xii GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 345 



vertical scale. If we considered the continents and their 

 adjacent oceans separately they would differ a little, but not 

 very materially, from this diagram ; in some cases the propor- 

 tion of land to ocean -would be a little greater, in others a little 

 less. 



Now, if we try to imagine a process of elevation and 

 depression by which the sea and land shall completely change 

 places, we shall be met by insuperable difficulties. "\Ve must, 

 in the first place, assume a general equality between ele- 

 vation and subsidence during any given period, because if 

 the elevation over any extensive continental area were not 

 balanced by some subsidence of approximately equal amount, 



Diagram of proportionate mean height of Land and depth of Oceans. 



\ 





Land 



Area. -28 of area 



of Globe. 



— : 









... 















Ocean 

 Area. -12 of area of Globe. 



Fig. 32. 



an unsupported hollow woidd be left under the earth's crust. 

 Let us now suppose a continental area to sink, and an adjacent 

 oceanic area to rise, it will be seen that the greater j:>art of 

 the land will disappear long before the new land has approached 

 the surface of the ocean. This difficulty will not be removed 

 by supposing a portion of a continent to subside, and the 

 immediately adjacent portion of the ocean on the other side 

 of the continent to rise, because in almost every case we find 

 that within a comparatively short distance from the shores of 

 all existing continents, the ocean floor sinks rapidly to a depth 

 of from 2000 to 3000 fathoms, and maintains a similar depth, 

 generally speaking, over a large portion of the oceanic areas. 

 In order, therefore, that any area of continental extent be 

 upraised from the great oceans, there must be a subsidence of 

 a land area five or six times as great, unless it can be shown 

 that an extensive elevation of the ocean floor up to and far 



