As concerns the “survival of the fittest” (or “natural selection"), we can, I think, at once conclude that the organism which best fulfils the organic law under the circumstances of supply is the “fittest,” ipso facto. In many

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cases this is contained in the commonsense consideration, that to be strong, consistent with concealment from enemies which are stronger, is best, as giving the organism mastery over foes which are weaker, and generally renders it better able to secure supplies.  Weismann points out that natural selection favours early and abundant reproduction.  But whether the qualifications of the “fittest” be strength, fertility, cunning, fleetness, imitation, or concealment, we are safe in concluding that growth and reproduction must be the primary qualities which at once determine selection and are fostered by it.  Inherent in the nature of the organism is accelerated absorption of energy, but the qualifications of the “fittest” are various, for the supply of energy is limited, and there are many competitors for it.  To secure that none be wasted is ultimately the object of natural selection, deciding among the eager competitors what is best for each.

In short, the facts and generalisations concerning evolution must presuppose an organism endowed with the quality of progressive absorption of energy, and retentive of it.  The continuity of organic activity in a world where supplies are intermittent is evidently only possible upon the latter condition.  Thus it appears that the dynamic attitude of the organism, considered in these pages, occupies a fundamental position regarding its evolution.

We turn to the consideration of old age and death, endeavouring to discover in what relation they stand to the innate progressiveness of the organism.

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THE PERIODICITY OF THE ORGANISM AND THE LAW OF PROGRESSIVE
ACTIVITY

The organic system is essentially unstable.  Its aggressive attitude is involved in the phenomenon of growth, and in reproduction which is a form of growth.  But the energy absorbed is not only spent in growth.  It partly goes, also, to make good the decay which arises from the instability of the organic unit.  The cell is molecularly perishable.  It possesses its entity much as a top keeps erect, by the continual inflow of energy.  Metabolism is always taking place within it.  Any other condition would, probably, involve the difficulties of perpetual motion.

The phenomenon of old age is not evident in the case of the unicellular organism reproducing by fission.  At any stage of its history all the individuals are of the same age:  all contain a like portion of the original cell, so far as this can be regarded as persisting where there is continual flux of matter and energy.  In the higher organisms death is universally evident.  Why is this?

The question is one of great complexity.  Considered from the more fundamental molecular point of view we should perhaps look to failure of the power of cell division as the condition of mortality.  For it is to this phenomenon—­that of cell division—­that the continued life of the protozoon is to be ascribed, as we have already seen.  Reproduction is, in fact, the saving factor here.