Chapter II —17— History 



beneath the sea, which may have served as halting-places for plants 

 and for many animals during their migration", Darwin nevertheless 

 considers impossible "such prodigious geographical revolutions within 

 the recent period, as are necessary on the view advanced by Forbes 

 and admitted by his followers. The nature and relative proportions of 

 the inhabitants of oceanic islands are likewise opposed to the belief of 

 their former continuity with continents. Nor does the almost uni- 

 versally volcanic composition of such islands favour the admission that 

 they are the wrecks of sunken continents; — if they had originally 

 existed as continental mountain ranges, some at least of the islands 

 would have been formed, like other mountain summits, of granite, 

 metamorphic schists, old fossiliferous and other rocks, instead of con- 

 sisting of mere piles of volcanic matter" (pp. 505-6). 



Starting from these premises, Darwin proceeds, with the pains- 

 taking care so characteristic of him, to assemble facts on the distribu- 

 tion of plants and to test some of them experimentally. He presents 

 data of experiments on the resistance of seeds to the action of sea- 

 water and on the length of time fruits or parts of plants with fruits 

 may float, and he recounts his observations on the transport of seeds 

 on drift timber and icebergs and also by birds, on the distribution of 

 fresh-water plants and animals, and on the inhabitants of islands and 

 their relation to those of the nearest mainland. 



Nevertheless, despite the existence of such means of seed dispersal, 

 he concludes: "The floras of distant continents would not by such 

 means become mingled; but would remain as distinct as they now 

 are." In spite of the occasional cases of seeds being transported across 

 the ocean, "how small would be the chance of a seed falling on favor- 

 able soil, and coming to maturity! . . . Out of a hundred kinds of 

 seeds or animals transported to an island, even if far less well-stocked 

 than Britain, perhaps not more than one would be so well fitted to its 

 new home as to become naturalised. But this is no valid argument 

 against what would be effected by occasional means of transport, dur- 

 ing the long lapse of geological time, whilst the island was being 

 upheaved, and before it had become fully stocked with inhabitants. 

 On almost bare land, with few or no destructive insects or birds living 

 there, nearly every seed which chanced to arrive, if fitted for the 

 climate, would germinate and survive" (pp. 514-5). 



Subsequent authors, however, gave to such chance factors primary 

 significance, resorting to them in all cases of an otherwise inexplicable 

 station separated from the main area, not reahzing that in reality they 

 were merely substituting such chance transport for the dogma of 

 multiple centers of species creation without supplying any factual 

 proof. From this impasse only in our own times has there been found 

 a way out, in the form of Wegener's hypothesis of continental drift, 

 according to which, assuming the permanence not of the separate 

 continents and oceans but of the areas occupied by them, we are at 

 the same time able to consider that they were formerly connected, thus 

 solving a number of puzzling problems of biogeography. 



Not less important and instructive for subsequent investigators 

 were Darwin's views on the dispersal of plants during the Ice Age or 

 Glacial Period, on the presence of identical species on isolated moun- 



