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Mere size acts in plants and animals both, in rendering them less susceptible to the cold of the wet season, or the winter, and thus protracts life.
The production of woody fibre in plants is a condition of longevity, and anything directly favouring this may extend life. It enables the plant to resist breakage by wind or other violence, and protects it from cold. Thus bulbs continue the individual life of an annual flower for many years, and thus the trunks and branches of trees and shrubs live, whilst the leaves and flowers die. Obviously the influence on age of the development of wood is but a part of the law of relation of evolution and longevity; but it is a special correlation, of very wide application.
12. Some Experimental Evidence. There are some experimental proofs of the influence of generative and personal expenditure on longevity which may be now cited. By preventing plants from reproducing, that is, by cutting off their flower-buds, the gardener increases the bulk and the longevity of some plants; leaves and wood being produced in place of generative products. By change from a warm to a colder climate, this may similarly be effected. The American aloe reproduces and dies in about five years in Mexico; in England it elaborates leaves for a hundred years before flowering. Again, the axolotl reproduces in warm Mexico as a branchiferous amphibian; in colder climates its fertility is diminished, it becomes a salamandroid before reproducing, thus lengthening life by delaying genesis. It is rarely that we can point to such cases as these, where the diminution of warmth affects sexual dėvelopment. Usually it will kill the animal or plant experimented upon-as in the case of the mignonette (a shrub in Barbary), and the palma Christi (a tree in India), which both die annually in our severe climate ; the longevity of the individual being in these cases diminished rather than the fertility delayed.
1 Some persons would wish to regard tree-stems, bulbs, &c. as a kind of asexual reproductive mass, and would look upon the flowers of each year as successive generations of individuals. The compound nature of perennial plants is one of the difficulties met with in attempting to define individuality.
The two cases are interesting to compare with man, who is believed to live longest in cold countries. Like the American aloe, as is seen, when it is taken to still colder climates than our own, or like the mignonette in England, man ceases to gain in longevity when a certain limit of cold is attained. Beyond the cold of temperate regions his longevity is probably injuriously affected, as is that of the palma and the mignonette in England, and that of the aloe in regions farther north. The general action of cold lies no doubt in the production of a sluggishness of the chemico-vital changes, which, if carried
1 It is stated (Aitken's • Medicine'), but not on statistical evidence, that the longevity of the Icelanders is greatly reduced by catarrh.
far, may destroy, but if moderated must extend, length of life (at the expense of intensity). The coldness of water, together with its diminished power of oxygenation, as compared with the atmosphere, is one of the direct causes of diminished expenditure in aquatic animals, rendering their life necessarily less intense than that of terrestrial forms, and so longer.
In keeping animals in menageries, in rearing pets and domesticated animals, man performs an experiment by diminishing personal expenditure. He frequently does the same in his own case, leading a careless, labourless existence; but there is in this as in other experiments (which are rarely so good in physiological enquiry in their results as natural comparisons) a disturbing cause, for Luxury, 'the fertile parent of a whole family of diseases,' as Galen termed her, steps in and works against the diminished expenditure. When man in his own person, or in the organisms he interferes with, so far baulks Nature's provisions that the organs become, as it were, rusty through the suspension of that personal expenditure, which is usually necessary to keep up the warmth by oxygenation, and to obtain necessary food, then he shortens rather than increases the length of life, disease attacks his victim, and death follows. This is seen exemplified in the case of domesticated animals,
1 Mr. Darwin informed the writer that he did not know of any reliable data admitting of a comparison between domestic animals and their nearest wild representatives (their actual wild forms being unknown in every case).
which are fattened for eating, and are believed to be short-lived in consequence. It is clearly the case in pets, such as small dogs, whose life is shortened by luxury. Hounds are the longest lived amongst dogs. On the other hand, there are cases in which man, by his care in avoiding expenditure, has lengthened his own and other animals' tenure of life ; and it appears, from the little that is known, that experimental evidence does support the proposition, that longevity is lengthened by diminution of personal expenditure.
13. Summary. Hence, in spite of the great complication of the case, we may conclude, on both deductive and inductive grounds, that the high or low potential longevity of different species, as a general law, is necessitated by those conditions of life which necessitate high or low individual development, as the case may be, whether of mere bulk, or complexity, or both; that it is directly subject to those conditions which cause personal expenditure to fluctuate, or which affect generative expenditure, being high when these are low, and low when these are high; that these relations interacting and contending variously according to the special case, determine the potential longevity of the various species of lower animals.
From the intricacy of these relations we may conclude that potential longevity is a very delicately balanced quantity, and that very slight causes may produce great fluctuations in it, and be almost impossible to trace; the magnitude of the result being far larger than in proportion to the magnitude of the initial cause, as is so often found to happen in Biological Science.
THE LONGEVITY OF MAN.
14. Preliminary. In this part of the essay, our object is to apply the conclusions we have obtained from the study of organisms generally to the case of man, and especially to observe how far his obedience to the general law is affected by, or dependent on, the different phases of civilization which he exhibits.
Man presents the most marked contrasts with animals generally in many of the chief conditions of existence affecting longevity. Civilized man lives in societies, one of the most essential bonds of union in which is the maintenance, to a greater or less extent, by the community of the feeble. The security which the healthy and vigorous man hopes for himself, when grown old and feeble, he naturally extends to others, and thus the aged are fed and protected as the result of a specific habit or characteristic among men (the most barbarous excepted).
Further, the intellect of man renders him utterly unlike animals in much that relates to age; for