CONDITIONS ON WHICH DISTRIBUTION DEPEND. 51 



spreading, as does the fairy ring extend itself, but with 

 this difference that instead of dying out in the centre, and 

 on the ground on which they first grew, they continued to 

 grow, and to grow it may be with increased luxuriance, in 

 the locality in which they have longest been produced and 

 reproduced abundantly ; and that in some of the out- 

 posts where they now grow isolated from their congeners, 

 they may have been severed from the centres whence 

 they sprung by geological changes, such as the elevation 

 or the depression of intervening land. 



1 am aware that these views have been opposed by 

 Schouw and others ; but the facts, upon the observation 

 of which they are founded, remain the same and unim- 

 pugned j it is these with which I am concerned ; my 

 allusion to the views of Forbes is only incidental ; and they 

 are cited in aid of my illustration. Schouw, it is stated by 

 Professor Balfour, considered that the existence of the same 

 species in far distant countries is not to be accounted for 

 on the supposition of a single centre for each species. 

 The Ubual means of transport, and even the changes which 

 have taken place by volcanic and other causes, are inade- 

 quate, he thought, to explain why many species are common 

 to the Alps and the Pyrenees, on the one hand, and to the 

 Scandinavian and Scotch mountains on the other, without 

 being found on the intermediate plains and hills; why the 

 flora of Iceland is nearly identical with that of the Scan- 

 dinavian mountains; and why Europe and North America, 

 especially the northern parts, have various plants in com- 

 mon, to which they have not been communicated by human 

 aids. Still greater objections to this mode of explanation, he 

 thinks, are founded on the fact that there are plants at the 

 Straits of Madgalhaeris, and in the Falkland and other 

 Antarctic islands, which belong to the flora of the Arctic 

 pole; and that several European plants appear in New 

 Holland, Van Dieman's Land, and New Zealand, which 

 are not found in intermediate countries. Schouw, there- 

 fore, supposes that these were originally not one but many 

 primary individuals of a species. 



