In 1859 Retzius announced that he found a close resemblance between the skulls of the Guanches of the Canaries and the Guaranas of Brazil, and recalled the Atlantis story to explain it.[421] In 1846 Forbes declared his belief in the former existence of a bridge of islands in the North Atlantic, and in 1856 Heer attempted to show the necessity of a similar connection from the testimony of palæontological botany. In 1860, Unger deliberately advocated the Atlantis hypothesis to explain the likeness between the fossil flora of Europe and the living flora of America, enumerating over fifty similar species; and Kuntze found in the case of the tropical seedless banana, occurring at once in America before 1492 and in Africa, a strong evidence of the truth of the theory.[422] A condensed review of the scientific side of the question is given by A. Boué in his article _Ueber die Rolle der Veränderungen des unorganischen Festen im grossen Massstabe in der Natur_.[423] The deep-sea soundings taken in the Atlantic under the auspices of the governments of the United States, England, and Germany resulted in discoveries which gave a new impetus to the Atlantis theory. It was shown that, starting from the Arctic plateau, a ridge runs down the middle of the Atlantic, broadening toward the Azores, and contracting again as it trends toward the northeast coast of South America. The depth over the ridge is less than 1,000 fathoms, while the valleys on either side average 3,000; it is known after the U. S. vessel which took the soundings as the Dolphin ridge. A similar though more uniformly narrow ridge was found by the “Challenger” expedition (1873-76), extending from somewhat north of Ascension Island directly south between South America and Africa. It is known as the Challenger ridge. There is, beside, evidence for the existence of a ridge across the tropical Atlantic, connecting the Dolphin and Challenger ridges. Madeira, the Canaries, and the Cape Verde Islands are cut off from these ridges by a deep valley, but are connected by shoals with the continent. Upon the publication of the Challenger chart (_Special Report_, vii. 1876), those who favored the theory of communication between the continents were not slow to appropriate its disclosures in their interests (_Nature_, Dec. 21, 1876, xv. 158). In March, 1877, W. Stephen Mitchell delivered a lecture at South Kensington, wherein he placed in juxtaposition the theory of Unger and the revelations of the deep-sea soundings, when he announced, however, that he did not mean to assert that these ridges had ever formed a connecting link above water between the continents.[424] Others were less cautious,[425] but in general this interpretation did not commend itself as strongly to conservative men of science as it might have done a few years before, because such men were gradually coming to doubt the fact of changes of great moment in the earth’s surface, even those of great duration.