14^ LIFE AND DEATH. [III. 



In an earlier work ^ I have attempted to show how exactly 

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

 is surrounded ; how it is lengthened or shortened during the 

 formation of species, according to the conditions of Hfe 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 con- 

 sideration 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 phe- 

 nomenon 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 Monoplastides 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 immortahty 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 

 by the conditions of the external world, and this renders 

 necessary the process which we call adaptation. Thus it is 

 just as inconceivable that either a homogeneous or a hetero- 

 geneous cell-colony possessing the physiological value of a 

 multicellular individual should continue to grow to an un- 

 limited extent by continued cell-division, as it is incon- 

 ceivable that a unicellular being should increase in size to 

 an unlimited extent. In the latter case the process of cell- 

 division imposes a limit upon the size attained by growth. 

 In the former, the requirements of nutrition, respiration, and 

 movement must prescribe a limit to the growth of the cell- 

 colony which constitutes the individual of the higher species, 

 just as in the case of the unicellular Monoplastides, and it does 

 not affect the argument if we consider this limitation to be 

 governed by the process of natural selection. It would only 

 ^ See the first essay on ' The Duration of Life.' 



