36 DISTRIBUTION OF ANIMALS. [PART I. 
which greatly increases its importance, namely, that the mean 
height of the land is very small compared with the mean depth 
of the sea. It has been estimated by Humboldt that the mean 
height of all the land surface does not exceed a thousand feet, 
owing to the comparative narrowness of mountain ranges and the 
great extent of alluvial plains and valleys; the ocean bed, on the 
contrary, not only descends deeper than the tops of the highest 
mountains rise above its surface, but these profound depths are 
broad sunken plains, while the shallows correspond to the moun- 
tain ranges, so that its mean depth is, as nearly as can be esti- 
mated, twelve thousand feet.!. Hence, as the area of water is three 
times that of the land, the total cubical contents of the land, 
above the sea level, would be only 34, that of the waters which 
are below that level.. The important result follows, that whereas 
it is scarcely possible that in past times the amount of land surface 
should ever greatly have exceeded that which now exists, it is 
just possible that all the land may have been at some time 
submerged; and therefore in the highest degree probable that 
among the continual changes of land and sea that have been 
always going on, the amount of land surface has often been 
much less than it is now. For the same reason it is probable 
that there have been times when large masses of land have been 
more isolated from the rest than they are at present; just as 
South America would be if North America were submerged, or 
as Australia would become if the Malay Archipelago were to 
sink beneath the ocean. It is also very important to bear in 
mind the fact insisted on by Sir Charles Lyell, that the shallow 
parts of the ocean are almost always in the vicinity of land; and 
that an amount of elevation that would make little difference to 
the bed of the ocean, would raise up extensive tracts of dry land 
in the vicinity of existing continents. It is almost certain, 
therefore, that changes in the distribution of land and sea 
must have taken place more frequently by additions to, or 
1 This estimate has been made for me by Mr. Stanford from the materials 
used in delineating the contours of the ocean-bed on our general map. It 
embodies the result of all the soundings of the Challenger, Tuscarora, and 
other vessels. obtainable up to August, 1875, 
