GEOGRAPHICAL DISTRIBUTION 321 



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 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 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 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, 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 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 

 continents which are now 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 differ- 

 ence 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, — a certain degree of relation 

 (as we shall hereafter see) between the distribution of 

 mammals and the depth of the sea, — these and other 

 such facts seem to me opposed to the admission of such 

 prodigious geographical revolutions within the recent 



