192 THE POPULAR SCIENCE MONTHLY 



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 reproduc- 

 tion 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 repro- 

 duction. It may well be that both length of life and rate of reproduc- 

 tion are dependent upon the duration and rate of assimilation and dis- 

 similation in somatic and germinal cells. 



There is also an undoubted relation between longevity and adapta- 

 bility, 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 proto- 

 plasm 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 genera- 

 tions ; 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 gen- 

 erations, 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 



