EARLY CORRESPONDENCE. 55 



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 di- 

 oxide 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. 



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, al- 

 though 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 ot 



