1916] CHILD—GRADIENTS IN ALGAE gt 
In such a metabolic or irritability gradient it is evident that the 
region of highest rate of reaction must control or dominate other 
regions within a certain range, simply because the changes trans- 
mitted from it possess greater energy or physiological effectiveness 
than those from other regions, and therefore constitute the chief 
factor in determining the general metabolic rate in these other 
regions within the range of its action. In plants the dominance 
of the apical region over other levels of the axis is a familiar fact, 
and I have shown that in at least the simpler animals similar rela- 
tions exist along the main axis, the region of highest metabolic 
rate being the dominant region. 
This relation of dominance and subordination dependent upon 
a gradient in metabolic rate or in the irritability of the proto- 
plasm is, as I believe, the simplest kind of physiological’ unity and 
order in living protoplasm, these transmitted changes are the 
fundamental form of physiological correlation, and the physiological 
individual as an expression of dynamic unity and order consists 
fundamentally of one or more such metabolic gradients. 
According to this conception, the individual originates as one 
or more quantitative gradients in the dynamic processes character- 
istic of the specific protoplasm concerned. Morphogenesis and 
differentiation result from the quantitative differences existing at 
different levels of the gradient because the substances which remain 
as constituents of or inclosures in the protoplasm differ not only 
in amount but in kind according to the rate or intensity of meta- 
bolic activity. In this way qualitative differences arise from 
quantitative, and what we call differentiation results. This con- 
ception of differentiation is supported by the fact that it is possible 
experimentally to alter widely the character of morphogenesis 
by means of conditions which act primarily in a quantitative way. 
If this view be correct, the physiological axiate individual is 
fundamentally a unity resulting from transmitted dynamic changes, 
* The word “physiological” is used here by way of distinction, Other kinds of 
individuality, of unity and order in aggregates, exist in the world and some of them 
undoubtedly exist in organisms, but by physiological unity and order is meant that 
kind which seems to be peculiarly characteristic of living things, and which determines 
orderly space and time relations with respect-to an axis; in short, an orderly form and 
an orderly course of development. 
