492 ON THE GEOGRAPHY OF PLANTS. 



the temperate zone there are fewer than in the tropics, and the total number 

 continues to decrease as we approach the pole; but as a cold country, Lapland 

 for instance, produces species that have a greater power of resisting low tempe- 

 rature than the great mass of phoenogamous plants, it happens that, in 

 Lapland, the relative proportion borne by ferns to the rest of the Flora, is 

 greater than in France or Germany. The numerical relations which appear in 

 the tables which I shall produce, are entirely unlike the relations indi- 

 cated by an absolute comparison of the species that vegetate under different 

 parallels of latitude. The variation which is observable in proceeding from 

 the equator, to the poles is consequently different in those two methods. In 

 that of fractions, which is adopted by Mr. R. Brown and myself, there are two 

 causes of variation; that is to say, the total numbers of phoenogamous plants do 

 not vary in passing from one parallel of latitude, or rather from one isothermal 

 zone to another, in the same proportions as the number of species of a given 

 family. 



If from species or individuals of the same form, which reproduce them- 

 selves in conformity to certain fixed laws, we pass to those divisions of the 

 natural systems which are abstractions of different degrees of importance, we 

 may either confine ourselves to genera, or orders, or sections of a still higher 

 degree. There are certain genera and families which belong exclusively to 

 certain zones, and a particular combination of the conditions of climate ; but 

 there is also a great number of genera and families, of which we find repre- 

 sentatives under all zones and at all elevations. The earliest researches upon 

 the geographical distributions of forms were those of M. Treviranus, published 

 in his ingenious work on Biology, and the object of these was the stations of 

 genera upon the globe. But it is more difficult to obtain general results from 

 such a method than from that which compares the number of species of each 

 family, or the great groups of a particular family to the whole mass of phreno- 

 gamous plants. In the frozen zone, the variety of genuine forms does not 

 diminish in any thing like the degree of decrement of species ; a greater 

 number of genera, in a given number of species, is always to be found in such 

 countries ; and so it also is with the summits of high mountains, which are 

 colonised by a great number of genera supplied by the more abundant vegetation 

 of the plains. 



It is very instructive to study the vegetation of the tropics and of the 

 temperate zone between the parallels of 40 and 50, in two different ways: 

 first, in determining the numerical properties of the Flora of a large extent of 

 country, including both mountains and plains ; and, secondly, in ascertaining 

 those proportions for the plains only of the temperate and torrid zones. As in 

 our herbaria we have indicated, by barometrical measurement, the elevation 

 of each plant in more than 4000 cases above the level of the sea in equi- 

 noctial America, it will be easy, when the account of the species is completed, 

 to separate those which grow at or above an elevation of 6000 feet from such 

 as are inhabitants of a lower region. This operation will affect most sensibly 



