Chap. XI. MEANS OF DISPEESAL. 357 



No geologist will dispute that great mutations of level, 

 have occurred within the period of existing organisms. 

 Edward Forbes insisted that all the islands in the 

 Atlantic must recently have been connected with 

 Europe or Africa, and Europe likewise with America. 

 Other authors have thus hypothetically bridged over 

 every ocean, and have united almost every island to 

 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 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 of level in 

 our continents ; but not of such vast changes in their 

 position and extension, as to have united them within 

 the recent period to each other and to the several inter- 

 vening oceanic islands. I freely admit the former ex- 

 istence 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, as I believe, by rings of coral or atolls standing 

 over them. Whenever it is fully admitted, as I believe 

 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 dis- 

 tribution, 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 continents which are now quite separate, 

 have been continuously, or almost continuously, united 



