XX 



mode in which certain plants have spread over the globe, 

 and as concerns the occurrence of others in detached islands, 

 to which it is difficult to understand their transmission. 



De Candolle indeed himself evinced his consciousness of 

 these perplexing cases, in pointing out, as he has done, the 

 existence of the same aquatic plants in two distinct basins 

 destitute of all possible communication; as, for instance, 

 that of the Aldrovanda vesiculosa in the valley both of the 

 Po and of the Rhone, notwithstanding the intervening bar- 

 rier which the Alps oppose. 



These difficulties however have recently been more fully 

 dwelt upon by Dr. Joseph Hooker, in the masterly essay he 

 has appended to his Botany of New Zealand. Nor indeed 

 can they be effectually grappled with, except by calling in, 

 as he has done, the aid of two principles, each of which 

 is suggestive of many important inferences, the one with 

 reference to the former state of our globe, the other as re- 

 lating to the influence of meteoric conditions, operating 

 over vast periods of time, in modifying those characters 

 which many botanists have relied upon for establishing dis- 

 tinctions in species. 



The former of these speculations will be found alluded to 

 in the body of this Work, where it is stated that the flora 



