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worth while speculating as to what may best serve this purpose, since in no animal or plant are we in a position to make use of any decision on the matter, and in the case of man it will be seen that it is not much wanted. The day may, however, come when sufficient observations will have been made on lower organisms to render such a fixed point of comparison useful.
Potential longevity differs then in different species (as a glance at the statements as to longevity below will fully prove), and agrees within certain limits in individuals of the same species. Why is this? It is no doubt because the particular structure and habits of each species in some way require or entail the particular limit or lease of life. But how is this effected ? Does the life of a given species receive its limit simply through the operation of the particular or specific external agencies (to which the species is born and specially constructed to meet) on each individual born ? Undoubtedly this is so to a large extent. For man may take an animal lower than himself in the scale of life, or a plant, and by his care and attention, by removing the agencies to which the creature is born, and carefully substituting others, may cause it to live much longer than it could possibly do if left to its natural conditions. Thus man may take a bird, and by providing it with food, and protecting it from competition with its fellows, from accidents and enemies, from the want caused by weakness in old age, protract its life. Parrots, thus, live even one hundred years, and goldfinches twenty-three, which there is good reason to believe is far beyond their length of life when in a state of nature.' So lions have lived in menageries to be forty to sixty years old (Haller), being fed after the loss of their teeth and the blunting of their claws. Insects have been so kept for three or four years; and many plants by attention are made perennial or biennial, whereas in natural conditions they would be annual. It will probably be admitted that man has this power in many cases without further illustration.
Hence we must again qualify or analyse potential longevity as applied to species; for there is one period which is proper to the species in its normal conditions, which it cannot by any struggles of its own extend, hedged in as it is by those very conditions in relation to which it has either been created, or by which it has been evolved. There is a second period which is equally proper to a species (as far as experiments tell us), which man can make evident by removing some of the natural conditions and substituting others, which however has its limit, beyond which limit. no power that is known can extend the life. The first period may be called Normal Potential Longevity, the second Absolute? Potential Longevity.
1 Absolute is used for want of a better term; it is only absolute' within man's experience.
Man himself, in his civilized form, is continually bringing his intelligence to bear on his own longevity, thus changing conditions as no other organism can, and consequently in his case normal and absolute potential longevity are merged. It is the development of unprecedented and overpowering intelligence which interferes in the case of man, and separates him in this as in other matters so greatly from other organisms. His intelligence enables him to take many precautions with advancing years; it leads him to form communities and organizations in which the active and young protect and minister to the aged. This great peculiarity in man, and the more than specific differences of condition which his all-adapting brain renders possible in various groups of individuals with less than specific difference of structure, makes it desirable to consider him apart from the rest of the organized world in such a matter as longevity.
When man exerts the greatest care to protract the life of certain organisms, he yet finds that death will come and limit the period. There is a limit to absolute potential longevity clearly enough in many organisms, and this limit, which may be termed an inherent one, must of course act in limiting normal potential longevity. What is it that constitutes this limit, are all organisms subject to it, and how does it become inherent ? It appears that in some organisms we cannot clearly say from observation that there is such an inherent limit; in fact, their absolute potential longevity appears to be very nearly practically unlimited ; but we may suppose that it has a remote limit which is difficult to observe on account of its distant character. Such organisms are fish, molluscs, large crustacea, annelids, many trees and sea-weeds. In other organisms, on the contrary, there is distinctly observable a natural inherent limit to life, which is inevitable, however carefully injurious and destructive influences are kept off, which makes its approach felt with the advance of years, in that state which is called 'natural decay' or 'senility.' Men, other mammals and birds, some reptiles, insects, some lower invertebrata, and many plants, exhibit this condition of things very obviously. In some, as insects, and some low worms and protozoa, the action of this natural decay' is far more powerful than it is in the other cases, and we see these creatures dying clearly under its influence; in others it is less obvious, and hence we may suppose that in the former group, where natural decay appears to play no part, its apparent absence is merely a matter of degree, and that it is simply reduced to a minimum.
That the time of the on-coming of this period of natural decay, i. e. the limit of absolute potential longevity, varies strictly and largely in different species, and proportionately to the normal potential longevity, is difficult of absolute proof in the absence of experiment; but it will probably be admitted from common experience as to ageing, and some facts bearing on it which it is needless to particularize are given in the ‘Statements as to Longevity,' below.
What we are then endeavouring to examine in various species of lower animals and in man, viz. normal potential longevity, varies in accordance with two sets of influences, the external agencies or specific 'milieu,' acting directly on individuals, and an inherent limiting agency. To the first, all organisms are severally subject; the second seems to possess a very small power in some. Both these are hereditary influences, as is implied in their truly specific character; the inherent is so by hypothesis, the external agencies are less obviously so, being indirectly inherited by the transmission of structural capacities and necessities involving the same details of life in the offspring as in the parent.
Not less hereditary is that average longevity which was spoken of as constituting the study known as 'mortality. It, equally with potential longevity, is a specific character as truly as a tuft of feathers or an additional antennary joint, and is determined by the reciprocal relations of the environment' and the 'organism,' and with a constant organism it cannot vary, whilst, if the 'environment is not constant, the organism must become a new species on the evolution hypothesis, or cease to exist on the specialcreation hypothesis being no longer fitted to its