First, let us see a little of the history of the matter. In the first third of the nineteenth century a number of pre-Darwinian ideas of evolution flitted about like ghosts in natural philosophy, as I have already said. The evolutionary ideas of Goethe and Lamarck are well known to-day. Another thinker of great influence was Lorentz Oken, who established the custom of holding scientific congresses. Oken had been constantly occupied with embryology, the science of the development of the individual organism. He was at all events acquainted with all that was known at the time on the subject. I open an old volume, wretchedly printed on blotting-paper, of Oken’s _General Natural History for all Readers_ (1833), and turn to a passage in the fourth volume (the first to be issued) on page 470. We read that the caterpillar of the butterfly resembles the animal form at a stage of development that lies below the insect—the worm. Oken says: “There is no doubt that we have here a striking resemblance, and one that justifies us in thinking that the development in the ovum is merely a repetition of the story of the creation of the animal groups.” Oken was quite aware that the chick in the egg had gill-slits like the fish. He bases his idea on that fact. He was very close indeed to the theory that Haeckel has so wonderfully elaborated. However, he was greeted with laughter. His theory was treated as an absurdity from 1833 to 1866. It cannot be denied that he was himself partly to blame for this. Oken made two serious mistakes. On both points Haeckel is perfectly clear and sound. Moreover, the theory of natural evolution that made it possible for us to speak of “ancestors” was still a Cinderella in the days of Oken. No sooner was it rehabilitated than the principle of the old theory of embryonic forms returned once more. Darwin himself at once appealed to it, but it was reserved for Haeckel to develop its full importance. He corrected it in two particulars. Oken and his admirers had made an unfortunate mistake. They believed in a genealogical tree of all living things, but they conceived it on the lines of the old classification. Linné had enumerated in succession: mammals, birds, amphibia, fishes, insects, and worms. He put them in one straight line, which is certainly the best arrangement for general purposes. But when Oken came with the idea of natural evolution, he at once took this series as the outline of a genealogical tree. The mammals descended from the birds; the fishes from the insects; and so on. If that were really the case, the highest animals would be expected to reproduce all the animal and plant stages in the course of their embryonic development, on the lines of the theory. The human being would have to be, successively, not only a lizard and a fish, but even a bird, a beetle, a crab, and so on. This was by no means borne out by the facts, and so the theory seemed to be discredited.