Ehrenberg's theory of life was entirely wrong, and was radically destroyed in the hour of its birth (1838) by the cell-theory which was then formulated, and to which he never became reconciled. Once Matthias Schleiden had shown the composition of all the plants, tissues, and organs from microscopic cells, the last structural elements of the living organism, and Theodor Schwann had done the same for the animal body, the theory attained such an importance that Kölliker and Leydig based on it the modern science of tissues, or histology, and Virchow constructed his cellular pathology by applying it to diseased human beings. These are the most important advances of theoretical medicine. But it was still a long time before the difficult question of the relation of these microscopic beings to the cell was answered. Carl Theodor von Siebold had already maintained (in 1845) that the real infusoria and the closely related rhizopods were _unicellular organisms_, and had distinguished these _protozoa_ from the rest of the animals. At the same time, Carl Naegeli had described the lowest algæ as "unicellular plants." But this important conception was not generally admitted until some time afterwards, especially after I brought all the unicellular organisms under the head of "protists" (1872), and defined their psychic functions as the "cell-soul." I was led to make a very close study of these unicellular protists and their primitive cell-soul through my research on the radiolaria, a very remarkable class of microscopic organisms that float in the sea. I was engaged most of my time for more than thirty of the best years of my life (1856-87) in studying them in every aspect, and if I came eventually to adopt a strictly Monistic attitude on all the great questions of biology, I owe it for the most part to my innumerable observations and uninterrupted reflections on the wonderful vital movements that are disclosed by these smallest and frailest, and at the same time most beautiful and varied, of living things.