672 DISTEIBUTION IN DIFFERENT PAKTS OF THE GLOBE. 



seem to imply that the same was the general law. Analogy would 

 lead us to believe that the extension of species over the earth originally 

 took place on the same plan on which it is'conducted at present when 

 a new island starts up in the midst of the ocean, produced either by 

 a coral reef or a volcano. In these cases the whole surface is not 

 at once overspread with plants, but a gradual progress of vegetation 

 is traced from the accidental introduction of a single seed, perhaps of 

 each species, wafted by winds, or floated by the currents. The re- 

 markable limitation of certain species to single spots on the globe 

 seems to favour the supposition of specific centres. Professor E. 

 Forbes says, the hypothesis of the descent of all the individuals of a 

 species, either from a first pair or from a single individual, and the 

 consequent theory of specific centres being assumed, the isolation of 

 assemblages of individuals from their centres, and the existence of 

 endemic or very local plants, remain to be accounted for. Natural 

 transport, the agency of the sea; rivers, and winds, and carriage by 

 animals, or through the agency of man, are insufficient means in the 

 majority of cases. It is usual to say that the presence of many 

 plants is determined by soil or climate, as the case may be ; but if 

 such plants be found in areas disconnected from their centres by con- 

 siderable intervals, some other cause than the mere influence of soil 

 or climate must be sought to account for their presence. This cause 

 he proposes to seek in an ancient connection of the outposts or isolated 

 areas with the origiaal centres, and the subsequent isolation of the 

 former through geological changes and events, especially those depend- 

 ent on the elevation and depression of land. Selecting the flora of the 

 British Islands for a first illustration of this view. Professor Forbes 

 calls attention to the fact, well known to botanists, of certain species 

 of flowering plants being found indigenous in portions of that area, at 

 a great distance from the nearest assemblages of individuals of the 

 same species in countries beyond it. Thus, many plants peculiar in 

 the British flora to the west of Ireland, have the nearest portion of 

 their specific centres in the north-west of Spain ; others confined with 

 us to the south-west promontory of England, are, beyond our shores, 

 found in the Channel Isles and the opposite coast of France ; the vege- 

 tation of the south-east of England is that of the opposite part of the 

 continent ; and the Alpine vegetation of Wales and the Scotch High- 

 lands is intimately related to that of the Norwegian Alps. The great 

 mass of the British flora has its most intimate relations with that of 

 Germany. He believes, therefore, that these isolated outposts were 

 formerly connected together by chains of land, and that they have 

 been separated by certain geological convulsions. Islands may be 

 considered as the remains of mountain chains, part of the flora of 

 which they still exhibit, and the farther they are from continents the 

 more likely are the plants to be peculiar. 



