1858.] NATURALIZED PLANTS OF GRBAT BRITAIN. 457 



hand there is but one aquatic, man being but little interested in 

 that class. 



This leads to an inquiry as to the time when these naturaliza- 

 tions were effected, and the dates are given as follows : — 



Since 1724 there have been introduced into Britain 55 spe-. 

 cies. 



Between the discovery of America and 1724, 9 species, viz. : — 



Anemone apennina. Anchusa sempervirens. 



Antennaria margaritacea. Asperugo proeumbens. 



Erigeron canadense. ' Rumex piilclier. 



Datura Stramonium. -^ Mercurialis annua. 

 Linaria Cymbalaria. 



Between the time of the Roman dominion in Britain and the 

 discovery of America, 10 species : — 



Dianthus Caryophjllus. Aristolochia Clematitis. 



Saponaria officinalis. Narcissus biilorus. 



Tragopogon porrifolius. Narcissus incomparabilis. 



Silybum Marianum. Lilium Martagon. 



Vinca major. Allium Ampeloprasum. 



During the rule of the Romans, 1 species : — 



Urtica pilulifera. 



Perhaps earlier than the Roman epoch, 7 species : — 



Isatis tinctoria. Sempervivum tectorum. 



Cochlearia rusticaua. Sedum dasyphyllum. 



Cheiranthus Cheiri. Sedum album. 

 Pyrethrum Parthenium. 



At a date that cannot be ascertained, 1 species: — 



Ononis reelinata. 



To resume, the present dispersion of plants by natural means 

 seems only to take effect very gradually, and almost exclusively, 

 over tracts of continuous land, though marine currents do trans- 

 port a few species to great distances in the tropics. So far as 

 human experience goes, an arm of the sea, be it ever so narrow, 

 presents an almost insuperable barrier. This has also been ob- 

 served by Gussone in Sicily and Iscliia ; species abundant on the 

 neighbouring coast of Italy are never known to be carried across 

 the straits by any agency of winds, currents, or birds. Indeed 

 it seems as if the natural means of transport had exhausted 

 themselves during the long ages which preceded the human 



N. S. VOL. II. 3 it 



