GrtOWTII. 157 



equal, a larger fish had no mechanical advantage over a 

 smaller, a larger fish could not exist — could not catch the 

 requisite amount of prey. 



§47. Obviously this antagonism between accumulation and 

 expenditure, must be a leading cause of the contrasts in size 

 between allied organisms that are in many respects similarly 

 conditioned. The life followed by each kind of animal is 

 one involving a certain average amount of exertion for the 

 obtainment of a given amount of nutriment — an exertion, 

 part of which goes to the gathering or catching of food, part 

 to the tearing and mastication of it, and part to the after- 

 processes requisite for separating the nutritive molecules — an 

 exertion which therefore varies according as the food is 

 abundant or scarce, fixed or moving, according as it is me- 

 chanically easy or difficult to deal with when secured, and 

 according as it is, or is not, readily soluble. Hence, while 

 among animals of the same species having the same mode of 

 life, there will be a tolerably constant ratio between accumu- 

 lation and expenditure, and therefore a tolerably constant 

 limit of growth, there is every reason to expect that different 

 species, following different modes of life, will have unlike 

 ratios between accumulation and expenditure, and therefore 

 unlike limits of growth. 



Though the facts as inductively established, show a general 

 harmony with this deduction, we cannot usually trace it in 

 any specific way; since the conflicting and conspiring factors 

 which affect growth are so numerous. 



§ 48. One of the chief causes, if not the chief cause, of 

 the differences between the sizes of organisms, has yet to be 

 considered. We are introduced to it by pushing the above 

 inquiry a little further. Small animals have been shown to 

 possess an advantage over large ones in the greater ratio 

 which, other things equal, assimilation bears to expenditure; 

 and we have seen that hence small animals in becoming large 



