GROWTH. 127 



§ 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 atoms — an 

 exertion which therefore varies according as the food is abund- 

 ant or scarce, fixed or moving, according as it is mechani- 

 cally easy or difficult to deal with when secured, and accord- 

 ing 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 accumulation 

 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 be- 

 tween 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 this 

 harmony in any specific way ; since the conflicting and con- 

 spiring causes which affect growth are so numerous. The 

 only contrast which seems fairly to the point, is the before- 

 named one between the vertebrates which fly, and the most 

 nearly-allied vertebrates which do not fly : the differences 

 in degrees of organization and relations to food, being not such 

 as seriously to affect the comparison. If it be admitted that 

 birds habitually expend more force than mammals and rep- 

 tiles, then it will follow a priori, that, other things being 

 tolerably equal, they should have a lower limit of growtli 

 than mammals and reptiles ; and this we know to be the fact 

 a posteriori. 



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



