146 LIFE AND DEATH. 



processes until we have ascertained the molecular structure of cells, 

 together with the changes that occur in this structure and the con- 

 sequences of the changes, we shall probably never understand either 

 the one or the other. The complex processes of life can only be 

 followed by degrees, and we can only hope to solve the great 

 problem by attacking it from all sides. 



Therefore it is, in my opinion, an advance if we may assume that 

 length of life is dependent upon the number of generations of 

 somatic cells which can succeed one another in the course of a 

 single life ; and, furthermore, that this number, as well as the 

 duration of each single cell -generation, is predestined in the germ 

 itself. This view seems to me to derive support from the obvious 

 fact that the duration of each cell-generation, and also the number 

 of generations, undergo considerable increase as we pass from the 

 lowest to the highest Metazoa. 



In an earlier work * I have attempted to show how exactly the 

 duration of life is adapted to the conditions by which it is sur- 

 rounded ; how it is lengthened or shortened during the formation 

 of species, according to the conditions of life in each of them ; in 

 short, how it is throughout an adaptation to these conditions. A 

 few points however were not touched upon in the work referred 

 to, and these require discussion ; their consideration will also throw 

 some light upon the origin of natural death and the forms of life 

 affected by it. 



I have above explained the limited duration of the life of 

 somatic cells in the lower Metazoa Orthonectides as a pheno- 

 menon of adaptation, and have ascribed it to the operation of 

 natural selection, at the same time pointing out that the existence 

 of immortal Metazoan organisms is conceivable. If the Mono- 

 plastides are able to multiply by fission, through all time, then their 

 descendants, in which division of labour has induced the antithesis 

 of reproductive and somatic cells, might have done the same. If 

 the Homoplastid cells reproduced their kind uninterruptedly, equal 

 powers of duration must have been possible for the two kinds of 

 Heteroplastid cells ; they too might have been immortal so far as im- 

 mortality only depends upon the capacity for unlimited reproduction. 



But the capacity for existence possessed by any species is not 

 only dependent upon the power within it ; it is also influenced 

 1 See the first essay on ' The Duration of Life." 



