40 THE GENESIS OF SPECIES. [CHAP. 



in strength ; for, as Mr. Herbert Spencer says, " It is de- 

 monstrable that the excess of absorbed over expended nu- 

 triment must, other things equal, become less as the size of 

 an 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, while it is unchanged in 

 proportions and structure what are the necessary con- 

 comitant changes that have taken place in it ? It is eight 

 times as heavy ; that is to say, it has to resist eight times 

 the strain which gravitation puts on its structure; and in 

 producing, as well as in arresting, every one of its move- 

 ments, it has to overcome eight times the inertia. Mean- 

 while, the muscles and bones have severally increased their 

 contractile and resisting powers, in proportion to the areas 

 of their transverse sections ; and hence 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 have 

 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 ex- 

 pended." Again, as to the cost at which nutriment is dis- 

 tributed through the body, and effete matters removed from 

 it, " Each increment of growth being added at the periphery 

 of an organism, the force expended in the transfer of mat- 

 ter must increase in a rapid progression a progression 

 more rapid than that of the mass." 



There is yet another point. Vast as may have been the 

 time during which the. process of evolution has continued, 

 it is, nevertheless, not infinite. Yet, as every kind, on the 

 Darwinian hypothesis, varies slightly but indefinitely ni 

 every organ and every part of every organ, how very gen- 



9 "Principles of Biology," vol. i., p. 122. 



