HUMBOLDTS MODE OF CALCULATION. 343 



The method to be adopted has been indicated by Alex- 

 ander von Humboldt in his " Ansichten der Natur " (" Pictures 

 of Nature "). His method consists in the comparison of the 

 vegetable families whose numerical relations are known, with 

 the number of species contained in our herbariums, or culti- 

 vated in our botanical gardens. 



But here, at the outset, a difficulty confronts us. Does any 

 relation exist between the classification of plants by natural 

 families and by their geographical distribution ? 



To group plants according to their analogies of structure, 

 we study them from an abstract point of view, and without 

 any regard to the medium in which they flourish. The ques- 

 tion grows complicated if we also take into consideration 

 their characteristic conditions and their distribution over 

 the terrestrial surface. Families are then split asunder, 

 and the importance of our scientific classifications disap- 

 pears. 



The gathering together of a small number of species, repre- 

 sented by innumerable individuals, confined within the same 

 area, may suffice to communicate to a landscape its character- 

 istic physiognomy: as, for example, is the case with the 

 Asiatic steppes, the landes of Brittany, the moors of Scotland, 

 the palm-groves and the clumps of Cactacese of tropical 

 America. By the side of species which impress us by their 

 mass that is, by the frequent reproduction of the same 

 individuals at an infinitesimal distance from one another 

 are placed those much more numerous species which are 

 everywhere very thinly sown. 



