GEOGRAPHY OF PLANTS. 465 



on tlie north and south, and exhibiting a cowespondence between the modifica- 

 tions of temperature observed successively in our progress from the equator to 

 the poles, and the geographical distribution of vegetable and, generally also, of 

 animal life on the earth. 



The geographical distribution of plants is a subject of vast extent and im- 

 portance. The nature of the vegetation covering the earth varies, as vre have 

 remarked, according to the climate and locality; and plants are fitted for dif- 

 ferent kinds of soils, as well as for different amounts of temperature, light, and 

 moisture. From the poles to the equator this constant variation in the nature 

 of the flora is a shifting scene, passing from the lichens and mosses (the lowest 

 vegetable forms in the Arctic and Antarctic regions) to the noble palms, 

 bauanas, and orchids of the tropics by a scries of regulated changes through 

 all the multiform aspects of the vegetable kingdom. The same progress and 

 graduated fitness is observed in the vegetation of lofty mountains under the 

 equator, when descending from the summit to the base. From the scanty 

 vegetation of Greenland, where the only woody plants are the Arctic willows, 

 trees scarcely a finger length in height, Ave may trace the expansion of vegeta- 

 tion as we move southward over the lichens and mosses to the saxifrages and 

 cruciferous plants, or those which resemble the cabbage and the turnip in their 

 mode of flowering ; then to grassy pastures, and by coniferous or fir-like trees, 

 and amentaceous or birch and alder-like trees, to the noi'thern borders of the 

 United States. Extending our glance further southward, we shall perceive 

 that we enter the region of oaks, hickory, and ash, of tulip-poplar, buttonwood, 

 walnut, red and white cedars, sugar and other maples, sassafras, sumac, laurel, 

 and many other trees and shrubs characteristic of the temperate regions of 

 North America. In the districts further south we find an increase both of 

 species and of genera, and more tropical forms show themselves, such as mag- 

 nolia, Osage orange, honey locust, cypress, holly bay, wax myrtle, the cotton 

 plant, rice, the live oak, and enter the borders of the regions of the palmetto 

 and the orange ; thence to those of the sugar-cane and pineapple, the coffee 

 plant and cocoa-nut, and the luxuriant vegetation of the equator and torrid 

 heats. "In this progress," as Humboldt, the father of geographical botany, 

 remarks, "we find organic life and vigor gradually augmenting with the in- 

 crease of temperature. The number of species continues to increase as we 

 approach the equator, and each zone presents its own peculiar features ; the 

 tropics their variety and grandeur of vegetable forms ; the north its meadows 

 and green pastures, its evergi-een firs and pines, and the periodical awakening 

 of nature in the spring time of the year." 



Many causes intimately connected with the aspects of our globe have an 

 influence in modifying the conditions of climate, and thus affecting the dis- 

 tribution of animals and plants on its surface. The geographical forms of 

 contour, the relief or elevation and depression of the terrestrial surface, the 

 relations of size, extent, and position, each exert a very marked effect upon the 

 climatic peculiarities of a district. The bearing or direction of the shores of a 

 continent, the elevation of a mountain in one place rather than in another, the 

 subdivision of a continent into islands or peninsulas, and other minor differ- 

 ences, have very important bearings upon the climate of a district. The de- 

 pression of a few hundred feet over some wide areas would reduce some regions 

 to the level of the sea, or sink them beneath its waves, or so modify the climate 

 of the higher portions left above the waters as to render them no longer tenable 

 by the life that once enjoyed a congenial clime. This is shown by the obser- 

 vation that some low islands scattered in clusters are covered with a vegetation 

 entirely different from that of extensive plains, though lying in the same lati- 

 tude. A change in the bearing of the shores would modify the currents of the 

 ocean, which would react upon vegetation. Mountain chains have oftentimes 



36 A. 



