32 THE DURATION OF LIFE. [I. 



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

 ism, viz. reproduction, follows upon the attainment of full size 

 — a fact which induced Johannes Miillerto 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'. 



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

 number of cell-generations, although perhaps a number which 

 varies widely within certain limits, we may now refer to the 

 operation of natural selection upon the relation of surface to 

 mass, and upon other phj'siological necessities which are pecu- 

 liar to the species. Because a certain size is the most favour- 

 able 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 objec- 

 tion 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 rapid increase of which first gave rise to the charac- 

 teristic form of the mature body, and then continued at a slower 

 f rate. In the mature animal, cell-reproduction still goes on, buTX 

 it no longer exceeds the waste ; for some time it just compen- j 

 \^ates for loss, and then begins to decline. The waste is not/^ 

 compensated for, the tissues perform their functions incom- 

 pletely, and thus the way for death is prepared, until its final 

 appearance by one of the three great Atna mortis. 



I admit that facts are still wanting upon which to base this 

 hypothesis. It is a pure supposition that senile changes are 

 due to a deficient reproduction of cells : at the same time this 



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



