OCR Text |
Show ANNOTATIONS A D ADDITIONS. 297 merely tells how many of the species of one and the same family are indigenous in each country or each quarter of the world. The results of this method are on the whole more exact, because they are obtained by the careful study of single families without the necessity of being acquainted with the whole number of the phanerogamre belonging to each country. The most vivid forms of Ferns, for example, are found between the tropics; it is there, in the tempered heat of moist and shaded places in mountainous i lands, that each genus presents the largest number of species: this variety of species in each genus diminishes in passing from the tropical to the temperate zone, and decreases still farther in approaching nearer to the pole. Nevertheless, as in the cold zone-in Lapland, for example-those plants succeed best which can best resist the cold, so the species of Ferns, although the absolute number is less than in France or Germany, are yet relatively more numerous than in those countries ; i. e. their number bears a greater proportion to the sum total of all the phanerogamous plants of the country. These proportions or ratios, given as above mentioned by quotients, are in France and Germany 7)- and 7 1 11 and in Lapland '1\· I published numerical ratios of this kind-( i. e. the entire quantity of phrenogamous 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 phanerogamre change in the same proportion as does the number of species belonging to a particular family. |