To appreciate the present position of biological science, it is necessary to have a clear conception of the history. For this, abundant historical materials are indeed available, and reach their highest level in the standard works of Sachs and Carus. Such detailed histories, however, produce, by their very completeness, a measure of embarrassment. Moreover, the existence of numerous distinct lines of research, often equally prominent at the same time and in the same work, is apt to obscure the fact that the science has really had a simple and natural evolution. What the student demands is not so much any detailed chronological survey, but rather a sketch which will show how the whole system of modern biology, with its increasingly exhaustive analysis of detail, lies within a few essential lines of research, as laid down by a definite succession of original thinkers.