EXPLANATIONS OF THE FORMULA. 61 



pages of this volume. It may be as well again to remind 

 those who use this work, that such a mode of gi'ouping spe- 

 cies, though founded upon fact, can be only a rough view 

 or approximation to nature ; many of the species being so 

 mtermediatc in the character of their- distribution, as to 

 render it doubtftd, if not optional, to which of the types 

 they should be refen-ed. The first species of the series, 

 the conspicuous Clematis, is an example of a doubtful 

 tpye ; while the second one, the easily overlooked Thahc- 

 trum alpinum, is an unquestionable example of the High- 

 land type. 



The sixth line gives the region or regions, and the zonal 

 range of the species. The regions and zones have also 

 been explained in former pages. In the case of species 

 which occur in both the Ai'ctic and Agi-aiian zones, an 

 abbreviation is made into the two initial letters " A. A." 

 The zonal range is indicated by naming the highest and 

 lowest zones inhabited by the species, if growing in more 

 than one of them, as is more usually the case. 



The seventh line is intended to show the lowest ascer- 

 tained limit of the species. Comparatively few British 

 species are wholly restricted to localities much above the 

 level of the shores. Hence, for the greater number, the 

 lower limits are indicated with reference to the most 

 southerly provinces withui which they descend nearly or 

 quite to the sea level. The expression " coast level " is not 

 to be construed as meaning the actual level of the tides, 

 but simply low situations not much above the sea. For 

 species which do not descend to the coast level in any part 

 of Britain, the lowest altitude is indicated by yards, and 

 usually reckoned by .steps of 50 yards. 



The eighth line shows the highest limit ascertained for 

 the species, also by Rte])s of 50 yards, without affecting 

 more mhiute exactness. There arc many jilants of the 



