383 MEANS OF DISPERSAL. 



disputes that great mutations 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 Em'ope 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 authorized in admit- 

 ting 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 ex- 

 tension of our continents, as to have united them within 

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

 ing 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, aa 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 

 difEerence 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 



