22 INTRODUCTION. 
satisfactory upon paper, nature refuses to trace them on her own 
domains. 
“To take an example: — Gnaphalium supinum has been seen 
at the level of a corv-field by Loch Callater (1600 feet) in Aber- 
deenshire, and Pteris aquilina has been found much above the 
same altitude, at a distance of half a dozen miles; namely, at or 
near 1700 and 1900 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 under equal conditions of 
exposure, &. 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 2000 feet of elevation ; rather below 
which, and at a slight distance, some weakly plants of Sawifraga 
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 
1000 feet below the lower-line of Saaifraga 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 ‘trespasses’ 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 kept cool by the dripping and oozing 
of water. In such situations, Silene acaulis may be found a 
thousand feet lower than it is seen as a constituent of the drier 
and sun-exposed sward. And there is also another circumstance 
which makes the lower limits appear more irregular and ex- 
ceptional than the upper ; namely, the fact that mouutain species 
