II] GEOGRAPHICAL DISTRIBUTION 21 



and became a serious pest. Elodea affords an admir- 

 able instance of the serious interference with the 

 balance of Nature by the introduction of a new 

 competitor into an environment conducive to 

 vigorous development. Another foreign water-plant, 

 Naias graminea, for the importation of which 

 Egyptian cotton may be responsible, has been 

 recorded from the Reddish canal near Manchesterds). 

 This African and Asiatic species occurs in Europe only 

 as a colonist ; it is said to have been introduced into 

 Italy with East Indian rice. A more recent case of alien 

 immigration due to unintentional human agency is 

 that of Potamogeton ^yennsglvanicus, a pond- weed of 

 Canada, the United States, Jamaica, and elsewhere. 

 Specimens of this species were first noticed in 1907 

 in a canal near Halifax close to the effluent from 

 a cotton mill. Since its discovery the plant has 

 slightly extended its range. It is suggested by 

 Mr Bennett, who first identified the alien, that its 

 fruits were brought to this country in goods from 

 the United States (le). 



Of the introduction of these and other foreign 

 plants we have satisfactory records ; but there are 

 many others which may owe their presence to man's 

 agency, though we have no information as to their 

 arrival. 



It has long been recognised that several members 

 of the British flora are related to Scandinavian 



