telis quibusdam Astaci fluviatilis_, and was printed in March, 1857. It appeared the same year in an enlarged form in Müller’s _Archiv_, with the title _The Tissues of the Craw-fish_. On March 7th he received his medical degree, Ehrenberg, the great authority on the infusoria, presiding. In the customary way the young doctor had to announce and defend several theses. One of them is rather amusing in view of later events. He most vigorously contested the possibility of “spontaneous generation.” The meaning of the phrase is that somewhere or at some time a living thing, animal or plant, has arisen, not in the form of a seed or germ or sprout from a parent living thing, but as a direct development out of dead, inorganic matter. Haeckel had not made a personal study of the subject. What he said in his thesis was merely a faithful repetition of Müller’s opinion. At that time it was believed that science had empirically disproved spontaneous generation. An old popular belief held that fleas and lice were born every day from non-living dirt and dust, but that had been refuted long before. No egg, no animal: every living thing develops from an egg. This had been laid down as a fixed rule. When the microscope revealed an endless number of tiny creatures in every drop of stagnant water, in the air and the dust and the soil, it was a question whether the rule was not wrong. Surely these simplest of all living things, apparently, were born by spontaneous generation? However, the question was believed to have been settled in two ways. Schwann, the co-discoverer of the cell-theory, had made certain experiments which seem to prove directly that even these tiny beings, the infusoria and bacteria, were never formed in a vessel containing water and dead matter, if it had been carefully assured beforehand that the minute living germs of these animals that floated in the air could not penetrate into the vessel. At the same time Ehrenberg and others stoutly denied that the infusoria were the “simplest” organisms, or that they could conceivably be born in that way. They declared that the infusoria were “perfect organisms” in spite of their smallness. The belief that these tiny creatures consisted of “one cell,” and so formed, as it were, the ultimate elements of the plant and animal worlds on the lines of the cell-theory, was seriously menaced, and apparently on the way to be destroyed. Finally, the tapeworm and similar parasites had been declared to evolve by a kind of spontaneous generation from the contents of the intestines. But this also was proved to be untrue. Thus there was ample material for a solid dogma: there was no such thing as spontaneous generation. The dogma, moreover, harmonised with the prevailing belief in a special vital force and a radical distinction between the living and the dead, which was still shared in a subtle form by even a man like Müller. The dogma was formulated. Spontaneous generation was struck out of the scientific vocabulary as