Chap. XII.] MEANS OF DISPERSAL. 141 



connected with Europe or Africa, and Europe likewise 

 witli 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 continents, as to have united 

 them within the recent period to each other and to the 

 several intervenmg 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 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 separate, 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 



