IN ASCENDING REGIONS. 55 



stages are in some measure imaginary, or at least arbi- 

 trary, and can never be precisely described or delineated 

 by exact lines. Still, they are useful as general indica- 

 tions, and for the most part answer the intended purpose. 

 A single isolated hill may be divided into as many stages 

 as wished, and with much exactness. Add adjacent hills, 

 and local differences in the comparative order or sequence 

 of species speedily appearing, broader stages must be 

 taken, or the exceptions confuse the design. Groups of 

 hills apart from each other are attended with yet wider 

 variations in the comparative ranges of the same species. 

 And when distant countries are compared together, such 

 differences become so great and numerous, that only the 

 broadest general distinctions can be adopted with success. 

 Our criterion, therefore, of the fitness of any imaginary 

 zones or regions of vegetation must be sought, on the one 

 hand, in their general applicability to all parts of the tract 

 or country to which they relate, without being attended 

 with so many local exceptions as in effect to nullify them. 

 But, on the other hand, they must not be so wide and 

 vague as to express nothing. A few exceptions are to be 

 preferred to the other alternative of vague inutility. 



Britain extends over many degrees of latitude, has 

 several distinct mountain tracts, and forms as it were a 

 sort of centre, where the Greenlandic, Scandinavian, and 

 Lusitanic climates meet together, or merge in each other. 

 These peculiarities very greatly interfere with artificial 

 systematising. The phenomena of vegetable distribution 

 are thereby rendered so complex and interconfused, that 

 it becomes a very difficult matter to say what are the 

 most convenient general divisions of its vegetation. By 

 bringing together such materials or data as I have been 

 able to accumulate, it appears to me that the following 

 scale will represent something like a general average, 

 although not without certain exceptions. 



d 4 



