GEOGRAPHICAL DISTRIBUTION 331 



dispersal of the same species to the most distant points, and re- 

 moves many a difficulty; but to the best of my judgment we are 

 not authorized in admitting such enormous geographical changes 

 within the period of existing species. It seems to me that we have 

 abundant evidence of great oscillations in the level of the land or 

 sea; but not of such vast changes in the position and extension 

 of our continents, as to have united them within the recent pe- 

 riod to each other and to the several intervening oceanic islands. 

 I freely admit the former existence of many islands, now buried 

 beneath the sea, which may have served as halting-places for 

 plants and for many animals during their migration. In the coral- 

 producing oceans such sunken islands are now marked by rings of 

 coral or atolls standing over them. Whenever it is fully admitted, 

 as it will some day be, that each species has proceeded from a 

 single birthplace, and when in the course of time we know some- 

 thing definite about the means of distribution, we shall be anabled 

 to speculate with security on the former extension of the land. 

 But I do not believe that it will ever be proved that within the 

 recent period most of our continents which now stand quite sepa- 

 rate, have been continuously, or almost continuously united with 

 each other, and with the many existing oceanic islands. Several 

 facts in distribution — such as the great difference in the marine 

 faunas on the opposite sides of almost every continent — the close 

 relation of the tertiary inhabitants of several lands and even seas 

 to their present inhabitants — the degree of affinity between the 

 mammals inhabiting islands with those of the nearest continent, 

 being in part determined (as we shall hereafter see) by the depth 

 of the intervening ocean — these and other such facts are opposed 

 to the admission of such prodigious geographical revolutions 

 within the recent period, as are necessary on the view advanced by 

 Forbes and admitted by his followers. The nature and relative 

 proportions of the inhabitants of oceanic islands are likewise op- 

 posed to the belief of their former continuity of continents. Nor 

 does the almost universally volcanic composition of such islands 

 favor the admission that they are the wrecks of sunken continents; 

 if they had originally existed as continental mountain ranges, 

 some at least of the islands would have been formed, like other 

 mountain summits, of granite, metamorphic schists, old fossilifer- 

 ous and other rocks, instead of consisting of mere piles of vol- 

 canic matter. 



I must now say a few words on what are called accidental means, 

 but which more properly should be called occasional means of 

 distribution. I shall here confine myself to plants. In botanical 



