II PERMANENCE OF THE GREAT OCEANIC BASINS 31 



means of communication between these continents. All 

 that is necessary to maintain, therefore, is that existing con- 

 tinents with their included seas, and their surrounding 

 oceanic waters as far as the 1,500-fathom, or in some ex- 

 treme cases the 2,000-fathom line, mark out the areas 

 within which the continental lands of the globe have been 

 built up ; while the oceanic areas beyond the 2,000-fathom 

 line, constituting, according to Mr. Murray's data, 71 per 

 cent, of the whole ocean, have almost certainly been 

 ocean throughout all known geological time.^ 



It will now be seen that this is a problem which deals 

 with the very broadest contrasts of the earth's surface, and 

 that its fundamental data are on so vast a scale as not to 

 be materially affected by the smaller details of physical 

 geography, or by differences of opinion as to the exact 

 meaning of certain terms. Whether a particular island is 

 more correctly classed as oceanic or continental, whether 

 a certain portion of the ocean should be placed within the 

 oceanic or the continental area, and whether certain rocks 

 were formed in very deep or in comparatively shallow water, 

 are of slight importance, except in so far as they may 

 throw light on the real question, which is, whether the 

 vast expanses of ocean beyond the 1,000-fathom line 

 (as shown by the map in Chap. XII. of my Darwinism), 

 forming about 92 per cent, of the whole oceanic area, have 

 ever been occupied, or extensively bridged over, by con- 

 tinental land. It is towards the solution of this great 

 problem that I now propose to submit certain general 

 considerations which appear to me to lie at the root of 

 the whole matter. 



Comparison of Oceanic and Continental Masses. — In the 

 paper already referred to. Sir John Murray has carefully 

 estimated both the area of the land and of the water on 

 the earth's surface, and their bulk as deduced from the 

 best available data. Taking the whole area of the globe 

 as 100, he finds the land surface to be 28, the water sur- 

 face 72. But the mean height of the land above sea-level 



^ Sir J. Murray gives his results for oceans and inland seas together. 

 The above percentage refers to the oceans only as usually understood ; 

 but the difference is not great. 



