These arguments did not convince Lyell; and they have only received an almost universal acceptance after the confirmatory evidence afforded by the voyage of the _Challenger_. Dredgings over many parts of the ocean showed that all the continental deposits are collected on a fringing shelf not more than 200 miles wide, and that beyond this in the ocean bed proper an entirely different kind of deposit is accumulating, composed of the shells, bones, and teeth of swimming or floating organisms, or the products of their decomposition, of volcanic and cosmic dust, and the products--_e.g._ manganese dioxide--of the decomposition of these and of floating pumice. Hence, the depths of the ocean afford no indications of a lost continental area, but are covered by a peculiar deposit unknown among the rocks of continents which were formed in comparatively shallow water round and not far from coasts, or in land-locked or nearly land-locked seas like the Mediterranean. [Sidenote: EARLY CORRESPONDENCE.] On July 20th, 1856, he wrote to Asa Gray, giving some account of his views, and stating his belief in evolution, but only hinting at natural selection. About this time we meet with evidence of the great difficulty with which Darwin’s ideas were thoroughly understood, even by his intimate friends, to whom he often wrote on the subject. Later on, when the “Origin of Species” was published, although the arguments in favour of natural selection were given in considerable detail, many years passed before the theory itself was understood by the great body of naturalists. This particular case of misunderstanding is of such great interest that it is desirable to consider it in detail.