GEOGRAPHY OF PLANTS. 467 



incipient vegetation of spring, and prolong the growing season by preventing 

 the recurrence of early autumn frosts. 



Under the iullaeuce of the physical agencies of heat, moistiire, &c., vegeta- 

 tion exhibits a regular progression, whether we ascend from the equator towai'ds 

 the poles, or from the base of a high mountain, under the equator, towards its 

 summit to the regions of perpetual snow. In both these directions there is a 

 striking general agreement in the order of the succession of the phenomena, so 

 tliat the natural productions of any given latitude may be properly compared 

 with those prevailing at any given height above the level of the sea. The 

 elevation at which any variety of plant will grow spontaneously will, of course, 

 vary with the latitude of the mountain on which it exists. There may then 

 be said to exist a corresponding similarity of climate and vegetation between 

 the successive degrees of latitude and the successive heights above the sea. 

 There is a class of plants which a slight advance beyond the freezing point 

 calls forth from their winter sleep, which have a peculiar stamp, and constitute 

 a peculiar flora, termed Alpine. We find this Alpine flora in regions where 

 snow covers the earth, Avherc the lakes are frozen most of the year, as in 

 northern Lapland, northern Siberia, Arctic North America, on the Alps, the 

 Pyrenees, the Carpathians, the Caucasus, in the mountains of Norway, on 

 Scotch and Icelandic summits, in the Himalayas, the Sierra Nevada, and on 

 our own Mount Washington. On the latter, Alpine plants identical with those 

 of Lapland in latitude C7° north, where they grow at a height of three thousand 

 feet and less above the level of the sea, are found at a height of not less than 

 six thousand feet in latitude 44° north, while beloAV this limit, in the wooded 

 valleys of New Hampshire, there is not one species which occurs also about 

 North Cape. The Alpine flora appears almost ubiquitous whei'ever a tempera- 

 ture sufiiciently reduced, and yet adequate to sustain life, is found. Scandinavian i 

 genera and even species reappear everywhere from Lapland and Iceland to the 

 tops of the Alps of Van Dieman's Laud, in rapidly diminishing numbers, it is 

 true, but in vigorous development throughout. They abound on the Alps and 

 Pyrenees, pass on to the Caucasus and Himalaya, thence extend to those of 

 the peninsula of India, to Ceylon and the Malayan Archipelago, (Java and 

 Borneo,) and reappear on the Alps of the New South Wales, Victoria, audi 

 Tasmania, and beyond these again on those of New Zealand and the Antarctic 

 islands, many of the species remaining unchanged throughout. It matters not 

 what may be the vegetation of the bases and flanks of these mountains, the- 

 northern species may be associated with Alpine forms of Germanic, Siberian, 

 Oriental, Chinese, American, Malayan or Australian, and Antarctic types, which, 

 are all more or less local assemblages ; the Scandinavian, from Sweden, Lapland, 

 and Norway, asserts its prerogative of ubiquity from Iceland, beyond its 

 antipodes, to its Antarctic congener of high southern latitudes.* 



The correspondence between the ascending vegetation on monntain sides and 

 the distribution of trees, especially over the whole extent of the temperate 

 zones, is so close that it may be considered a universal law. On lofty 

 mountains climates seem arranged, as it were, in strata, one above another,. 

 Avith a regularity that has strongly impressed the attentive observer. He has 

 noticed that the trees resembling the chestnut and the walnut, or most of those 

 that flower in aments or catkins, occur in the lower latitudes under the influence- 

 of a genial clime, but disappear entirely before he reaches a latitude or an 

 elevation where agriculture ceases. Further north he finds a variety of 

 poplars, willows, oaks, maples, and other deciduous trees interspersed with 

 pines, which begin to form continuous forests until they become an almost 

 uniform pine and birch forest, covering in unbroken continuity the more northern 

 districts as far as trees extend. So likewise, in ascending higher and higher on 



* J. D. Hooker, Flora of Tasmania. — Silliman'e Joimial of Science, XXIX, pp. 323, 324. 



