830 THE AMERICAN NATURALIST.  [Vou. XXXII. 
the physicist abstracts the thing-in-itself, or the substance of 
things, as matter, which in its ultimate nature, as atoms, is 
unchangeable — immutable. In like manner, he abstracts from 
experience the substantial basis of observed actions and calls 
it energy. Thus in the working hypothesis of physics, the 
substantial basis of particular acts is energy, exactly as the 
substantial basis of particular things is matter. But the physi- 
cist finds no place in this hypothesis for any variation in the 
ultimate kind, amount, or constitution of these two substantial 
bases of experience. All differences in phenomena are, to this 
theory, different arrangements of immutable units. 
The application of this hypothesis, of the immutability of the 
essence of things, to the phenomena of living organisms requires 
us to assume that there are some kinds of ultimate immutable 
units back of organisms, which necessarily behave uniformly, 
except as they may be diverted by the action of some force 
from outside. Thus, I suppose, has arisen the almost universal 
belief that the uniform behavior of organic bodies, which we 
define by the term heredity, is a necessary and fundamental 
characteristic of organic units, just as inertia is a characteristic 
of inorganic bodies. The final outcome of this view is the 
assumption of the existence of separate and immutable units 
for every divergent phenomenon of organisms. Darwin evi- 
dently adopted some such view, and I do not see that the 
followers of Darwin have escaped this fundamental conception 
in elaborating the general evolution hypothesis. But the first 
step toward correcting it was taken when the idea of immuta- 
bility was dissociated from the organic species. 
Cuvier was the last of the great naturalists to maintain the 
immutability of species. It was the recognition of the intrinsic 
mutability of the organic species that made a rational theory 
of evolution possible, and it is my sincere conviction that a 
consistent theory of evolution cannot be built up which stops 
here. I cannot discover that there is any halting place. In 
order to explain the wonderful phenomena of organisms, the 
principle of mutability must be extended to the ultimate units 
of which every living body is composed. Not only the species, 
but the individual, the cell, the units which constitute the living 
