XI. CLIMATAL ZONES. 495 



that of the Calluna to be entered too low. The same 

 remark will also appl}^ to the Erica and Pteris, compara- 

 tivelj'- with other less conspicuous plants that usually 

 cease at similar altitudes or rather higher. 



Partly on account of the circumstances here mentioned, 

 it will be found that some discrepancies exist between the 

 assignment of the species to their zones in the ' summary 

 of distribution ' and their relative positions in the lists of 

 ' upper limits.' Those lists are local ; and they show the 

 local extremes, not the usual or average limits ; still less 

 the relative positions, as determined after allowing for 

 sources of uncertainty and error. The positions of the 

 three names — Calluna, Erica, Pteris — in the lists of up- 

 per limits on the Grampian mountains and in North 

 England, will not divide those lists correctly into zones, 

 always con-esponding with the nos. given for the same 

 species in the summary of distribution. In the list for 

 the North of England especially, where the liills are 

 insufficiently high, and the extent of much-elevated sur- 

 face is very restricted, the true uppermost zones of the 

 plants are shown quite imperfectly. 



The list of upper limits on the Grampian mountains 

 will be found to correspond much better with the zonal 

 assignments. But there is an inexactness in this list 

 also which demands some fui-ther exposition. The tract 

 , of the Grampians includes a large area, extending over 

 about one degree of latitude, and stretching ahnost 

 across Scotland longitudinally on both sides of the line 

 of 57 north latitude. The climate cannot be equal over 

 this wide tract. Its northern declivities, in the North- 

 east Highlands, are doubtless colder than its southern 

 declivities in the South-east Highlands. And the limits 

 of the same species appear more depressed near the 

 western coast, on and about Ben Nevis, than they ai-e 



