42 INTRODUCTORY EXPLANATIONS. 



graphical maps. Though such Unes may be made to look 

 very clear and satisfactory upon paper, nature refuses to 

 trace them on her own domains. 



To take an example : — Gnaphalium supinum (of British 

 authors) has been seen at the level of a corn-field by Loch 

 Callater (1,600 feet) in Aberdeenshire, and Pteris aquilina 

 has been found much above the same altitude, at the dis- 

 tance of half a dozen miles ; namely, at 1,700 and 1,900 

 feet. Yet in ascending the gradual declivities of the same 

 mountains, under equal conditions of surface and shelter, 

 the Pteris always ceases before we attain the altitude at 

 which the Gnaphalium commences. On this account, they 

 are assigned to different zones, although their limits can 

 thus cross in respect of absolute elevation. They belong 

 to different climates, and are so placed by nature when un- 

 der equal conditions of exposure, &c. So, again, a bush of 

 Corylus Avellana has existed during many years against 

 the sunny face of a steep rock on the Clova Mountains, at 

 nearly 2,000 feet of elevation ; rather below which, and at 

 a slight distance, some weakly plants of Saxifraga nivalis 

 have been observed. Nevertheless, this solitary instance 

 cannot warrant the assignment of these two species to the 

 same zone; because the upper-line of the Corylus is usually 

 500 to 1,000 feet below the lower-line of Saxifraga nivalis. 

 In studying the climatic affinities of plants, we must reason 

 as well as observe, and estimate as well as measure. 



Though examples of each may thus occur, it is more 

 usual to find " tresspasses " downwards than upwards. The 

 descending course of water conduces much to this result in 

 the mountain districts, not only by bringing down the seeds 

 of plants from the higher zones, but also by bringing down 

 the cold of the mountains; so that the first appearance of 

 alpine species, as we ascend the mountains, is usually by 

 the side of the water-courses, or upon wet rocks which are 



