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looked upon as abnormal, and not credited with a false significance. The life of a dried Rotifer is rather potential or latent life than efficient life, and it is with efficient life that our definition of longevity deals. 1 Doubtless efficient life, by approximating more closely, from time to time, or continuously, to this merely potential life, has its term extended in various cases ; ? but the normal and natural course of events must not be confounded with what is abnormal and accidental. With these qualifications, longevity may be defined as the length of duration of life of an individual. With this meaning of the term we may now endeavour to see how the longevity of various organisms may be compared,

4. The various kinds of Longevity.

Great as is our ignorance with regard to longevity in all that relates to accuracy and detail, yet there are a few patent facts within everyone's experience which it is well to consider at once. Firstly, various individuals enjoy various durations of life. That men, cats,

1 Professor Owen remarks in a recent note (“Monthly Microsc. Journal,' vol. i. p. 294)—“There are organisms (Vibrio, Rotifer, Macrobiotus, &c.) which we can devitalize and revitalize-devive and revive-many times. As the dried animalcule manifests no phenomenon suggesting any idea contributing to form the complex one of “life" in my mind, I regard it to be as completely lifeless as is the drowned man whose breath and heat have gone, and whose blood has ceased to circulate.'

Ex. gr. Parasitic worms which become encysted: hybernation approaches this condition in great measure.

mice, bees, and buttercups live for different periods of time, is matter of experience, and not only this, but all men do not live equally long, nor all cats, nor all bees and flowers. Hence every individual has its own longevity, if we understand that term to mean duration of life. On looking a little further, we readily discover that there is a closer agreement as to duration of life (though we cannot deal with accurate numbers) between the individuals of the same species than there is between the individuals of different species; and though the individuals of the same species exhibit great variation in their length of life, yet there is a probable duration which characterizes the species, and is the same therefore for all the individuals. We thus, then, have individual and specific longevity. But when we try to form some more definite notion of this specific longevity,' great difficulties have to be encountered.

By 'specific longevity' we may mean the average longevity of the individuals of a species, that is, the average duration of life of all the individuals born ; and had we data for various organisms as we have for some groups of mankind, we should speak of this period as the expectation of life at birth, and could assign to it a fixed quantity, as is done for men. On the other hand, a very different term is that which we usually speak of as the longevity' of this or that race, family, or species. Howsoever ignorant we are of numbers in this matter, though it is even difficult to define what are the limits of the period to which we refer, yet, in speaking of 'longevity' of groups

of beings, we usually mean the potential longevity—or ‘lease of life,' as Mr. Grindon terms it and do not allow the average longevity, affected as it is by disease and accident at all periods of life, to enter into our consideration.

The term 'mortality' is usually applied to the question of average longevity, and hence, in accordance with general convention, longevity may be understood to refer to potential longevity. Once for all, here it may be pointed out how slightly these quantities can affect each other, though they are to a certain extent related. Mortality has been largely studied in the case of man, and much more is known of it than of longevity in his case ; but among animals and plants generally, vastly important as mortality is in regard to the necessities of life and of organisms, there is as little known as in the matter of longevity. That the average longevity of a group of individuals is but slightly related to the potential longevity, appears from these considerations. From enemies preying upon the 'young ones,' or from disease, or from a severe struggle for food, or from the accidents of dispersion, vast numbers of the individuals of the group may die at a very early age; those, on the other hand, which do survive, may live to a period of time quite unaffected by the conditions which acted

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on them in early stages of existence. Thus from great destruction of young the average longevity may be brought very low, and not indicate directly at all the potential longevity. It is clear that a very high potential longevity will materially raise the average longevity, whilst a low one will somewhat diminish it; on the other hand, the chances of life may be the better in each individual of the survivors from the fact that the average longevity has been lessened by the destruction of numbers of the weaker and unhealthy among the young. It is clear that the subject of mortality is so distinct from that of longevity that it cannot enter largely into consideration on the present occasion.

Whilst we have fixed terms to give us the means of comparing average longevities, what have we that corresponds in the case of potential longevity?

his matter has not been fixed by any authority, even in the case of man, who is indeed the only animal of which there are sufficient facts known to enable one to use in any way such a definite indication of potential longevity for comparison.

Statisticians frame tables for various groups and classes of men, in which the probable 1 after-lifetime or expectation of life is calculated for any given age. The expectation of life at birth obviously indicates the

1 Mr. Neison has proposed rather to compare the lifetime, of which there is an equal chance, at different ages, but this proposal has not been made use of.

average longevity of the group, but at what period of life does the expectation fairly indicate the potential longevity? It might be answered at once that, as a matter of course, the highest age attained by any individual of the group, that is, the greatest individual longevity, is the measure of the potential longevity of the group; but we must remember, in dealing with a large number of cases, not to mistake abnormal or exceptional cases for normal ones, and not to base conclusions for a group on such cases. In the case of man, as noted again below, this may be of less importance, but with the various organisms of the animal and vegetal kingdoms we cannot justly say that the longevity proper to a species is indicated by the greatest longevity attained by an individual of the species. In searching for some terms to be used as indicating the potential longevity where statistics are available (and where they are not, guesses and estimations based on the few existing data must take their place), the probable after-lifetime of an individual, when it has attained the average longevity of the species, might be taken arbitrarily as fixing the potential longevity of the species. But it seems better, though less precise, to use the probable after-lifetime of an individual at that age when it has passed some crisis, such as the maturity of the reproductive organs, or other similar crises, as the case may be, for the purpose of giving fixed terms of comparison as to potential longevity. It is perhaps scarcely

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