He never pretended that there was the faintest corroborating evidence visible to the microscope in the organ, in the blood, or in the germ cell. It was not an accounting for what is, but for what it seemed possible to him might be. This theory of Pangenesis, in the shape in which Darwin promulgated it, has dropped out of consideration almost entirely. DeVries of recent years has revised it, but with distinct modifications, and most biologists pay no attention to it. There is a school of biologists, headed by Weissman, who have come to be known as Neo-Darwinians. These men have insisted that Natural Selection, if properly understood and developed, is quite sufficient to account for the fact of evolution, including the appearance of variations. Weissman himself is a microscopist of more than common skill. He is thoroughly accomplished in the most modern methods of killing, fixing, staining, and mounting. This worker's acquaintance with the intimate structure of the cell is probably as great as that of any other man in the world. Weissman asserts that he has seen inside the nucleus all the machinery necessary to explain how the father hands over his qualities to his children. He insists, equally strongly, that this process is such that no father can hand to his child any qualities which he himself did not have at least in potentiality at his birth. Everything the individual acquires during his lifetime is his own possession, which he may use and develop to the utmost extent, but it dies with him. His children, born after he possesses it, can no more inherit it than those born before. Weissman expressed this in his famous statement that "There is no inheritance of acquired characters." The biological world has had no shock equal to this since Darwin's time, and there are few other questions to which scientists to-day return with such constant vigor.