Variation and Heredity 287 
establish itself, either because it is adapted to live under con- 
ditions somewhat different from the parent form, so that the 
dangers of intercrossing are lessened, or because the new 
form may absorb the old one. It is also clear, from what 
has gone before, that the new form can only cease to be fer- 
tile with the parent form, or with its sister forms, after it has 
undergone such a number of changes that it is no longer 
able to combine the differences in a new individual. This 
result will depend both on the kinds of the new characters, as 
well as the amounts of their difference. This brings us to 
a consideration of the results of De Vries, who has studied 
the first steps in the formation of new species in the “ muta- 
tions”’ of the evening primrose. 
THE Mutation THEORY OF DE VRIES 
De Vries defines the mutation theory as the conception 
that “the characters of the organism are made up of elements 
(‘Einheiten’) that are sharply separated from each other. 
These elements can be combined in groups, and in related 
species the same combinations of elements recur. Transi- 
tional forms like those that are so common in the external 
features of animals and plants do not exist between the 
elements themselves, any more than they do between the 
elements of the chemist.” 
This principle leads, De Vries says, in the domain of the 
descent theory to the conception that species have arisen 
from each other, not continuously, but by steps. Each new 
step results from a new combination as compared with the old 
one, and the new forms are thereby completely and sharply 
separated from the species from which they havecome. The 
new species is all at once there; it has arisen from the parent 
form without visible preparation and without transitional steps. 
The mutation theory stands in sharp contrast to the selec- 
tion theory. The latter uses as its starting-point the com- 
