INTRODUCTORY EXPLANATIONS. 35 



amples. These thi-ee sluaibs are distributed through Bri- 

 tain, from the Peninsula to the North Highland provhice ; 

 the two latter extending mto Shetland; and all three as- 

 cending the Highland momitains above the limit of cultiva- 

 tion. But they ascend to rmequal altitudes, for (in round 

 numbers) the Vaccinium grows even at 4,000 feet of eleva- 

 tion, while the Calluna fails at 3,000, and the Genista at 

 2,000. 



As a mode of indicating the climatic range of species 

 rather more precisely than can be done under two regional 

 divisions only, it is expedient to sub-divide the regions 

 each into tln-ee subordinate zones. But in attempting this 

 sub-division, the practical difficulty before adverted to (pp. 

 30-31) comes in the way of any rigidly precise definition. 

 On a single conical mountain of some thousands of feet in 

 elevation, it might be easy to distinguish six zones, or even 

 six times six zones, by the successive tenninations of dif- 

 ferent species. But when this is tried on different moun- 

 tains, local variations present themselves in the compara- 

 tive hmits of species, such as are shown in the examples 

 before given (pp. 26-28) . The local variations are always 

 found to beco?ne more wide and numerous, as the moun- 

 tains ai-e distant fi-om each other, and dissimilar in their 

 physical chai'acters and geographical position. Within the 

 the length of Britain, we have several distinct gi'oups of 

 mountains, differing among themselves very widely in re- 

 spect of latitude, of maritime proximity, of elevation, of 

 geological character and other conditions. Accordingly, 

 our six zonal sub-divisions \vill present many local differ- 

 ences in the species which constitute their respective floras; 

 and even where the same species are repeated in the same 

 zones on different moimtains, as will more usually be the 

 case, their limits will often differ somewhat relatively to 

 each other. It is ncccssaiy for the student in botanical 



