404 ORIGIN OF SPECIES 



one to the other. No geologist disputes that great muta- 

 tions of level have occurred within the period of existing 

 organisms. Edward Forbes insisted that all the islands in 

 the Atlantic must have been recently connected with Europe 

 or Africa, and Europe likewise with America. Other authors 

 have thus hypothetically bridged over every ocean, and 

 united almost every island with some mainland. If indeed 

 the arguments used by Forbes are to be trusted, it must be 

 admitted that scarcely a single island exists which has not 

 recently been united to some continent. This view cuts the 

 Gordian knot of the dispersal of the same species to the most 

 distant points, and removes many a difficulty; but to the best 

 of my judgment we are not authorised 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 con- 

 tinents, as to have united them within the recent period 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 migra- 

 tion. 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 something definite about 

 the means of distribution, we shall be enabled 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 sep- 

 arate, 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 con- 

 tinent, — the close relation of the tertiary inhabitants of sev- 

 eral 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 



