Biologist, born at Frankfort-on-the-Main; studied medicine at Göttingen; devoted himself to the study of zoology, the first-fruit of which was a treatise on the "Development of Diptera," and at length to the variability in organisms on which the theory of descent, with modifications, is based, the fruit of which was a series of papers published in 1882 under the title of "Studies on the Theory of Descent"; but it is with the discussions on the question of heredity that his name is most intimately associated. The accepted theory on the subject assumes that characters acquired by the individual are transmitted to offspring, and this assumption, in his "Essays upon Heredity," he maintains to be wholly groundless, and denies that it has any foundation in fact; heredity, according to him, is due to the continuity of the germ-plasm, or the transmission from generation to generation of a substance of a uniform chemical and molecular composition; b . 1834.