570 ORIGINAL VEGETATION OF THE GLOBE. 



ous such centres situated in different parts of the world, each centre 

 being the seat of a particular number of species ; they thus admit great 

 vegetable migrations similar to those of the human races. Those who 

 adopt the latter view, recognise in the distribution of plants some of 

 the last revolutions of our planet, and the action of numerous and 

 varied forces which impede or favour the dissemination of vegetables 

 in the present day. They endeavour to ascertain the primitive flora 

 of countries, and to trace the vegetable migrations which have 

 taken place. Daubeny says, that analogy favours the supposition 

 that each species of plant was originally formed in some particular 

 locality, whence it spread itself gradually over a certain area, rather 

 than that the earth was at once, by the fiat of the Almighty, 

 covered with vegetation in the manner we at present behold it. The 

 human race arose from a single pair, and the distribution of plants 

 and animals over a certain definite area, would 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 remarkable limitation of 

 certain species to single spots on the globe, seems to favour the suppo- 

 sition 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 considerable intervals, some other cause than the mere in- 

 fluence of soil or climate must be sought to account for their presence. 

 This cause he proposes to seek in an ancient connexion of the outposts 

 or isolated areas with the original centres, and the subsequent isolation 

 of the former through geological changes and events, especially those 

 dependent 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 



