358 MEANS OF DISPERSAL. 



MEANS OF DISPERSAL. 



Sir C. Lyell and other authors have ably treated this sub* 

 iect. I can give here only the briefest abstract of the more 

 inportant facts. Change of climate must have had a power- 

 fu influence on migration. A region now impassible to 

 certain organisms from the nature of its climate, might have 

 beei a high road for migration, when the climate was differ- 

 ent, J shall, however, presently have to discuss this branch 

 of tne subject in some detail. Changes of level in the land 

 must also have been highly influential : a narrow isthmus 

 now separates two marine faunas ; submerge it, or let it 

 formerly have been submerged, and the two faunas will now 

 blend together, or may formerly have blended. Where ths 

 sea now extends, land may at a former period have con- 

 nected islands or possibly even continents together, and 

 thus have allowed terrestrial productions to pass from one 

 to the other. No geologist disputes that great mutations 

 of level have occurred within the period of existing organ- 

 isms. Edward Forbes insisted that all the islands in the 

 Atlantic must have been recently connected with Europe or 

 Africa, and Euiope likewise with America. Other authors 

 have thus nypcthetically bridged over every ocean, and 

 united almost every island with some mainland. If, iudeed, 

 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 01 

 sea ; but not of such vast changes in the position and exten- 

 sion of our continents, 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 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 



