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. We 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. 
Ocean 
Area. •72 of area of Globe. 
Fig. 32. 
an unsupported hollow would 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 part 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 v r e 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 
