Page:Aspects of nature in different lands and different climates; with scientific elucidations (IA b29329668 0002).pdf/114

 *[Footnote: this kind,—(i. e. the entire quantity of phænogamous plants in each of the different Floras divided by the number of species in each family)—in my Prolegomenis de distributione geographica Plantarum, in 1817; and in the Memoir on the distribution of plants over the Earth's surface, subsequently published in the French language, I corrected my previously published numbers by Robert Brown's great works. In advancing from the Equator to the Poles, the ratios taken in this manner vary considerably from the numbers which would be obtained from a comparison of the absolute number of species belonging to each family. We often find the value of the fraction increase by the decrease of the denominator, while yet the absolute number of species has diminished. In the method by fractions, which I have followed as more instructive in reference to the geography of plants, there are two variables; for in proceeding from one isothermal line, or one zone of equal temperature, to another, we do not see the sum total of all the phanerogamæ change in the same proportion as does the number of species belonging to a particular family.

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]*