ON THE GEOGRAPHY OF PLANTS. 



491 



borne by one family to another vary on the same isothermal line 1 and are such 

 proportions the same on either side of the equator? These are, properly speak- 

 ing, questions of geographical botany : they are connected with the most 

 important problems of meteorology, and the physics of the globe in general. 



In studying the geographical distribution of particular forms* we can 

 pause either at a consideration of particular species, genera, or natural families. 

 It often happens, that, a particular species, especially of those kinds which 

 I have called social, covers a vast extent of country: such, for instance are, in 

 the north, the heaths and forests of pines ; such are, in equinoctial America, 

 the assemblages of multitudes of Cactus, Croton, Bambusa, and Brathys, of 

 the same species. It is curious to examine such instances of multiplication 

 and organic development. We may enquire what species, in a given zone, 

 produces the greatest number of individuals ? and we may mark the families 

 to which the predominant species belong in different climates. 



In a northern climate, where Composite and ferns are to phcenogamous 

 plants in the relation of one to thirteen, and of one to twenty-five, one single 

 species of fern may occupy ten times as much land as all the Composite put 

 together. In such a case, ferns would exceed Composite by their mass, by the 

 number cf individuals belonging to particular species of Pteris or Polypodium ; 

 but they would not exceed them if a comparison were instituted between dif- 

 ferent forms exhibited by the two groups of Composite and ferns, and the sum 

 total of phcenogamous species. As the multiplication of all species does not 

 follow a single law, and as these do not all produce an equal number of indivi- 

 duals the quotients obtained by dividing the total number of phcenogamous 

 plants, by the number of species of different families, do not by themselves 

 determine the aspect, or, it might almost be said, the nature, of the monotony 

 of vegetation in different quarters of the world. A traveller is often surprised 

 at the continual repetition of individuals of one species, and of the masses of 

 such individuals which are continually occurring ; but he has equal reason to 

 wonder at the rarity of other species which are useful to mankind. Thus 

 in countries where whole forests are formed by Rubiaceae, Leguminosse, and 

 Terebinthaceae,.the cinchonas, logwood, and balsam trees are comparatively very 

 rare. 



In the consideration of species, the subject may also be viewed in an 

 absolute manner, with reference to the number of species which prevail in parti- 

 cular zones. This interesting kind of comparison has been made in M. De 

 Candolle's grand work, and M. Kunth has carried it into effect with more 

 than 3,500 Composite? now known. It does not, indeed, indicate what families 

 predominate, in a given degree, over other phcenogamous plants, either with 

 regard to the number of species, or the mass of individuals ; but it determines 

 the numerical relations of species of the same family in different latitudes. 

 The most varied forms of ferns, for instance, are found in the tropics ; it is in 

 the mountainous, temperate, humid, and shady regions of those parts of the 

 world that the family of ferns produces the greatest number of species.' In 



