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.”
Organisation is due to internal causes. Structural characters crystallise out, as it were. “Orthogenesis,” or the definitely determined tendency of evolution to advance in a few directions, is a law for the whole of the animate world. In active response to the stimuli and influences of the environment the organism expresses itself in “organic growth” without any relation to utility. Butterflies in particular, and especially their markings and coloration, are taken as illustrations. In the Darwinian theory of “mimicry” these played a brilliant part. The great resemblance to leaves, to dried twigs, or to well-protected species which are secure from enemies, was regarded as the most convincing proof of the operation of natural selection. But Eimer shows that markings, striping, spots, the development of pattern, and the alleged or real resemblances to leaves, are really subject to definite laws of growth, in obedience to which they gradually appear, developing according to their own internal laws, varying and progressing altogether by internal necessity, and without any reference to advantage or disadvantage, In association with this orthogenesis, Eimer recognises halmatogenesis, correlation and “genepistasis” (coming to a standstill at a fixed and definite stage), and these seem to him to make the Darwinian theory utterly impossible. The text and the illustrations of the book show how, in the sequence of evolution (according to Eimer’s laws of transformation), the groupings of stripes, bands, and eye-spots must have appeared on the butterfly’s wing, how convex or concave curvings of the contour must have come about at certain points, so that the form of a “leaf” and the lines of its venation resulted, how the eye-spots must have been moulded and shunted, so that they produced the effect of rust or other spots on withered leaves. Particular interest attaches to the detailed arguments against the idea that the butterfly must receive some advantage from its “mimicry.” Even the Darwinians have to admit that in a whole series of cases the advantage is not obvious. They talk with some embarrassment of “pseudomimicry.” Some butterflies that are supposed to be protected have the protective markings on the underside, so that these are actually hidden when the insects are flying from pursuing birds. Many of the leaf-like butterflies are not wood-butterflies at all, but meadow species,(47) and so Eimer’s arguments continue.