92 THE ORIGIN OF THE NERVOUS SYSTEM 



previously, and therefore a new product of reaction. 

 Such changes determine others, and there is not the 

 slightest doubt that a change in protoplasmic condition, 

 originating as a purely quantitative alteration in some 

 factor or factors, may become the starting-point of 

 extensive qualitative changes. 



When we take into account the material exchanges 

 between protoplasm and its environment, various other 

 possibilities of change in concentration of substances in 

 the protoplasm appear. An increase in rate of oxidation 

 in the protoplasm may oxidize certain substances more 

 rapidly than they enter the protoplasm, and, as they 

 disappear, other substances, previously inactive, may 

 enter reaction and different products be formed. As a 

 matter of fact, even with our present fragmentary 

 knowledge of living protoplasm, almost endless possi- 

 bilities appear for the origin of qualitative differences 

 from primarily quantitative changes in the system. It 

 may even be questioned whether it is possible to bring 

 about a change in protoplasm which remains through- 

 out purely quantitative. 



Differentiation in the organism consists, at least very 

 largely, in the appearance in protoplasm of substances 

 not present originally in appreciable quantities, and such 

 substances differ in amount or in kind in different regions 

 or cells. In many such cases a simple difference in 

 relation between the rate of nutritive intake and the 

 rate of oxidation is sufficient to account for the appear- 

 ance of a particular substance in a certain region or at 

 a certain time, or its non-appearance or disappearance 

 in another region or at another time. The accumulation 

 and disappearance of fat and various other so-called 



