SOME CURRENT THEORIES 453 



weight or volume undergoes more or less continuous increase and 

 the proportion of active substance to total weight or volume becomes 

 less and less. Consequently the percentage increment of growth 

 decreases more or less continuously from the beginning of these 

 changes, and the absolute increment, while at first increasing, must 

 sooner or later decrease. It is, in fact, not the increase in the 

 autocatalyst of growth, but the increase of other products of reac- 

 tion and the transformation of active protoplasm into other less 

 active forms which retards growth, and these changes are going on 

 and the proportion of these substances is increasing more or less 

 continuously from the beginning of the growth period. Enriques 

 ('09), in a critique of the autocatalytic theory of growth, has 

 emphasized the fact that in consequence of differentiation a "dilu- 

 tion" of the actively growing substance occurs and the rate of 

 growth decreases, until finally the total growth is insufficient to 

 balance the losses, and senile atrophy occurs. Senescence, senile 

 atrophy, and death result from changes of this kind, not from the 

 autocatalytic changes, and there is no need of assuming, as the 

 adherents of the autocatalytic theory of growth are forced to do, 

 that the conditions which determine senile atrophy are different 

 from those which are concerned in growth. Senile atrophy is in 

 reality merely the necessary result of continued growth in organisms 

 with a relatively stable substratum. 



Growth is not a simple chemical reaction and cannot be con- 

 sidered as such: it is a complex physico-chemical process in which 

 changes in the physical character of the substratum as well as 

 chemical conditions are concerned. The rate of growth is deter- 

 mined, not simply by the laws of autocatalysis, but by a complex 

 of factors of different kinds. The decrease in the absolute growth- 

 increment in later stages does not represent approach toward a 

 chemical equilibrium, but rather a continued dilution and physical 

 change of the protoplasm. 



The question whether reduction and dedifferentiation are 

 reversals in the chemical sense of growth and differentiation has 

 already been raised (see pp. 38, 56). If it were possible to regard 

 the whole life cycle of the organism as a reversible chemical reac- 

 tion it would doubtless simplify very greatly our conception of 



