apt to be short lived, though the latter is by no means universally true—length of life being conditioned by duration of the ascendency of assimilation over dissimilation, whereas size is conditioned also by rate of assimilation as contrasted with dissimilation.
Weismann has pointed out a relation between longevity and the rate of reproduction—animals in which there is a slow rate of reproduction being in general long lived, while those in which the rate of reproduction is rapid are generally short lived. Numerous exceptions to this rule may be cited, though in many cases it is undoubtedly true; but Weismann has not proved that length of life is the result of slow reproduction. It may well be that both length of life and rate of reproduction are dependent upon the duration and rate of assimilation and dissimilation in somatic and germinal cells.
There is also an undoubted relation between longevity and adaptability, or the power of regulation. If life is continuous adjustment of internal conditions to external conditions, length of life may be said to depend upon the duration and perfection of such adjustment. The power of regulation is much less perfect in some animals than in others, and at certain stages of the life cycle than at other stages. But in all animals this power is greatest where the relative proportion of protoplasm to metaplasm, or differentiation products, is greatest, and where the protoplasm is most labile. In Protozoa this power of regulation is shown at every division and it suffers no abatement in successive generations; in Metazoa generally the power of regulation is greatest in early stages of development and in tissues in which protoplasm is abundant, and it diminishes as life advances and as the products of differentiation more and more replace the protoplasm. In the fission of a Paramecium there is a certain amount of dedifferentiation preceding division and of redifferentiation succeeding it, and as a result of this the two halves of the original Paramecium become alike; furthermore, in successive generations, there is no accumulation of the products of differentiation. In the division of the eggs of Metazoa the cleavage cells sooner or later become unlike, owing to the differentiations present in the mother cell and the failure of complete regulation in the daughter cells. This progressive differentiation is accompanied by a progressive loss of the power of regulation, and when the general protoplasm is so completely transformed into differentiation products that the power of regulation is completely lost, the organism as a whole must lose the power of adjustment to external conditions, and hence of indefinitely continued life.
Many different hypotheses have been advanced to account for the running down of the vital machine. That death is not a necessary corollary of life is evidenced by the potential immortality of Protozoa and of the germ cells of Metazoa. Senescence, like all other processes
