EXPLANATIONS OF THE FORMULA. 59 



West Highlands, are still incompletely made out; and 

 there are several dubious species, whose provincial dis- 

 tribution can be very imperfectly known at present. 

 The number of un-enclosed figm-es in the first line, in 

 contrast with the nmnber given as the estimate, will show 

 where additions have been made to the latter, on as- 

 sumed probabilities. For counties, the estimated nmnber 

 will often be given much higher than has been actually as- 

 cei-tained ; more especially for the common species, which 

 ai-e seldom noticed, except in local floras and catalogues 

 which profess to include everything. The census for 

 counties is estimated by, first, reckoning up those within 

 which the species has been ascertained to occur ; secondly, 

 reckoning those fi-om which it is believed to be absent ; 

 and, thii-dly, adding the rest to one or other side, according 

 to presumed probabihties. Though much is here assumed, 

 pai-ticularly with respect to the distribution of the common 

 species, yet the result of this estimate, it is believed, will 

 come very near the truth ; the author's attention having 

 been so long directed to the distribution of plants in Bri- 

 tain, that he can usually guess pretty accurately whether 

 any given species will or will not be fomid in any given 

 county. Of com-se, there are some exceptions to the ac- 

 cm-acy of such guesses, especially in the cases of confused 

 or neglected species, such as OEnanthe peucedanifoha or 

 Ranmiculus Lenonnandi. As an example, where much has 

 been assumed without certam knowledge, the census of 

 Stellaria media may be cited. On putting together all 

 his local lists, the author cannot show, on authority, that 

 this very common plant grows in so many as sixty coun- 

 ties ; but he unhesitatingly assumes it to occur in eveiy 

 coimty. The whole number of c;ounties is taken at 82 ; 

 some small counties (Rutland, Kimoss, &c.) being sunk 

 into those adjacent, and some isles or groups of isles (Man, 



