30 THE DURATION OP LIFE. 



The above-mentioned considerations show us that the degree 

 of reproductive activity present in the tissues is regulated by 

 internal causes while the natural death of an organism is the 

 termination the hereditary limitation of the process of cell- 

 division, which began in the segmentation of the ovum. 



Allow me to suggest a further consideration which may be com- 

 pared with the former. The organism is not only limited in time, 

 but also in space : it not only lives for a limited period, but it can 

 only attain a limited size. Many animals grow to their full size 

 long before their natural end : and although many fishes, reptiles, and 

 lower animals are said to grow during the whole of their life, we do 

 not mean by this that they possess the power of unlimited growth 

 any more than that of unlimited life. There is everywhere a 

 maximum size, which, as far as our experience goes, is never sur- 

 passed. 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, con- 

 stituted as it is, it would be incapable of existence if it attair :d 

 such dimensions. But nevertheless the relations between surf; ice 

 and mass do not form the only reason why any given individ lal 

 does not exceed the average size of its species. Each indivic 1 lal 

 does not strive to grow to the largest possible size, until \>he 

 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 constructed can also be earned 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, 

 r These observations point to the conclusion that the size of the 



