For this he wrote the first adequate account in German of the Darwinian theory of natural selection, which drew a warm letter of appreciation from Darwin himself.
The view that instinct is the hereditarily fixed result of habit derived from experience long dominated all inquiry into the subject, but we may now expect to see a renewed and careful study of animal instincts carried out with the view of testing the applicability to each instance of the pure Darwinian theory without the aid of Lamarckism.
Of far-reaching importance was, on the other hand, his foreshadowing of the Darwinian theory in his works on the metamorphosis of plants and on animal morphology.
Not only did Planck oppose the idealism of his confreres; his views were, in another aspect, directly antagonistic to the Darwinian theory of descent, which he specifically attacked in Wahrheit and Flachheit des Darwinismus (Ndrdlingen, 1872).
