| OCR Text |
Show ANNOTATIONS AND ADDITIONS. 295 dependent on the predominance of particular families of plants, which render it either desolate or adorned, smiling or majestic. Grasses forming extensive savannahs, Palms and other trees affording food, or social Coniferre forming forests, have powerfully influenced nations in respect to their material condition, to their manners, to their mental dispositions, and to the more or less rapid development of their prosperity. In studying the geographical distribution of forms, we may consider pecies, genera, and natural families, separately. In social plants, a single species often covers extensive tracts of country; as in northern regions forests of Pines or Firs and extensive heaths ( ericeta), in Spain cistus-covered grounds, and in tropical America assemblages of the same species of Cactus, Croton, Brathys, or Bambusa Guadua. It is interesting to examine these relations more closely, and to view in one case the great multiplicity of individuals, and in another the variety of organic development. We may inquire what species produces the greatest number of individuals in a particular zone, or we may ask which are the families to which, in different climates, the greatest number of species belong. In a high northern region, where the Compositre and the Ferns are to the sum of all the phrenogamous plants in the ratio of 1 : 13 and 1 : 25 (1". e. where these ratios are found by dividing the sum total of all the Phanerogamre by the number of species belonging to the family of Compositre or to that of Filices or Ferns), it may nevertheless happen that a single species of Fern covers ten times more ground than do all the species of Compositre taken together. In this case Ferns predominate over Compositre by their mass, or by the number of individuals belonging to the same species of Pteris or Polypodium; but they do not so predominate if we only compare the number of the different specific forms of Filices and Compositre with the sum of all the pbrenogamous plants. Since, then, multiplication of plants does not follow the same law in all species,-that is to say, all species do not produce the same number of individuals,therefore the quotients given by dividing the sum of the phrenogamous plants by the number of species belonging to one family, do not suffice by themselves to determine the character of the landscape, or the physiognomy which Nature assumes in different regions of the |
