PHYSIOLOGICAL GRADIENTS 91 
some to constitute a formidable difficulty for this con- 
ception, but I believe the difficulty is only apparent. It 
is of course true that differences in the velocity alone of 
a pure chemical reaction do not give rise to qualitatively 
different products of reaction. In protoplasm, however, 
we are concerned, not with a pure chemical reaction, 
but with an exceedingly complex system, involving both 
physical and chemical factors. The coefficients of 
quantitative alteration of the different factors of the 
protoplasmic system in relation to quantitative external 
factors, such, for example, as temperature, are of very 
different magnitudes. The temperature coefficients of 
most chemical reactions differ widely in magnitude from 
those of many physical processes, and there is even 
considerable difference in the temperature coefficients 
of different chemical reactions. A change in tempera- 
ture then does not alter the different component factors 
of the protoplasmic system to the same degree, but alters 
some much more than others. Consequently such a 
change, which is nonspecific and quantitative for each 
factor, may alter the system as a whole in such a way" 
that chemical reactions which could not take place 
before the change may occur after it, and vice versa. 
Changes in concentration of reacting substances do in 
many cases determine different products of reaction, 
and change in temperature may bring about changes 
in concentration of substances in protoplasm in various 
ways, e.g., by alteration in the water content, the dis- 
persion of the colloids, adsorption, and by change in the 
rate of chemical reaction itself, which may change the 
concentration of the products of a given reaction and so 
determine a second reaction which could not take place 
