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1. Introductory.—2. Writers on Longevity and General Sources of Information.-A. Longevity in Organisms generally. 3. Longevity defined.-4. The various kinds of Longevity. 5. Inherent Death.–6. Elements of the Life Period.-7. High Individuation favours Longevity.-8. Small Expenditure favours Longevity.9. Why Longevity is thus influenced.-10. Inductive.-11. Other Relations of Longevity.–12. Some Experimental Evidence.-13. Summary.-B. Longevity of Man. 14. Preliminary.-15. Sources of Information as to Human Longevity.-16. Statements as to the Duration of Human Life.-17. General Conclusions from the foregoing Statements and Tables.—18. Interpretation of the Law. -19. Duration of Life in Past Time.—20. The Influence of various States of Civilization.-21. Abnormal Longevity in Man.




I. Introductory.

THE periodic phenomena observable in organisms have always a special interest for students of Nature on account of the extreme obscurity of their relations, as well as from the practical importance which they possess for mankind. There is probably but small room for doubt that ultimately the various recurring periods of death, of reproduction, of sleep, of hybernation, of gestation, of puberty, are all related to or derive their origin from those great astronomical cycles of change in the relative positions of sun, moon, and earth—which we know as year, month, and day. Whilst in some of the periodic phenomena of organisms this relation is clear and direct, in others it is obscured by the introduction


of most complex factors, which do not permit us to trace in the majority of organisms the operation of the astronomical cause. The duration of lifel is vastly influenced by varying conditions in various organisms, but the prime factor in all cases is the influence of changing day and night, of alternate winter and summer, or wet and dry season. How this influence is modified by the creation of correspondences between organisms and their surroundings, or how, in the words of the philosophy of evolution, new factors have arisen in the progressive development of organisms by the law of the survival of the fittest, is to be the subject of enquiry in this essay.

Although it is vain (with present knowledge) to expect to gain a complete insight into those agreements between beings and their environment, of which the duration of life is one, yet certain facts and considerations have been pointed out which go far towards enabling us so to do. From the time of Aristotle onwards, observers and philosophers have accumulated facts and multiplied speculation on the causes of longevity: the field is a well-trodden one, and for many years to come any increased knowledge of it must be looked for rather from the examination of long-acquired facts and their re-arrangement, than from new or unexpected observations by individual workers.

1 The great importance which man attaches himself to long life, gives the enquiry into longevity in animals a greater importance than it deserves as a physiological or philosophical question. The varying intensity of life in different species, and the average mortality of a species, are more clearly influential quantities in nature than the possible length of life. Time does not appear in the organic world as an easily recognisable factor, for though in life as in levers what is lost in power is gained in time, it is difficult to distribute the amount of life of any given species rightly between intensity and length.

2 In other words, astronomical cycles have furnished the unit for organic cycles. The cyclical character has been as it were impressed upon organic matter, by the great cycles of the universe. No further implication is intended in the text.

2. Writers on Longevity, and general Sources of

Information. In consequence of the general nature of the enquiry proposed in this essay, we have little in common with those who in former ages have enlarged upon the possible means of prolonging human life; nor are we concerned specially with those questions as to the possible and extreme periods of man's tenure of existence, which to-day occupy the attention of many literary men of an antiquarian or curious turn of mind, In the writings of these men we cannot expect to find more than one limited class of facts bearing on comparative longevity; and in too many cases the facts so called are not supported by scientific evidence. In a recent article in

1 It will be seen below how vast an amount of enquiry has yet to be made, as to length of life in animals and men. No single individual can do much in this matter in less than a lifetime.

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