346 DARWINISM chap. 



above the surface could occur without an equivalent depression 

 elsewhere. The fact that the waters of the ocean are sufficient 

 to cover the whole globe to a depth of two miles, is alone 

 sufficient to indicate that the great ocean basins are permanent 

 features of the earth's surface, since any process of alterna- 

 tion of these with the land areas would have been almost 

 certain to result again and again in the total disappearance of 

 large portions, if not of all, of the dry land of the globe. But 

 the continuity of terrestrial life since the Devonian and Car- 

 boniferous periods, and the existence of very similar forms in 

 the corresponding deposits of every continent — as well as the 

 occurrence of sedimentary rocks, indicating the proximity of 

 land at the time of their deposit, over a large portion of the 

 surface of all the continents, and in every geological period — 

 assure us that no such disappearance has ever occurred. 



Oceanic and Continental Areas. 



When we speak of the permanence of oceanic and conti- 

 nental areas as one of the established facts of modern research, 

 we do not mean that existing continents and oceans have 

 always maintained the exact areas and outlines that they now 

 present, but merely, that while all of them have been under- 

 going changes in outline and extent from age to age, they 

 have yet maintained substantially the same positions, and 

 have never actually changed places with each other. There 

 are, moreover, certain physical and biological facts which 

 enable us to mark out these areas with some confidence. 



We have seen that there are a large number of islands 

 which may be classed as oceanic, because they have never 

 formed parts of continents, but have originated in mid-ocean, 

 and have derived their forms of life by migration across the 

 sea. Their jDeculiarities are seen to be very marked in com- 

 parison with those islands which there is good reason to 

 believe are really fragments of more extensive land areas, and 

 are hence termed "continental." These continental islands 

 consist in every case of a variety of stratified rocks of various 

 ages, thus corresponding closely with the usual structure of 

 continents ; although many of the islands are small like 

 Jersey or the Shetland Islands, or far from continental 

 land like the Falkland Islands or New Zealand. They all 



