ILLUSTRATIONS (13). DISTRIBUTION OF PLANTS. 283 



each family, or the great leading divisions (acotyledons, mono- 

 cotyledons, and dicotyledons), with the sum total of the phanero- 

 gamia. In the frigid zone, the variety of forms, or the number 

 of the genera, does not decrease in an equal degree with that 

 of the species, there being in these regions relatively more 

 genera and fewer species.* The case is almost the same 

 on the summits of high mountain-chains, where are sheltered 

 individual members of many different genera which one 

 would be disposed to regard as belonging exclusively to the 

 vegetation of the plain. 



I have deemed it expedient to indicate the different points 

 of view from which the laws of the distribution of vegetable 

 forms may be considered. It is only when these points of 

 view are confounded together, that we meet with contradic- 

 tions, which have been unjustly attributed to uncertainty of 

 observation. f When expressions like the following are em- 

 ployed: "This form, or this family diminishes as it approaches 

 towards the cold zone," or "the true habitat of this form is 

 in such or such a parallel of latitude;" or "this is a southern 

 form," or, again, "it predominates in the temperate zone;"' 

 it should be definitely stated whether reference is made to 

 the absolute number of the species, and the proportion of their 

 predominance according to the increase or decrease of lati- 

 tude; or whether the meaning conveyed is, that a family, 

 when compared with the whole number of the phanerogamia 

 of a flora, predominates over other families of plants. The 

 impression conveyed to the mind of the predominance of forms, 

 depends literally on the conception of relative quantity. 



Terrestrial physics have their numerical elements as well 

 as the cosmical system, and it is only by the united labours 

 of botanical travellers that we can hope gradually to arrive 

 at a knowledge of the laws which determine the geogra- 

 phical and climatic distribution of vegetable forms. I have 

 already observed that in the temperate zone of the northern 

 hemisphere, the Compositee ^Synantheree) and the Glumaceae 

 (in which latter division I place the three families of the 

 Gramineae, the Cyperoideae, and the Juncaceae) constitute the 

 fourth part of all phanerogamia. The following numerical 



* Decandolle, ThZorie elementaire de la Botanique, p. 190; Hum- 

 boldt, Nova genera et species Plantarum, t. i. pp. xvii. 1. 



t Jahrbucher der Gewachskunde, bd. i. Berlin, 1818, s. 18, 21, 30, 



