150 THE INDUCTIONS OF BIOLOGY. 



energy, it results that as much of them as is used up for the 

 genesis of energy is taken away from the means of growth, 

 and as much as is economized by diminishing the genesis of 

 energy, is available for growth. Given that limited quantity 

 of nutritive matter which the pre-existing structure of an 

 organism enables it to absorb ; and it is a necessary corollary 

 from the persistence of force, that the matter accumulated as 

 growth cannot exceed that surplus which remains unde- 

 composed after the production of the required amounts of 

 sensible and insensible motion. This, which would 



be rigorously true under all conditions if exactly the same 

 substances were used in exactly the same proportions for the 

 production of force and for the formation of tissue, requires, 

 however, to be taken with the qualification that some of the 

 force-evolving substances are not constituents of tissue; and 

 that thus there may be a genesis of force which is not at the 

 expense of potential growth. But since organisms (or at 

 least animal organisms, with which we are here chiefly 

 concerned) have a certain power of selective absorption, 

 which, partially in an individual and more completely in a 

 race, adapts the proportions of the substances absorbed to the 

 needs of the system; then if a certain habitual expenditure 

 of force leads to a certain habitual absorption of force- 

 evolving matters that are not available for growth; and if, 

 were there less need for such matters, the ability to absorb 

 matters available for growth would be increased to an equi- 

 valent extent; it follows that the antagonism described does, 

 in the long run, hold even without this qualification. Hence, 

 growth is substantially equivalent to the absorbed nutriment, 

 minus the nutriment used up in action. 



This, however, is no answer to the question — why has 

 individual growth a limit? — why do the increments of 

 growth bear decreasing ratios to the mass and finally come 

 to an end ? The question is involved. There are more causes 

 than one why the excess of absorbed nutriment over ex- 

 pended nutriment must, other things equal, become less as 



