INTRODUCTION 



THE term alien is used to designate any species 

 which, though now spontaneous, originated in 

 Britain through human agency. Cultivated plants are 

 only included when recorded as escapes from, or 

 survivals of, cultivation. Now although alien plants 

 are usually defined as above, and are frequently for that 

 reason called " introduced plants," it is seldom possible 

 to obtain any definite information as to the manner 

 in which they actually arrived in the country. The 

 term "introduced plant" or "introduction," moreover, is 

 not really distinctive, for all plants, native and otherwise, 

 must have been originally introduced to their present 

 habitats. In the great majority of cases botanists arrive 

 at their conclusions as to the status of a species by 

 a careful observation of its present circumstances in 

 the British Isles, and also of its geographical distribution 

 beyond them. Thus a species which exists in perfectly 

 wild and natural surroundings, both here and in the 

 neighbouring parts of the world, is deemed indigenous, 

 for there is no reason to suppose that its presence is 

 due to any agent but natural dissemination at the time 

 when the flora of North-West Europe originated. If, 

 on the other hand, a species is always found to be 

 connected with artificial surroundings, it is classed 

 as an alien. It cannot often be proved to have been 

 originally introduced by man, but, in the other alter- 

 native — viz., that it existed in the country prior to 

 man's advent — it must once have grown in perfectly 

 wild habitats. In order to suppose, therefore, that the 



