variability and mutability; the
former manifesting itself in gradual and isolated changes, the latter in
saltatory changes on a larger scale. The mistake made by Wallace and by
the later Darwinians has been that they regarded this latter form ("single
variation") as unimportant and not affecting evolution, and the former as
the real method of evolutionary process. That fluctuating individual
variations do occur De Vries admits, but only within narrow limits, never
overstepping the type of the species. Here De Vries utilises the recent
statistical investigations into the phenomena of individual variation and
their laws, as formulated chiefly by Quetelet and Bateson, which were
unknown to Darwin and the earlier Darwinians. The actual transition from
"species to species" is made suddenly, by mutation, not through variation.
And the state of equilibrium thus reached is such a relatively stable one
that individual variations can only take place within its limits, but can
in no way disturb it.

De Vries marshals a series of facts which present insurmountable
difficulties to the Darwinian theory, but afford corroboration of the
Mutation theory. In particular, he brings forward, from his years of
experiment and horticultural observation, comprehensive evidence of the
mutational origin of new species from old ones by leaps, and this not in
long-past geological times, but in the course of a human life and before
our very eyes. The main importance of the book lies in the record of these
experiments and observations, rather than in the theory as such, for the
way had been paved for it by other workers.

In contrast to Darwinism, De Vries states the case for "Halmatogenesis"
(saltatory evolution) and "Heterogenesis" (the production of forms unlike
the parents), taking his examples from the plant world, but his attitude
to Darwinism is conciliatory throughout. Eimer, on the other hand, is
sharply antagonistic, especially to Weismann; he takes his proofs from the
animal kingdom, and in the second volume of his large work already
mentioned, which deals with the "orthogenesis of butterflies," he attempts
to set against the Darwinism "chance theory," a proof of "definitely
directed evolution," and therefore of the "insufficiency of natural
selection in the formation of species."

Eimer's Orthogenesis.

Organisation is due to internal causes. Structural characters crystallise
out, as it were. "Orthogenesis," or the definitely