62 INTRODUCTION. 
4. Alien species are those certainly or very probubly of foreign 
origin; though several placed in this category are now well 
established amid the indigenous flora of this island; others less 
perfectly so. 5. Casual species are chance stragglers from culti- 
vation ; those occasionally imported and sown with agricultural 
seeds ; those introduced among wool, oil-seeds, or other mer- 
chandize; foreign plants found on ballast heaps deposited from 
ships; and generally such alien species as are most uncertain in 
place or persistence. 
7. EXPLANATIONS OF THE FoRMULA. 
In treating various questions of phyto-geography, and in forming 
comparative tables and lists for illustration, it is found convenient 
that the distribution of each and every species in a flora should be 
shown under some uniform plan or method, all treated alike. 
Besides its more direct uses, such an uniformity of treatment has 
the considerable incidental recommendation of being easily read 
and understood by foreigners, whose knowledge of our language 
may be too limited for easily following a more varied diction. And 
strict condensation of many details being an object especially 
sought to be attained in the present work, a fixed formula has 
been adopted, which will be found to express much within a 
compass comparatively short; showing the distribution of each 
species under various aspects, and in various relations to the 
physical geography of Britain; as well as tracing its area or 
geographic extension over many other countries. 
Allowing seven lines to each, it is found that four species can be 
conveniently got on a page, without the awkwardness of disjoining 
the lines relating to the same species, by the necessity of carrying 
some of them over to a succeeding page. Four of these lines will 
suffice for a condensed summary of distribution within Britain 
itself ; leaving three lines available for tracing out the geographic 
area beyond Britain, and in a manner which will exhibit the 
negative equally as the positive facts of distribution elsewhere ; 
a part of the inquiry far too much neglected by Authors who 
