MEANS OF DISPERSAL. II7 



time to fixed resting places or homes, and probably many 

 possess the same habit ; ^ it is doubtless true, however, 

 as Binney argued, that — although no individual can 

 have made any considerable progress — a species in the 

 course of the countless generations which have existed 

 may have wandered to vast distances from its original 

 birth-place.- Vast changes of climate, it will be re- 

 membered, have occurred from time to time, and, as 

 Mr. Darwin has remarked, a region now impassable 

 owing to the nature of its climate may have been " a 

 high-road for migration when the climate was different." 

 It will be remembered also that a great many changes 

 in the relative positions of land and sea have certainly 

 taken place during the existence of living genera, so 

 that uninterrupted highways may once have existed 

 where arms of the sea now absolutely preclude unaided 

 migration : Forbes held indeed, as Darwin puts it, that 

 " all the islands in the Atlantic must have been recently 

 connected with Europe or Africa, and Europe likewise 

 with America," and other authors, it is said, "have thus 

 hypothetically bridged over every ocean, and united 

 almost every island to some mainland ;" it is certain, 

 however, that the enormous geographical changes here 

 implied have never really occurred during the period of 

 recent organisms, but geologists are agreed that great 

 mutations of level have taken place within this period, 



' On the " Faculty of homing in Gastropods," see " Naturalist, ' 

 1890, pp. 307-18. 



'A. Binney, '-'Terrestrial air-breathing Mollusks," i. (1851), 

 p. 105. 



