AN ESS A V ON LONGEVITY. 37 



the matter of life, and prevents its accumulation) in- 

 creases more rapidly than growth ; there is not a direct 

 agreement between the increase of the one and of the 

 other. This appears from the following considerations. 

 It is demonstrable that the excess of absorbed over 

 expended nutriment must, other things being equal, 

 become less as the size of the animal becomes greater, 

 In similarly shaped bodies the masses vary as the cubes 

 of the dimensions, whereas the strengths vary as the 

 squares of the dimensions. ' Supposing a creature 

 which a year ago was one foot high, has now become 

 two feet high, what are the necessary concomitant 

 changes that have taken place in it.'' It is eight 

 times as heavy, but the muscles and bones have 

 increased their power only in proportion to the areas 

 of their cross sections ; hence they are severally but 

 four times as strong as they were. Thus, while the 

 creature has doubled in height, and while its ability 

 to overcome forces has quadrupled, the forces it has 

 to overcome hdcve grown eight times as great. Hence, 

 to raise its body through a given space, its muscles 

 have to be contracted with twice the intensity, at 

 a double cost of matter expended.' Mr. Spencer 

 shews that the same relation is true for the absorbing 

 surface, which has only increased fourfold, and for 

 the circulation of nutriment, which has to be trans- 

 mitted to an enlarged periphery. Thus, then, the 

 period of growth must be limited ; thus a period 

 must be reached when the germinal or living matter 



