INTRODUCTION. 



3 



plantation from one hemisphere to the other, especially from the southern to the 

 northern, to discover the suitable climate. It is true, in the warmer zones of one 

 and the same hemisphere, the different regions of the mountains, having greater 

 climatic relationship, exhibit many species belonging to the colder zones ; but 

 there are generally a few forms peculiar to warmer countries, which extend their 

 range high up these mountains, imparting to them their idiosyncrasy. This 

 mixture of plants appertaining to different climates imparts quite a peculiar 

 physiognomy. A district in the Mexican mountains 8000 or 9000 feet above the 

 sea, does certainly remind us by its oaks and firs of North America, or even 

 Europe; but only to render its Yuccas and Fourcroyas still more prominent 

 features. 



But it is not only latitude by which we measure differences in the typical 

 forms of vegetation, principally caused by the climate ; the longitude also works 

 decided changes in the vegetation, which, though subordinate to the former, and 

 apparently quite independent of climate, nevertheless present everywhere new 

 pictures.* Passing over the striking and numerous physiognomic differences 

 caused in one and the same zone by local circumstances, such as deserts, steppes, 

 and great swamps, we mention only those observable in regular and gradual trans- 

 itions in the same latitude, but different longitude, and under almost identical 

 climatic conditions. The phenomenon first presenting itself is that the geogra- 

 phical range of most species is not of sufficient extent to occupy longitudinally the 

 whole zone, of course most frequent where the zones are longest, i. e. in the lowest 

 latitudes, and least frequent where shortest. This is the reason why, in the neigh- 

 bourhood of the poles, the geographical range of species extends, without regard 

 to continents, over the whole zone, which is short enough to be filled up by it ; but 

 on nearing the equator, those species, the home of which is the entire zone, con- 

 tinually decrease in proportion to the other occupants of the soil, and thus an 

 increasing number of districts of distribution is ranged side by side, in order to 

 provide for the continually increasing length of the zones. 



Thus, in consequence of the spherical shape and position of our earth, con- 

 tinually increase, with the temperature of the climate, not only the capabilities of 

 vegetation and the number of species of each country, but also the space, so that 

 the principal character of the vegetation as imparted by the different climates 

 may obtain full play to divide in numerous variations longitudinally denned. The 



* It is true that the zones, of which the geo- 

 graphy of plants takes notice, are defined by the 

 wavy isothermal lines, &c. ; but these lines have on 



the whole the same direction as those straight ones 

 of which mathematical geography avails itself. 



