Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/352

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3. Those which grow within 30 or 35 degrees on each side of the Equator, 1200 species.

If we compare the amount of Ferns with the united numbers of other tribes of plants, we may form some idea of the relative importance of this family in the vegetation of the district, or period to which we apply such comparison. Thus, in the entire number of known species of plants now existing on the globe, we have 1500 Ferns and 45,000 Phanerogamiæ, being in the proportion of 1 to 30. In Europe this proportion varies from 1 : 35 to 1 : 80, and may average 1 : 60. Between the Tropics, Humboldt estimates the number in Equinoxial America at 1 : 36, and Mr. Browrrgives 1 : 20 as the proportion in those parts of intertropical Continents which are most favourable to Ferns.

Mr. Brown (Appendix to Tuckey's Congo Expedition) states that the circumstances most favourable to the growth of Ferns are humidity, shade, and heat. These circumstances are most frequently combined in the highest degree in small and lofty tropical islands, where the air is charged with humidity, which it is continually depositing on the mountains, and thereby imparting freshness to the soil. Thus in Jamaica Ferns are to the, Phanerogamiæ nearly in the proportion of 1 to 10; in New Zealand as 1 to 6; in Taiti as 1 to 4; in Norfolk Island as 1 to 3; in St. Helena as 1 to 2; in Tristan d'Acunha (extratropical) as 2 to 3. Ferns are also the most abundant Plants in the Islands of the Indian Archipelago.

It appears still further, that not only are certain Genera and Tribes of Ferns peculiar to certain climates, but that the enlarged size of the arborescent species depends in a great degree on Temperature, since Arborescent Ferns are now, found chiefly within, or near the limit of the Tropics.