280 VIEWS, &C. PHYSIOGNOMY OF PLANTS. 



In studying the geographical distribution of forms, we may 

 consider the species, genera, and natural families of plants 

 separately. A single species, especially among social plants, 

 frequently covers an extensive tract of land. Thus we have 

 in the north, Pine or Fir forests, and Heaths (ericeta}; in 

 Spain, Cistus groves ; and in tropical America, collections of 

 one and the same species of Cactus, Croton, Brathys, or Bam- 

 busa Guadua. It is interesting to study more closely these 

 relations of individual increase, and of organic development; 

 and here we may inquire, what species produces the greatest 

 number of individuals in one certain zone; or, merely what 

 are the families to which the predominating species belong in 

 different climates. In a very high northern latitude, where the 

 Composite and the Ferns stand in the ratios of 1 : 13 and 1 : 25 

 to the sum of all the phanerogamia (i. e., where these ratios are 

 found by dividing the sum total of all phanerogamia by the 

 number of species included in the family of the Composite, or 

 in that of the Ferns) ; one single species of Fern may, however, 

 cover ten times more space than all the species of the Com- 

 posite taken together. In this case the Ferns predominate 

 over the Composite by their mass, and by the number of the 

 individuals belonging to the same species of Pteris, or Poly- 

 podium; but they will not be found to predominate, if we 

 only compare the number of the different specific forms of the 

 Filices, and of the Composite, with the sum total of all Phane- 

 rogamia. As, therefore, multiplication of plants does not follow 

 the same laws in all species, and as all do not produce an equal 

 number of individuals, the quotients obtained by dividing the 

 sum of all phanerogamic plants by the species of one family, 

 do not alone determine the leading features impressed on the 

 landscape, or the physiognomy of nature peculiar to different 

 regions of the earth. If the attention of the travelling botanist 

 be arrested by the frequent repetition of the same species, 

 by its mass, and the uniformity of vegetation thus produced, it 

 will be still more forcibly arrested by the infrequency of many 

 other species useful to man. In tropical regions, where 

 the Rubiacese, Myrtles, Leguminosse, or Terebinthaceas, 

 compose the forests, one is astonished to meet with so few 

 trees of Cinchona, or of certain species of mahogany 

 (Swietenia), of Hoematoxylon, Styrax, or balsamic Myroxylon. 

 I would also here refer to the scanty and detached occurrence 

 of the precious febrifuge-bark trees (species of Cinchona) 



