GROWTH. 151 



the size of the animal becomes greater. In similarly- 



shaped bodies the masses, and therefore the weights, vary as 

 the cubes of the dimensions; whereas the powers of bearing 

 the stresses imposed by the weights vary as the squares of 

 the dimensions. Suppose a creature which a year ago was 

 one foot high, has now become two feet high, while it is 

 unchanged in proportions and structure ; what are the neces 

 sary concomitant changes ? It is eight times as heavy ; that 

 is -to say, it has to resist eight times the strain which gravi 

 tation puts upon certain of its parts; and when there 

 occurs sudden arrest of motion or sudden genesis of motion, 

 eight times the strain is put upon the muscles employed. 

 Meanwhile the muscles and bones have severally increased 

 their abilities to bear strains in proportion to the areas of 

 their transverse sections, and hence have severally only four 

 times the tenacity they had. This relative decrease in the 

 power of bearing stress does not imply a relative decrease in 

 the power of generating energy and moving the body; for in 

 the case supposed the muscles have not only increased four 

 times in their transverse sections but have become twice as 

 long, and will therefore generate an amount of energy propor 

 tionate to their bulk. The implication is simply that each 

 muscle has only half the power to withstand those shocks and 

 strains which the creature s movements entail ; and that con 

 sequently the creature must be either less able to bear these, 

 or must have muscles and bones having relatively greater 

 transverse dimensions : the result being that greater cost of 

 nutrition is inevitably caused and therefore a correlative 

 tendency to limit growth. This necessity will be seen still 

 more clearly if we leave out the motor apparatus, and con 

 sider only the forces required and the means of supplying 

 them. For since, in similar bodies, the areas vary as the 

 squares of the dimensions, and the masses vary as the cubes ; 

 it follows that the absorbing surface has become four times 

 as great, while the weight to be moved by the matter ab 

 sorbed has become eight times as great. If then, a year 



