruction is not so produced, one must suppose there is an actual inherent opposition in the thing, and that a residue is produced. For this reason the lesser flame is incidentally consumed by the larger one, because the food which the smaller one consumes in a long time in the form of smoke, is consumed by the larger one quickly. And so everything is in constant motion, constantly coming into existence and passing out of existence. And the environment either assists or opposes. Constantly changing things may last a longer or a shorter time than their own nature prescribes, but nothing lasts for ever, where opposites exist. For at the very start, matter contains in itself the principles of opposition, so that if one employs the category of place, spatial change is involved; if one employs the category of quantity, we have changes of growth and decay; if one employs passivity, then qualitative change.
Neither are the largest creatures less exposed to destruction than others (for the horse is shorter lived than man), nor the small animals (for many insects live only a year), nor, in general, are plants longer lived than animals (for some plants are annuals), nor are sanguineous animals, by virtue of their being sanguineous, long lived (for bees live longer than do certain sanguineous animals), neither are the bloodless animals, as such, long lived (for molluscs, which are bloodless, live only a year), nor land animals (for there are both plants and land animals that live only a year), nor sea animals (for the crustaceans and molluscs are short lived). On the whole, the longest lived organisms are found amongst plants, an example of which is the palm. Next the sanguineous live longer than the bloodless animals, and the land animals longer than those that live in water. So that the longest lived animals are those where we find the combined marks of having blood and living on the land, as instanced in man and the elephant. It is a rule also that the larger animals are longer lived than the smaller ones. And this characteristic of size applies to other examples of longest lived animals, as well as to the instances cited.
The cause of all this might be discovered in the following facts. One must understand that an animal is by nature moist and warm, and life is also moist and warm, whereas old age is dry and cold, and so is death. And this is plain to observation. The matter in living bodies has these qualities of warm and cold, dry and moist. As beings grow old they must then dry up, and so the moist should be constituted in such way as not to dry up easily. Now, fatty elements do not readily decay. The reason is that they contain air, and air compared with other elements is fire. But fire is not subject to decay. The amount of moisture should not be small, for a small amount is quickly dried out. Consequently, larger animals and plants are, as a rule, longer lived than others, as we said before. For it is reasonable to suppose that the larger creatures possess a greater supply of moisture. But it is not merely for this reason that they are longer lived, for there are two causes of long life, a quantitative and qualitative cause. Consequently, there must not merely be a certain quantity of moisture present, but this must also be warm, in order that it be not easily congealed or dried up. It is for this reason that man is longer lived than certain larger animals. For animals that are defective in the mass of moisture are longer lived, provided their excess in the quality of this moisture is relatively greater than their defect in its quantity. Some animals have an oily warmth, in consequence of which their moisture is not easily dried up or chilled. Others again have a moisture of a different sort. Further, whatever is meant to be difficult to destroy should not throw off much residue. For this, whether it be due to disease or to nature, destroys a thing. Residue has the significance of opposition and is destructive of a thing either in its entire nature or in some part of it. Consequently, salacious animals and such as abound in seed age quickly. For seed is a residue and when it is thrown off produces dryness. For this reason a mule lives longer than a horse or an ass, and women live longer than men, in cases where men are lascivious. And so male sparrows are shorter lived than females. Further, males subjected to hard labour are short lived, and on account of toil age more rapidly. For toil produces dryness, and old age is dry. In the ordinary course of nature, and taking it all in all, men live longer than women. The reason for this is that the male is a warmer animal than the female. The same classes of animals live longer in a warm than in a cold climate, for the same reason that the larger animals live longer than smaller ones. Particularly striking in this connection is the size of the naturally cold animals. So snakes, lizards, and rough-scaled reptiles found in warm localities and the testacea of the Red Sea attain a great size. For warm moisture is the cause of growth and life. In cold districts animal moisture is more watery and consequently easily congealed, so that animals with little or no blood, whether their habitation is the land or water, do not occur at all in the northern regions, or if they occur they are smaller and shorter lived. For frost impedes growth. Both plants and animals perish when they get no nourishment, for then they consume themselves. Just as a larger consumes and destroys a smaller flame by using up its food, so the natural warmth, whose primary function is digestion, consumes the matter in which it is found. Aquatic animals are shorter lived than land animals, not because they are moist unqualifiedly, but because they are watery. Moisture of this sort is very perishable, because it is cold and easily congealed. For the same reason the bloodless animal is very perishable, unless it is protected by great size, because it contains no oily or sweet element. I say sweet, for animal fat is sweet. Consequently bees are longer lived than other animals that are larger.
It is amongst plants that we find the longest lived organisms, and these attain a higher age than animals, in the first place because they are less watery and therefore not easily congealed. Secondly, they contain a viscous oily substance, and therefore, although they are dry and earthy, they nevertheless possess a moisture which is not easily dried out. We must now find an explanation for the great age attained by trees. For a peculiar explanation applies to them which does not apply to animals, excepting insects. This peculiarity is that plants constantly renew themselves and so attain great age. For new shoots are put forth from time to time and others grow old. And the same thing is true of the roots. But this renewal does not take place in all parts at once; sometimes only the trunk and branches die and others grow up alongside of them. And when this happens other roots spring from the remaining part. And so it continues, one part passing out of existence and another part coming into being. Consequently, they live long. Plants have a resemblance to insects, as already said. For life continues when they are divided, and out of one insect or plant two or several are produced. Divided insects, however, reach merely to the state of living, but are not able to continue long in life. For they have no organs, and the principle of life in the single part has no power to develop an organ. This principle in the plant, on the contrary, has the power of developing organs, for it contains in every case both root and stem potentially. Consequently, the new and the ageing branch keep growing from this, differing little in their length of life, just as it is with grafts. In the grafting of shoots, one would say that in a certain sense this same process occurs, for the shoot is part of a plant. In the grafting of shoots, however, the continuity of life occurs in a state of separation from the mother plant, while in the other cases the lives are conjoined. The reason is that the inherent potential principle in the plant is all-pervasive.
There is, however, a point of identity between animals and plants. In animals the males are, as a rule, longer lived than the females. For their upper parts are larger than the lower ones (the male is more dwarflike than the female); the warm element is found in the upper parts and cold in the lower ones. Also plants with large roots are longer lived than the others. Annuals are not of this kind, but trees are. For the upper part and head of a plant is the root, but annuals have their main growth towards the lower parts and the fruit. These questions will be examined in detail in the treatise On Plants. For the present we have explained the cause of longevity and shortness of life in animals. There remain for our consideration the subjects of Youth and Old Age, Life and Death. And after these have been investigated, our treatise on animals will have been finished.