IN ASCENDING REGIONS. 71 



springs preserve a mean temperature considerably higher 

 than that of the atmosphere, and thus force the plants, 

 although in the middle of summer such springs are some- 

 what colder than the air. Thus, in low situations, springs 

 (much cooler than the air in summer) cause the growth 

 of plants characteristic of an inferior climate ; while higher 

 up they are attended with the opposite phenomena, being 

 marked by a superior vegetation. However explained, I 

 have noticed such to be the fact. 



Having thus given a brief sketch of the several regions, 

 which will enable any botanist to recognise them, I may 

 repeat, by way of caution, that all such attempted divisions 

 are very imperfectly defined in nature; in fact, almost 

 arbitrary with the designer. But although the divisions 

 into special zones or regions is thus little else than an 

 artificial aid to description and memory, it is not the less 

 true that ascending stages of vegetation do occur, and 

 that the comparative limits of species may be pointed out 

 in an ascending or descending scale. Thus, taking each 

 genus or order by itself, we might place the species with 

 considerable precision ; but the whole flora of a country 

 can never be so arranged in consequence of the terminal 

 lines of species continually intersecting each other. Take 

 the following arrangement of the Ericaceae (without the 

 Pyroleae and Monotropeae) as an example ; the sequence 

 being probably correct for the actual distribution in Bri- 

 tain. But who is able correctly to incorporate the Saxi- 

 frageae with them ? Even this list is not quite certain, 

 for Arbutus alpina and Andromeda polifolia not occurring 

 under the same latitude within Britain, it is doubtful 

 whether the upper line of the latter does or does not cross 

 the lower line of the Arbutus. The outer column repre- 

 sents the upper, the inner column gives the lower lines. 



