Darwinian ideas were fermenting intensely in his mind at that time. The most audacious practical and theoretical problems arose from the fundamental theory, and forced themselves on him at every moment. A great deal was sketched in outline in the Stettin speech, but the serious scientific work would have to be begun on his return to Jena, in his view. First, he thought, two features of Darwin’s system must be given a completely new and original complexion. Firstly, the bottom of the tree, where life begins. Secondly, the crown of all terrestrial evolution: the manner in which man is connected with the tree. It was his philosophic vein that settled both points, the philosophy of unity that sought to replace God by natural development, both below and above, in regard to the primitive cell and in regard to man. But the way in which he set about it was very far removed from all conventional philosophy. The whole rigour of his professional zoology found expression in it. And that was really the novelty of it. The same conclusions might have been drawn by any dozen ordinary philosophers, once they got on the right track. Even they could see that, if two and two are four, one and one are two, and three times three nine. Haeckel went very differently, and much more profoundly, to work. [Illustration: ERNST HAECKEL, 1890. _From a relief-portrait modelled by Kopf, of Rome._ ] As an old pupil of Virchow’s he applies the cell-theory to Darwinism—in the lower stage. The first living things, the roots of the great tree of life, consisted of a single cell. The logic of the cell-theory itself went as far as this. But is the individual cell the simplest of all living forms? Here there was a long-standing controversy as to definitions. At first the cell was regarded literally as a kind of chamber, like the cell in the honeycomb. Then it was found that the jelly-like, mobile matter within the cell-chamber was the essential element, the vehicle of life. Finally, it was possible to conceive this slimy substance without any firm membrane, without a chamber. Inside it, however, there was always (it was then thought) a thick and hard substance, the nucleus. If that was the fundamental and only really essential form, the Darwinian primitive and initial type of all terrestrial life must have been a similar drop of living matter with a solid central nucleus, a nucleated individual cell.