264 ON THB IMPORTANCE OP THE STUDY, &C. 



Future observation will probably render them still more so, by- 

 associating them with important geographical phenomena It is an 

 indisputable fact that certain genera, and even orders, of plants are 

 mutually adapted, by that similarity in their organization which 

 occasions us to class them together, to inhabit, almost exclusively, 

 the different climates in which they are indigenous : particular pro- 

 portions in the mean temperature and moisture of the atmosphere, 

 perhaps combined with some other yet unappreciable influences, 

 acting under different parallels, or at different elevations upon the 

 surface of our globe, have prescribed limits which some of the natu- 

 ral groups of the vegetable kingdom have never yet passed, or 

 beyond which they are but very slowly progressing. Thus, the 

 extensive genus Erica, or Heath, consisting of about four hundred 

 known species, belongs almost exclusively to the southern extremi- 

 ty of Africa ; some eight or ten are scattered over Europe, but not 

 a single species Tias been hitherto discovered on the opposite conti- 

 nent of America, or its islands. There, however, the Cacti com- 

 municate a remarkable feature to the equatorial regions, and the 

 Oaks and Magnolias to the northern continent. To the neighbour- 

 hood of the Cape of Good Hope, the country of the Ericae, our col- 

 lections are, likewise, indebted for the multitudinous families of 

 Pelargonium and Mesembryanthemum ; while, on the other hand, 

 the PassiflorjE without, and the Bignonae with very few, exceptions, 

 are confined to the meridianal or central part of the new continent. 

 Of the Roses, not a single indigenous species is known either in 

 South America or Africa. Of the Aloes, only one species is a 

 doubtful native of the West-Indian islands — the Barbadoes Aloe of 

 the shops ; three or four species only of a genus originally very nu- 

 merous, though now divided, are met with out of Africa : while, 

 on the contrary, two allied genera, the Yucca and Agave, are ex- 

 clusively aborigines of the western continent. Other natural groups 

 and their species are found only at particular elevations above the 

 level of the ocean. The Cinchonaceae of the tropical forests, Hum- 

 boldt ascertained to be confined by limits of this description. And 

 though some of the epiphytic lichens are found to be indifferent 

 to temperature, others are only capable of existing at a certain 

 mean, and their presence, therefore, upon the dried bark would be 



