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Show 298 PHYSIOGNOMY OF PLANTS. We may, if we please, pass from the consideration of species to that of divisions formed in the natural system of botany according to an ideal series of abstractions, and direct our attention to Genera, to Families, and even to the still higher, i. e. more comprehensive, Classes. There are some genera, and even some entire families, which belong exclusively to particular zones of the Earth's surface; and this not only because they can only flourish under a particular combination of climatic conditions, but also because both the localities in which they originated, and their migrations, have been limited. It is otherwise with the greater number of genera and of families, which have their representatives in all regions of the globe, and at all latitudes of elevation. The earliest investigations into the distribution of vegetable forms related solely to genera; we find them in a valuable work of Treviranus, in his Biology (bd. ii. s. 47, 63, 83, and 129). This method is, however, less fitted to afford general results than that which compares either the number of species · of each family, or the great leading divisions (of Acotyledons, Monocotyledons, and Dicotyledons) with the sum of all the phanerogamre. We find that in the cold zones the variety of forms does not decrease so much if estimated by genera as if estimated by species; in other words, we find relatively more genera and fewer species. (Decandolle, Theorie elementaire de la Botanique, p. 190; Humboldt, Nova genera et species Plantarum, t. i. pp. xvii. and l.) It is almost the same in the case of high mountains whose summits support single members of a large number of genera, which we should have been a p?·iori inclined to regard as belonging exclusively to the vegetation of the plains. I have thought it desirable to indicate the different points of view from which the laws of the geographical distribution of plants may be considered. It is by confounding these different points of view that apparent contradictions are found, which are unjustly attributed to uncertainties of observation. (Jahrbiicher der Gewachskunde, bd. i. Berlin, 1818, s. 18, 21, 30.) When such expressions as the following are made use of-aThis form, or this 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 |
