viii INTRODUCTION 



plants now confined, for instance, to our hedges and 

 arable lands are not introduced, it is necessary to assume 

 that they once occupied, but have now become extinct 

 in, natural habitats. In the presence of a perfectly tenable 

 alternative, viz., human introduction, and considering 

 that very few provable cases of extinction of natives 

 are known, it seems more proper to regard such species 

 as alien. Even if the possibility of descent from 

 wild British stock is pressed, it is more accurate to 

 describe the species extinct as a native than to place 

 it among our extant indigenous flora. It should be 

 observed that a species like the Sweet Violet, now 

 naturalised in Northern Britain, though probably derived 

 from wild British stock, would not be called native 

 there, and even if, becoming extinct in Southern Britain, 

 it were reintroduced from the north, it would none the 

 less be considered extinct as a native though of unim- 

 peachable descent. The plea for nativity in artificial 

 habitats has been brought forward especially in the 

 case of certain waste ground plants. There are natural 

 wastes, such as the haunts of wild animals, offering 

 much the same conditions as those of domesticated 

 cattle, and it is urged that the natural waste ground 

 flora has been carried on by artificial conditions. 

 The same reasoning, however, holds, and, unless a 

 waste ground species is actually found on a natural 

 waste, it is here included among the aliens. 



More difficult is the problem presented by some of 

 the plants which abundantly accompany human opera- 

 tions but also occasionally appear in wild habitats in 

 their neighbourhood. In such cases it has been found 

 useful to carefully insert on a large-scale map all the 

 positions in which the plant grows over a few acres 

 of land. When that has been done in the case of 

 several different species it is easy to distinguish the 

 native diagrams from the alien ones. 



While this list will include, then, all species found 

 only in artificial habitats in Britain, it will also contain 

 some which occur in natural surroundings but which 



