The work that has probably excited most interest in this connection is De Vries' “Die Mutationstheorie: Versuche und Beobachtungen Über die Entstehung von Arten im Pflanzenleben.”46 In a short preliminary paper he had previously given some account of his leading experiments on a species of evening primrose (Œnothera lamarckiana), and the outlines of his theory. In the work itself he extends this, adding much concrete material, and comparing his views in detail with other [pg 173] theories. Darwin, he says, had already distinguished between 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 [pg 174] 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.”