570 Species According to the Theory of Mutation. 
more numerous. Every more highly organized being has, 
as a rule, more of them than its ancestors of long ago had. 
In applying this principle to the doctrine of elementary 
characters we see at once that the number of these units 
must increase with increasing differentiation; or, con- 
versely, that the degree of differentiation is ultimately 
determined by the number of elementary characters. 
Whenever a new unit is added to those already existing, 
differentiation advances a step forward. If it were pos- 
sible to count the units we should have a measure of the 
degree of differentiation of all organisms. 
Obviously the individual steps are only small ones, 
at the present time at least ; and any single one of them 
can hardly effect a noticeable increase in differentiation. 
At any rate we have at present no means of so exactly 
measuring the degree of differentiation, since we cannot 
estimate the possible influence of a single unit on a com- 
plex built up of thousands of them. Only groups of units 
produce clear and obvious differences in the degree of 
organization ; but within the limits of a small genus or 
of a multiform collective species the several types seem 
to us to be almost always equivalent. 
The individual steps into which, according to this 
view, the process of gradual differentiation can be ana- 
lyzed, we propose to call mutations ; and since they con- 
stitute an advance, progressive mutations. Each of them 
contributes a new character to the complex of hereditary 
qualities already present. 
Such a new character need not, however, become vis- 
ible as soon as it arises, since we are not dealing solely 
with external qualities but with the internal factors to 
which they owe their appearance. Even as the germ 
contains large numbers of qualities awaiting development, 
