The year 1877 was a critical one in this respect. In the middle of his struggles Haeckel retained all the simplicity of his nature. He saw that the idea of evolution was triumphing over all obstacles and rapidly securing the allegiance of the best men of the time. On the 18th of September, 1877, he spoke of this with unrestrained delight at the scientific congress at Munich. He described the theory of evolution as “the most important advance that has been made in pure and applied science.” Then Rudolf Virchow delivered a speech at the same congress. There is no doubt whatever that in the period since Virchow had indicated a neutral field in 1863, in which science might effect “its compromise,” Haeckel had boldly invaded that province. In the previous year he had published a little work called _The Perigenesis of the Plastidules, or the Generation of Waves in Vital Particles_. It was delivered in lecture-form at the medical-scientific congress at Jena in November, 1875, and then printed on the occasion of Seebeck’s jubilee, May 9, 1876. Possibly it is the least known of all Haeckel’s works, though in my opinion it is one of the most valuable in regard to the prophetic breadth of its intuition. It essays to establish a theory of heredity. In dealing with this deepest mystery of life psychic factors are pressed into service without reserve. Not only is the cell-soul put into prominence, but the cell in turn is resolved into a number of smaller units, the plastidules. Each plastidule is then conceived as a psychic unity. The souls of the plastidules are endowed with memory; that is the root of heredity. They learn; that is the psychological expression of adaptation. The little work offers a suggestion of a psychology of Darwinism that may very well become the nucleus of the whole Darwinian structure in the twentieth century. But at the time it was quite obvious that a man with such ideas as these was breaking with lusty fist through the sacred net that spread before Virchow’s reserved province. The hour had come, therefore, for Virchow to feel that he must expel the idea of evolution from the whole field of science, and not merely from embryology and anthropology.