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 
