The geographical distribution of animals, with a study of the relations ...

36 DISTRIBUTION OF ANIMALS. [part r.which greatly increases its importance, namely, that the meanheight of the land is very small compared with the mean depthof the sea. It has been estimated by Humboldt that the meanheight of all the land surface does not exceed a thousand feet,owing to the comparative narrowness of mountain ranges and thegreat extent of alluvial plains and valleys ;the ocean bed, on thecontrary, not only descends deeper than the tops of the highestmountains rise above its surface, but these profound depths arebroad sunken plains, while the shallows correspond to the mountainranges, so that its mean depth is, as nearly as can be estimated,twelve thousand feet. 1Hence, as the area of water is threetimes that of the land, the total cubical contents of the land,above the sea level, would be only -^ that of the waters whichare below that level. The important result follows, that whereasit is scarcely possible that in past times the amount of land surfaceshould ever greatly have exceeded that which now exists, it isjust possible that all the land may have been at some timesubmerged ; and therefore in the highest degree probable thatamong the continual changes of land and sea that have beenalways going on, the amount of land surface has often beenmuch less than it is now. For the same reason it is probablethat there have been times when large masses of land have beenmore isolated from the rest than they are at present; just asNorth America were submerged, orSouth America would be ifas Australia would become if the Malay Archipelago were tosink beneath the ocean. It is also very important to bear inmind the fact insisted on by Sir CharlesLyell, that the shallowparts of the ocean are almost always in the vicinity of land ;that an amount of elevation that would make little differenceandthe bed of the ocean, would raise up extensive tracts of dry landin the vicinity of existing continents. It is almost certain,therefore, that changes in the distribution of land and seamust have taken place more frequently by additions to, ortoiThis estimate has been made for me by Mr. Stanford from the materialsused in delineating the contours of the ocean-bed on our general map. Itembodies the result of all the soundings of the Challenger, Tuscarora, andother vessels; obtainable up to August, 1875.