Of still lower forms of life there is as little knowledge in most cases as in the higher forms. The Vegetable division of organisms among its higher and terrestrial members furnishes ample data; the ages of trees, shrubs, and such like forms appear to be well ascertained, but those whose condition and structure is diversified by aquatic habitat, leave us as much in ignorance as do similarly inaccessible animals.

Here, then, before entering on this enquiry, whilst but looking out on the road, we see how few are the guide-posts-how small the assurance we can have of taking the right turning. When that immense engine of scientific observation, which is wielded by statisticians, has been fairly and fully applied to the human species and to such of the many and varied forms of animal life as may be possible, not only so as to determine length, but other quantitative phenomena 1 of life also, then we may hope to see the problem, about to be discussed, definitely and clearly investigated by inductive methods.

1 Such are the time of gestation, incubation, metamorphosis, of hybernation, of sleep, of growth, the amount of deaths and births at various ages, of food consumed, of force exerted-phenomena, none of which can be measured or determined by isolated cases, but require, like longevity. the examination of vast numbers to give true results, varying as they may do in the individuals of a species.

A. LONGEVITY IN ORGANISMS GENERALLY.

3. Longevity defined.

It is very necessary to have a clear perception of the meaning of the principal terms involved in the consideration of the duration of life. By 'longevity' must be understood the length of time during which life is exhibited in an individual.1 Unless we introduce the term 'individual,' and assign to it a definite meaning, we become involved in numerous difficulties when making a comparison of the length of life in different species of organic beings. However important in a zoological sense the definition of 'individual' may be which regards the various forms and existences appertaining to a species between ovum and ovum, as the individual of that species, for physiological purposes, such a definition cannot be accepted. The whole product of a fertilized germ, whilst it no doubt, in many cases, agrees with all requirements as a definition of the individual of a species, is yet, in many other cases, open to much

1 Though longevity is thus limited on the present occasion, in accordance with the ordinary usage of the term, it is only right to remind the reader that there is a vastly important and most interesting aspect of longevity which has been but little written on or considered as yet, and which it is to be regretted we cannot now enter upon. The longevity of races and species (as such) is the subject to which allusion is made: in connection with the Darwinian theory and struggle for existence, the longevity of species will prove a most fertile field of research.

objection. In the Vertebrata, in Mollusca, in most Insects, such a definition is unobjectionable; but when we consider the numerous examples of asexual reproduction we are led into difficulty. 'It seems a questionable use of language to say that the countless masses of Anacharis alsinastrum which, within these few years, have grown up in our rivers, canals, and ponds are all parts of one individual.'1 And yet, as this plant does not seed in England, these countless masses, having arisen by asexual multiplication, must be so regarded, if the above definition be accepted. In the Hydrozoa are we to ascribe the same amount of individuality to those forms which give rise to separate polypidoms by the separation of budded offspring, as to those to which these remain attached, to form a compound polypidom? The same difficulty occurs with the lower Annelids and Vermes, with Aphis, and such asexually proliferous insects, and with multiaxial plants whose buds are capable of separation and the initiation of distinct existences. An illustration of the perplexity into which the above definition of the individual would lead us in regard to the question of longevity is furnished by such compound organisms as are known among the Actinozoa, 'Ehrenberg judges that certain enormous corals which he saw in the Red Sea, and parts of which are still tenanted by working polypes, were alive in the time of the Pharaohs, and have been growing and enlarging

1 Spencer.

ever since. Others of equally vast age have been observed in the waters of tropical America.'1 If we regard the whole product of the fertilized germ as an individual, then we must conclude that these corals have a longevity of more than 3000 years, though we well know that countless generations of asexually-produced polyps have succeeded one another in these great coral masses. Without further discussing the question here, we may adopt Mr. Spencer's view, that there is no possible definition of individual which is absolutely unobjectionable. 'Individualities merge and are distributed, in such cases as fusion and fission, which renders the estimation of their longevity a matter of great indefiniteness, and we shall find it most agreeable to all the facts in issue, to consider as individuals all those wholly or partially independent organized masses which arise by multicentral and multiaxial development that is either continuous or discontinuous.' 2

The period of life which we must compare in different species is then that presented by individuals as above defined. We have not to consider the life of attached gemmæ, nor of unlaid ova. Such life is not the life of distinct individuals. Thus then, for our purpose, all parts of a tree, as long as they remain attached to the original axis, are but one individual. But we have to consider and compare the duration

1 Leo Grindon: Life,' &c. p. 99.

2 Principles of Biology: Spencer, vol. i. p. 207.

C

of life of all separated and independent organized masses the branch is part of the individual tree while it is attached thereto; but if it be removed when young and grafted or bedded, it is a new individuality and has its own longevity. So with the bedded polyps or worm-segments: whilst attached, they have but one individuality; when thrown off or disunited by the death of their common parent or axis, they are distinct individualities. Though not fully satisfactory, such seems to be the fittest use of the word 'individual' in relation to our subject.

Certain difficulties are also involved in the term 'life.' For, whilst in this essay it is beside the subject to enter into an explanation or definition of that phenomenon, we are certainly called upon to consider whether, in defining longevity as the duration of life of an individual, we regard the suspended animation of such organisms as Rotifera, Tardigrada, and some Nematodes, as also the retarded development of seeds, such as those of Egyptian wheat obtained from the most ancient monuments, as coming into consideration as instances of duration of life. The best solution which can be given of this difficulty appears to be in regarding these cases as strictly exceptional, and as being rather examples of suspension of life than of its duration. Though the conditions under which this suspension occurs may furnish some evidence as to the conditions favourable to the retention of vitality by an organism, they must be