INTRODUCTORY EXPLANATIONS. 6 



A similar reason will suggest likewise a repetition of the 

 cut which was introduced in illustration of the " Zones," or 

 ascending stages of vegetation. 



Ascending Zones of Vegetation. — See vol. i. p. 41. 



I. Inferagrarian zone. 

 Midagrarian zone. 

 III. Superagrarian zone. 



IV. Inferarctic zone. 



V. Midarctic zone. 



VI. Superarctic zone. 



The present opportunity may be taken also to connect a 

 slight en-or which occurs in several places in the first 

 volume. On page 62 of that volume it was mentioned 

 that 46° (Fahr.) was assumed as the mean annual tempe- 

 rature of the north coast of Scotland, in estimating that 

 under which the several species had been observed to grow 

 wild in Britain. But, by a table given in Edmondston's 

 Flora of Shetland, it appeared that still the same mean 

 annual temperature prevails in Orkney ; while the mean of 

 the monthly means at Unst, in Shetland, from August, 1841, 

 to July, 1842 (a year of high temperature), was even at 485- 

 degrees of Fahrenheit's scale ; the winter months being 

 much milder, although the summer months were cooler, 



B 2 



