THE DURATION OP LIFE. 31 



individual is in reality pre-determined, and that it is potentially 

 contained in the egg from which the individual developes. 



We know further that the growth of the individual depends 

 chiefly upon the multiplication of cells and only to a slight extent 

 upon the growth of single cells. It is therefore clear that a 

 limit of growth is imposed by a limitation in the processes by 

 which cells are increased, both as regards the number of cells 

 produced and the rate at which they are formed. How could we 

 otherwise explain the fact that an animal ceases to grow long 

 before it has reached the physiologically attainable maximum of its 

 species, without at the same time suffering any loss of vital 

 energy ? 



In many cases at least, the most important duty of an organism, 

 viz. reproduction, follows upon the attainment of full size a fact 

 which induced Johannes Muller to reject the prevailing hypothesis 

 which explained the death of animals as due to ' the influences 

 of the inorganic environment, which gradually wear away the life 

 of the individual.' He argued that, if this were the case, ' the 

 organic energy of an individual would steadily decrease from the 

 beginning,' while the facts indicate that this is not so l . 



If it is further asked why the egg should give rise to a fixed 

 m/^nber of cell-generations, although perhaps a number which 

 va.-jes widely within certain limits, we may now refer to the opera- 

 tion of natural selection upon the relation of surface to mass, and 

 upc^n other physiological necessities which are peculiar to the species. 

 Because a certain size is the most favourable for a certain plan 

 of organization, the process of natural selection determined that 

 such a size should be within certain variable limits, characteristic 

 of each species. This size is then transmitted from generation to 

 generation, for when once established as normal for the species, the 

 most favourable size is potentially present in the reproductive cell 

 from which each individual is developed. 



If this conclusion holds, and I believe that no essential objection 

 can be raised against it, then we have in the limitation in space 

 a process which is exactly analogous to the limitation in time, 

 which we have already considered. The latter limitation the 

 duration of life also depends upon the multiplication of cells, the 



1 Johannes Muller, 'Physiologic,' Bd. I. p. 31, Berlin, 1840. 



