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Show ANNOTATIONS AND ADDITIONS. 299 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 phanerogamre of which a Flora consi ts. The impression of prevalence as conveyed by the eye depend on relative quantity. Terrestrial physics have their numerical elements, as has the System of the Universe, or Celestial Physics, and by the united labors 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 Compositre (Synantherere), and the Glumacere (including under this latter name the three families of Grasses, Cyperoidre and Juncacere), make up the fourth part of all phrenogamous plants. The following numerical ratios are the results of my investigations for 7 great families of the vegetable kingdom in the same temperate zone. Glumacere t (Grasses alone 1'2-) Compositre {r Leguminosre 1 11r Labiatre ·b: Umbelliferre ·io Amentacere (Cupuliferre, Betulinere, and Salicinere) q;1; Cruciferre 1 17i The forms of organic beings are in reciprocal dependence on each other. In the unity of natm·e these forms limit each other according to laws which are probably attached to periods of long duration. If on any particular part of the globe we know with accuracy the number of species of one of the great families of Glumacere, Leguminosre, or Compositre, we may with a tolerable degree of probability form approximative inferences, both as to the sum of all the phanerogamre of the country, and also as to the number of species belonging to the rest of the leading families of plants. The number of Cyperoidre determines that of Compositre, and the number of Compositre that of Leguminosre; they even enable us to judge in what classes or orders the Floras of countries are still incomplete, and teach us, if we are on our guard against confounding together very |