32 THE GENESIS OF SPECIES, [CHAP. 



ture ; and in producing, as well as in arresting, every one 

 of its movements, it has to overcome eight times the 

 inertia. Meanwhile, the muscles and bones have severally 

 increased their contractile and resisting powers, in propor- 

 tion 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 expended." Again, as to the cost at which nutri- 

 ment is distributed 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 matter 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 fhe 

 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 in 

 every organ and every part of every organ, how very 

 generally must favourable variations as to the length of 

 the neck have been accompanied by some unfavourable 

 variation in some other part, neutralizing the action of 

 the favourable one, the latter, moreover, only taking 

 effect during these periods of drought ! How often must 

 individuals, favoured .by a slightly increased length of 

 neck, have failed to enjoy the elevated foliage which they 

 had not strength or endurance to attain ; while other indi- 

 viduals, exceptionally robust, could -struggle on yet farther 

 till they arrived at vegetation within their reach. 



