- [Footnote: family, diminishes as the cold zones are approached;—it
has its true home in such or such a latitude;—it is a southern form;—it predominates in the temperate zone;" care should always be taken to state expressly whether the writer is speaking of the absolute number of species, and its increase or decrease with the change of latitude; or whether he means that the family in question prevails over other families of plants as compared with the entire number of phanerogamæ of which a Flora consists. The impression of prevalence as conveyed by the eye depends on relative quantity.
Terrestrial physics have their numerical elements, as has the System of the Universe, or Celestial Physics, and by the united labours of botanical travellers we may expect to arrive gradually at a true knowledge of the laws which determine the geographical and climatic distribution of vegetable forms. I have already remarked that in the temperate zone the Compositæ (Synanthereæ), and the Glumaceæ (including under this latter name the three families of Grasses, Cyperoidæ and Juncaceæ), make up the fourth part of all phænogamous plants. The following numerical ratios are the results of my investigations for 7 great families of the vegetable kingdom in the same temperate zone.
Glumaceæ 1/8 (Grasses alone 1/12)
Compositæ 1/8
Leguminosæ 1/18
Labiatæ 1/24
Umbelliferæ 1/40
Amentaceæ (Cupuliferæ, Betulineæ, and Salicineæ) 1/45
Cruciferæ 1/19
]*
