DISTRIBUTION OF BRITISH PLANTS. 327 



kind of difference is known to exist in species discovered 

 in a fossil state, when found at wide distances from each 

 other, which, like those last named, are not identical, but 

 only representative. 



And then attention is drawn to the opposite truth, that 

 when certain conditions, suited to particular plants, may be 

 traced over a continuous area (whether in the present state 

 of the surface of the globe, or, by the assistance of geology, 

 in some previous and far different one) species which are 

 identical range throughout. 



The inference which is drawn from these three facts is, 

 that when we find plants of positively identical species 

 growing in lands which are near to, but severed from, each 

 other, these lands must at some time have been connected ; 

 and that over the continuous area which they formed, plants 

 gradually spread from one original centre. 



There are many naturalists who, though they regard spe- 

 cies as permanently distinct, nevertheless account for their 

 extensive distribution by a plurality of parents, or original 

 centres ; but the contrary opinion, as was said before, is a 

 necessary basis of Professor Forbes' s theory. 



He further thinks that, in the first place, the character 

 of the English flora is not distinctive enough to warrant a 



