INTRODUCTION xi 



walls, and by roadsides" and "native in hedge banks" 

 would be shown to be inconsistent, for no species 

 could be aboriginal in these situations. 



The term wild is usually applied to all plants growing 

 spontaneously. 



The expressions casual, colonist, and denizen were used 

 by Watson to denote decreasing stages of dependence 

 on man. The first only is used in this work where 

 it is applied to the least independent introductions. 



The better established of our aliens can be roughly 

 classified in accordance with the special artificial habitats 

 which they affect, and names have been coined to 

 distinguish the groups. Thus those which inhabit road- 

 sides are sometimes known as viatical weeds, those of 

 cultivated fields agrestal, and so on, but the classes are 

 not clearly enough defined to derive much elucidation 

 from the use of these terms. 



If a species can spread into artificial localities in 

 regions over which it might have been expected to, but 

 has not, extended as a native, it must be supposed that 

 man gives it assistance which compensates it for the 

 natural advantages enjoyed only in its native area. 

 Different species find the necessary support for leaving 

 their native range in different artificial conditions. The 

 conditions accompanying human operations of which 

 plants seem especially to take advantage are the dis- 

 turbance of the ground and the unnatural supply of 

 plant-food. A more or less methodical series of localities 

 might be drawn up according to the degree of mecha- 

 nical disturbance or the supply of various plant-foods 

 obtaining in each, but a few examples to show what 

 is meant will be sufficient. Thus hedges afford an 

 occasional disturbance of the ground in the process of 

 cleaning the ditches and remodelling the banks. This 

 seems to be all that is required to attract the White 

 Dead Nettle, for instance, from its home in South- 

 T/ast Europe over most parts of the Continent, and 

 as far as England. Other herbaceous perennials might 

 be mentioned which have spread as aliens in hedge- 



