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conditions. The close relation of the average longevity to the welfare of the species is seen in cases where man has interfered with this quantity, as in game-preserving. Gamekeepers killed 'vermin,' i.e. hawks, weasels, foxes, &c., which were in the habit of diminishing the average longevity of the grouse, by destroying weakly birds. The vermin being destroyed, the average longevity was unduly raised, and as a consequence we had the grouse disease, which threatened the extinction of the species. Other such cases might be actually pointed to, or conjectured.1

We have then these three quantities of life—the normal, the absolute, and the average longevity, each one of which, in its unequal distribution, we are entitled to assume, is fitted to the requirements of the specific organism, either by special design, or by the gradual evolution of relations. By enquiring what the correspondences are, we may endeavour to frame some general propositions as to the causes affecting longevity, and thus be the better able to examine the question of man's longevity. It is seldom, on account of the small knowledge available, that the term absolute potential longevity' will have again to be used. In speaking of potential longevity, or longevity, henceforth, unless otherwise said, normal

1 Such, for example, are the diseases of domesticated animals, and of civilized man himself. The incapacity of some plants and animals to become established in a new country may be attributed, in many cases to the absence of some cause—nature's sanatory police—which would check undue average longevity, and thus maintain a healthy stock.

potential longevity, or what appears to belong to that term, as above explained, will be meant.

5. Inherent Death.

Let us

now parenthetically enquire as to this inherent cause of death—this something in the organism which, more clearly than the other structures and properties of the organism, limits life. We say,

more clearly,' for it is impossible to regard what was ascribed to the 'milieu,' 'environment,' or 'external agencies,' without remembering that they have their correlatives in the organism itself.

How is it that absolute potential longevity is made to have a limit by heredity? How is it that natural decay is hereditary as to time and effect? The whole subject of the hereditary transmission of specific characters has been recently treated of by Mr. Darwin in his volumes on 'Animals and Plants under Domestication, and the ingenious theory of Pangenesis started to explain and collect all these phenomena under one head. Though Mr. Darwin does not allude especially to senility, he mentions at length periodic developments agreeing as to their time of appearance in both parent and offspring. The theory of Pangenesis is thus stated : 'I assume that cells before their conversion into completely passive or “formed material,” throw off minute granules or atoms, which circulate freely thiroughout the system,

and when supplied with proper nutriment multiply by self-division, subsequently becoming developed into cells like those from which they were derived. They are supposed to be transmitted from the parents to the offspring, and are generally developed in the generation which immediately succeeds, but are often transmitted in a dormant state during many generations, and are then developed. Their development . is supposed to depend on their union with other partially developed cells or gemmules which precede them in the regular course of growth.' 'Gemmules are supposed to be thrown off by every cell or unit, not only during the adult state, but during all the stages of development. (Darwin, loc. cit. vol. ii. p. 374.)

We may use this theory to explain the hereditary character of senility. The gemmules, 'when supplied with proper nutriment, multiply. As long as there is nutriment for them they will continue to be produced, but when the superabundance of nutriment ceases, which, as we shall see, is soon after growth is quite completed, their production ceases; they are thus limited in number, and, being called upon in repair and reproduction, are gradually exhausted. But it is not necessary to have recourse to the pangenetic gemmules, which are only considered by Mr. Darwin as provisional hypotheses. The

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1 Mr. Darwin does not appear to connect the gemmules with ordinary repair (i. e. of waste, not injury), which it would be more satis

physiological units of Mr. Herbert Spencer, which he describes as follows, will suffice as an assumption ; or, indeed, we need go no further in explicitness than is involved in the assumption of 'a matter of life.' What we have to explain is why Mr. Spencer's units, or the matter of life,' should be limited in quantity in various organisms, so that life terminates at different periods, even when two species compared appear to have been subjected to the same external agencies. The old writers distinguished the 'vires in posse' and the 'vires in actu. The aged, they said,

factory to do. The material character seems also some objection to these gemmules. For take the case of a polyp reproducing asexually for three thousand years. According to Mr. Darwin's supposition, involved in his explanation of Atavism and Reversion, the last generation of polyps contain gemmules from every preceding generation. The persistence of the same material gemmule, and the vast increase in the number of gemmules, and consequently of material bulk, as later generations come on, make a material theory difficult. Modified forcecentres, becoming further modified in each generation, such as Mr. Spencer's physiological units, might be made to fit in with Mr. Darwin's hypothesis in other respects.

1 Mr. Spencer, after describing the organic 'polarity' seen in the phenomena of repair and development, says, “If then this organic polarity can be possessed neither by the chemical units nor the morphological units, we must conceive it as possessed by certain intermediate units, which we may term pbysiological. There seems no alternative but to suppose that the chemical units combine into units immensely more complex than themselves, complex as they are; and that in each organism, the physiological units produced by this further compounding of highly compound atoms, have a more or less distinctive character. We must conclude that in each case, some slight difference in their mutual play of forces produces a difference in the form which the aggregate of them assumes.'

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had not, as the young, this under-stratum of 'vires in posse' to call upon in cases of exhaustion. “We must never forget to insist,' says M. Reveillé Parise, upon this fundamental principle, that the unknown force of life, vis abdita quædam, diminishes more and more with the progress of age.' 'Ex viribus vivimus,' said Galen. A young man is commonly said to overtax his strength and to injure his constitution by great expenditure of force when young. The common idea expressed in these various statements of opinion is that a store of life-force or lifematerial exists, which the young accumulate, which increases up to a certain amount, but which ceases to do so at some period, and thenceforward dwindles. Professor Huxley has well expressed this in terms of life-material, in a lecture delivered at Edinburgh, in January, 1869. “At any rate,' says Professor Huxley, the matter of life is a veritable peau de chagrin, and for every vital act it is somewhat the smaller. All work implies waste, and the work of life results, directly or indirectly, in the waste of protoplasm.' Is there any direct evidence of the existence of such a store of force or material as is evidently usually supposed to exist in organisms? If we look at the question from the point of view of force, it makes little difference, for force implies matter in a particular condition. It could not be maintained that one

1 Quoted by M. Flourens, loc. cit.

Method. Medend.' Lib. xi.

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