GROWTH. 149 



will not produce extra growth, beyond a point very soon 

 reached. Deduction shows to be necessary, as induction 

 makes familiar, the truths that the value of food for purposes 

 of growth depends not on the quantity of the various organi- 

 zable materials it contains, but on the quantity of the mate- 

 rial most needed ; that given a right proportion of materials, 

 the pre-existing structure of the organism limits their 

 availability; and that the higher the structure, the sooner 

 is this limit reached. 



§ 46. But why should the growth of every organism be 

 finally arrested? Though the rate of increase may, in each 

 case, be necessarily restricted within a narrow range of varia- 

 tion — though the increment that is possible in a given time, 

 cannot exceed a certain amount; yet why should the incre- 

 ments decrease and finally become insensible? Why should 

 not all organisms, when supplied with sufficient materials, 

 continue to grow as long as they live? To find an answer to 

 this question we must revert to the nature and functions of 

 organic matter. 



In the first three chapters of Part I, it was shown that 

 plants and animals mainly consist of substances in states of 

 unstable equilibrium — substances which have been raised to 

 this unstable equilibrium by the expenditure of the forces we 

 know as solar radiations, and which give out these forces in 

 other forms on falling into states of stable equilibrium. 

 ]^eaving out the water, which serves as a vehicle for these 

 materials and a medium for their changes; and excluding 

 those mineral matters that play either passive or subsidiary 

 parts; organisms are built up of compounds which are stores 

 of force. Thus complex colloids and crystalloids which, as 

 united together, form organized bodies, are the same colloids 

 and crystalloids which give out, on their decomposition, the 

 forces expended by organized bodies. Thus these 



nitrogenous and carbonaceous substances, being at once 

 the materials for organic growth and the sources of organic 



