X. CLIIMATE OF PLANTS. 481 



which it may be expected to cease in another country with 

 dissimilar climate. But subject to those local differences 

 in the seasons and extremes, before commented on at 

 pages 166 to 174, mean annual temperature is not the 

 less available on that account for various comparisons 

 within Britain itself, and also for some comparisons be- 

 tween this island and the opposite coasts of the continent. 

 The relative limits of different species within this island 

 and on the adjacent European coasts, will admit of com- 

 parison through their ranges of mean yearly temperature, 

 although these may not exactly express the special cli- 

 mates for each species. We may know that a man of 42 

 years is older than a man of 41 years of age, although we 

 may be able to ascertain exactly neither the months nor 

 the days when they respectively complete a year of 

 added age. 



Annual means subserve also passably well for compa- 

 risons between the altitudinal ranges of the same species 

 for different divisions of the island, and even for testing 

 the correspondence between altitudinal and latitudinal 

 limits, as exemplified on pages 164, 165. Thus, when we 

 find Urtica dioica traced up the Grampian mountains of 

 North Britain to a height where the mean temperature is 

 supposed to be about 4.05 centigrade (page 329), we may 

 reasonably enough infer that the height represented by 

 6.00 in North England (page 336) is far below that at 

 which the same species might exist, if suitable soil and 

 site were there found for it higher up. So, again, on 

 assuming the mean temperature of Orkney and Shetland 

 to be equal with that experienced between 300 and 400 

 yaixls of altitude in North England, we should infer that 

 many of the species which grow on the coast-level in 

 those northern islets will also grow at or above 300 yards 

 in England ; and the fact is so. 



VOL. lY. 3 Q 



