12 INTRODUCTORY EXPLANATIONS. 



assuredly be found in many more of these square mile 

 sections, than would the Hj'jiericum. Or, at the other 

 extremity of the scale, Cynodon Dactylon and Erica vagans 

 have been ascertained only in one province ; and in sub- 

 di\dding that province into square miles, the Cynodon 

 might be confined in one of them, while the Erica would 

 extend into several. 



No test which it is possible to apply to all species alike, 

 can perfectly show their census — comparative rarity or 

 commonness. We cannot count the individual plants of a 

 species, as we would count the people of a kingdom ; and 

 in an island so large as Britain, we csjinot ascertain the 

 number of special locahties, unless it be for some few ex- 

 tremely local species. Their census must therefore be 

 vaguely estimated by the experience of individual ob- 

 servers, who will never agree in their estimates ; or it must 

 be tried by some test which is adapted to show the number 

 of spaces (not the number of precise places) in which each 

 one occm's. The nearer we can reduce the " spaces " into 

 " places," by increasing theii" number, and diminishing 

 their- extent, the more exact will the test become. As yet, 

 the want of recorded inibnnation, pai'ticulaiiy about the 

 local distribution of the more common species, imavoid- 

 ably throws us upon spaces which ai'e too large for much 

 precision. 



One object of the present work, will be that of showing 

 the area of each species with gr'eater exactness than has 

 liitherto been done, or even attempted by any other writer 

 on the plants of Britain ; and a second object, combined 

 with the fonner, will be to take the fir-st steps towai'ds form- 

 ing a census, by a numerical scale. Something of this 

 kind may be seen in the " List of Species," in the ' New 

 Botanist's Guide' (England and Wales, pp. 355 — 398, and 

 Scotland, pp. 521 — 539,) where all the coimtics are enu- 



