I.] THE DURATION OF LIFE, 3 1 



life. There is everywhere a maximum size, which, as far as 

 our experience goes, is never surpassed. The mosquito never 

 reaches the size of an elephant, nor the elephant that of a whale. 



Upon what does this depend ? Is there any external 

 obstacle to growth ? Or is the limitation entirely imposed 

 from within ? 



Perhaps you may answer, that there is an established relation 

 between the increase of surface and mass, and it cannot be 

 denied that these relations do largely determine the size of the 

 body. A beetle could never reach the size of an elephant, 

 because, constituted as it is, it would be incapable of existence 

 if it attained such dimensions. But nevertheless the relations 

 between surface and mass do not form the only reason why 

 any given individual does not exceed the average size of its 

 species. Each individual does not strive to grow to the largest 

 possible size, until the absorption from its digestive area 

 becomes insufficient for its mass ; but it ceases to grow because 

 its cells cannot be sufficiently nourished in consequence of its 

 increased size. The giants which occasionally appear in the 

 human species prove that the plan upon which man is con- 

 structed can also be carried out on a scale which is far larger 

 than the normal one. If the size of the body chiefly depends 

 upon amount of nutriment, it would be possible to make giants 

 and dwarfs at will. But we know, on the contrary, that the 

 size of the body is hereditary in families to a very marked 

 extent ; in fact so much so that the size of an individual depends 

 chiefly upon heredity, and not upon amount of food. 



These observations point to the conclusion that the size of 

 the individual is in reality pre-determined, and that it is poten- 

 tially contained in the ^gg 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 pro- 

 cesses 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? 



