INTRODUCTORY EXPLANATIONS. 43 



kept cool by the dripping and oozing of water. In such 

 situations, Silene acaulis may be found a thousand feet 

 lowei* 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 mountain 

 species will re-appear on maritime cliffs, although absent 

 from the open plains and low hills between the coast and 

 the mountains. Thus, on the southern acclivities of the 

 Grampian mountains, Saxifraga oppositifolia is seldom seen 

 below a thousand feet of elevation; although still farther 

 south, it was found on the west-coast of Scotland, by Pro- 

 fessor Balfour, very little above the sea-level. On the coast, 

 consequently, this " alpine plant" intermingles with species 

 which never occur at alpine elevations on the mountains. 



In addition to their distribution by provinces and clima- 

 tic zones, there is a third mode of indicating the geographi- 

 cal relations of plants, which may also require some ex- 

 planation. It has been before observed that certain species 

 are spread over the whole island, while others are limited 

 to one, two, three or more of the provinces. The same 

 holds true in the zones ; some species occurring in all of 

 them, others in one or more. Perhaps no two species have 

 exactly the same distribution or relative frequency ; and 

 yet certain general similarities may be traced, by which the 

 species may be grouped together under a few leading Types 

 of disirihntion . In the small volume before alluded to, by 

 the title of ' Remarks,' certain "geographic types" were in- 

 dicated (pp. %Q — 89) ; and in the ' Tabular Appendix ' to 

 the same volume (115 — 184) the species were severally 

 assigned to their peculiar types. This was simply an at- 

 tempt to express, by a single term, the leading character of 

 their distribution, with reference to geographical position 



