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; 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 usual 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 Arctie 
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. 
