330 MEANS OF DISPERS^IL. Chap. XI. 



\vill now Ijlend or may fonnerly have blended : where the sea 

 now extends, land may at former pciiods have connected 

 islands or possibly even continents together, and thus have 

 allowed terrestrial productions to pass from one to the other. 

 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 have 

 been recently connected with Europe or Africa, and Europe 

 likewise with America. Other authors have thus hj'potheti- 

 cally bridged over every ocean, and united almost every island 

 to some main-land. 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 conti- 

 nent. This view cuts the Gordian knot of the dispersal of the 

 same species to the most distant points, and removes many a 

 dilhculty : but, to the best of my judgment, we are not author- 

 ized in admitting such enormous geograplucal 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 exten- 

 sion of our continents, as to have imited 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-pi'oducing oceans such sunken islands 

 are now marked by rings of coral or atolls standing over them. 

 AVhcneverit is fully admitted, as no doubt it Avill some day be, 

 that each species has proceeded from a single birthplace, and 

 Avlien 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 be- 

 lieve that it Avill 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 oth- 

 er, and Avith 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 continent — the close re- 

 lation 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 con- 

 tinent, being in ])art determined (as we shall hereafter see) by 

 the depth of the intervening ocean — these and other such facts 



