Liebig's contradictions or to his joke. He bided his time. It came in a quarter of a century, when Schwann saw his theoryextended to cover a great variety of chemical and pathological actions, and almost universally accepted; and received in 1878, from Pasteur, who had carried it to its highest triumph, a letter recognizing him as the one who had opened the road by following which his own wonderful discoveries were made.
These researches might of themselves have sufficed to make the name of Schwann illustrious. But they are relatively but little known because their fame has been dimmed in the face of the incomparable luster of his great discovery of the cell theory. The publication of the book in which the basis of this theory was laid down opened a new era in biological study. We might search in vain, says Simon, in his History of the Natural Sciences, for an example of a more radical revolution in the direction and character of scientific labors than that which was effected in 1838 and 1839 by the publication of Schwann's histogenetic theory. The revolution was sudden, and triumphed, we might say, without resistance. As Henle has remarked, the scientific soil in which this theory took root and grew had been prepared from two different points of view: one, philosophical or ideal; the other, positive or histological. The philosophical preparation dated from the beginning of the study of Nature, and was illustrated in the propensity of the human mind to look for some simple cause for the diversity of phenomena. To this we owe the monads of Epicurus and Leibnitz, Oken's philosophy of Nature, and many other efforts ancient and modern. On the other side, certain histological researches, often very modest, but coming close to the facts, had prepared a way for the cell theory. Robert Brown had discovered the cellular nucleus in 1831; Mirbel, Von Mohl, and Unger had demonstrated that the organs and tissues of plants were at bottom aggregations of cells in different degrees of transformation. Schleiden had been studying the important part played by the nucleus in the formation of vegetable cells, and had given it the name of cytoblast; and other authors had found in animals organs formed of cells. But these were as yet only isolated facts.
Schwann has himself told the story of the way the idea of his discovery first occurred to him. "One day," he says, "when I was dining with M. Schleiden, that illustrious botanist spoke of the important part which the nucleus plays in the development of plant-cells. I at once recollected that I had seen a similar organ in the cells of the dorsal cord, and instantly appreciated the extreme importance the discovery would have if I could show that it plays the same part there as the nucleus of plants in the development of vegetable cells. It must follow, in fact, in consequence of the identity of so characteristic phenomena, that the
