66 lU. INTBODUCED SPECIES. 



as distinguished from those afforded to them hj human 

 agency. It is possible that none of these species were 

 aboriginal natives on the present surface of Britain. It 

 may be that all of them were immigrants into the British 

 islands, at different dates, from other lands ; those lands, 

 or some of them, having subsequently ceased to be. Such 

 uncertainties belong at present rather to geological, than 

 to geographical botany ; and they cannot here be dis- 

 cussed. The broad line of distinction is here to be 

 drawn between natural and human agency ; — natural 

 agency being assumed, where human agency is not ob- 

 vious or suspected. 



In the second category, that oi Introduced Species, are 

 placed all those -which are supposed to have been brought 

 into Britain through the instrumentality of mankind. In 

 some few instances they are known to have been so intro- 

 duced; notably in the case of some American species, 

 which were unknown in Europe before the discovery and 

 settlement of the Western Continent. In far the majo- 

 rity of instances botanists only infer or suppose that a 

 species has been introduced, because they can detect 

 some remaining indications of human agency in the con- 

 ditions under which it is still observed to exist here ; the 

 inferences suggested by the present conditions being 

 occasionally corroborated by historical or traditional evi- 

 dences also. But it seems quite within possibilitj', and 

 even within reasonable probability, that the indications of 

 human agency may have become obliterated in various 

 instances. And if such instances do occur, the plants so 

 situate, although only naturalised aliens, are now una- 

 voidably placed in the same category with the aboriginal 

 natives. 



If they have become established sufficiently well to 

 maintain themselves among the native vegetation, with- 



