170 
PHYSIOLOGY: C. M. CHILD 
is present. Undoubtedly other factors besides the gradients play a part 
in determining the character of the results in many cases, but the orderly 
course of events is dependent upon the gradients. 
The metaboHc gradient cannot of course arise or persist simply as a 
gradient in rate of chemical reaction, for the metabolic processes and 
the colloid substratum, the protoplasm, in which they occur are always 
associated. The persistence of the gradient when once established, is 
due to the changes in the substratum connected with the differences in 
rate of reaction. For example, there is evidence that, at least in certain 
forms, a gradient in amount or activity of oxidizing enzymes exists 
along the axis. 
According to this conception, the starting point of the process of dif- 
ferentiation is, in the final analysis, a quantitative gradient or gradients, 
i.e., a difference in rate rather than in kind of metabolic reaction. Is 
such a basis adequate? I believe it is. We know that in chemical reac- 
tions in vitro quantitative differences very often result in qualitatively 
different products. In the organism, where a great number of chemical 
reactions occur in a complex substratum which influences both their 
rate and character, the possibihty of differences in quality arising from 
differences in quantity is much greater. The surface of the earth, with 
its physiographic features and its Hving forms, shows a remarkable differ- 
entiation along the equatorial-polar axis, and this differentiation results 
from the action of quantitative factors, viz., differences in light and 
heat. Moreover, it is possible to determine differences in the course 
and results of development which are manifestly qualitative by changes 
in external conditions which affect primarily the rate rather than the 
kind of reaction in the organism. 
If the axiate individual is primarily a metaboHc gradient or gradients, 
the lengths of these gradients represent the physiological limits of size 
of the individual. If the actual size exceeds this limit, either as the 
result of growth or of decrease in the length of the gradient, that portion 
which lies beyond the limit of size becomes isolated physiologically^ 
This physiological limit of size is actually attained by the whole organism 
only in the plants and lower animals, where transmission is effective only 
over relatively short distances, because of the rapid decrement in inten- 
sity or energy. In the higher animals, where the chief conducting paths 
have become highly differentiated nerves, in which the decrement is 
very slight and transmission over very great distances is therefore pos- 
sible, size is limited by other factors, such as the progressive differentia- 
tion of cells, which limit division and growth. 
