According to the Wegener hypothesis, the continents originally formed a single block. This broke into sections in the Carboniferous, and, during the geologic periods that followed, the continents drifted apart. Wegener, as many others had done before him, originally conceived the idea that the similarity in the coast lines of Europe-Africa and America indicated connection previous to the Carboniferous. It is to his credit, however, that he investigated in detail all the possible consequences of this hypothesis. He considered especially the present-day relative movements of parts of the earth’s crust, and he started investigations of the movement of Greenland relative to Europe. He emphasized the importance of determining and explaining the changes in the land-bridge connections between the continents, which occurred from time to time, because of their bearing on the problem of continental origin and history. Similarities in the structure of certain regions, now far apart, . . .