Page:An Essay on the Antiquity of Hindoo Medicine.djvu/26

LauracecB, Myristicece, Scitaminece, Musacece, and Pipe- racecB, which prevail in equatorial forests, and perfectly characterise tropical vegetation ; but they do not do so more than the Anonacece, Dilleniacece, Guttiferce, Auran- tiacecB, Dipterocarpece, Sapindacece, Meliacece, Combre- tacece, Cinchonacece, Bignoniacece, Ebenacece, Sapotea:, CycadacecB, Bromeliacece, Pandanacece, and many others. As tropical climate is not terminated by an abrupt line, but, according to the influence of local causes, extended into higher latitudes, so does it carry with it the peculiarities of tropical vegetation. Thus, if the country be open towards the equator, the equinoctial rains will be blown with the periodical winds to much higher latitudes than where a mountain range intervenes, or the course of the winds is diverted by a high and shelving coast. Hence India, open to the south, is deluged by the periodical rains ; while Egypt, in the same latitude, owes its fertility only to the overflowing of the Nile. The same effects may ensue, if the same causes operate, along a valley or a mountain range. Thus, Humboldt has shewn that, in the basin of the Ohio, Gleditsia monosperma, the Catalpa, and Aristolochia Sipho, extend three degrees further north than on the coast of the Atlantic. So, along the base of the Himalayas, where there is considerable moisture of the soil, a vigorous vegetation has sprung up, which adds to the humidity of the atmosphere, as well by exhalation from its leaves as by preventing free evaporation from the ground. A greater equality of temperature is also preserved, from the umbrageous covering at once im- peding the ready absorption of heat by day, as it checks free radiation during the night. Hence, along this tropic- girt base, with greater equability of temperature, we find more uniform moisture than in the open plains; and in it many plants extending several degrees further north than they do in any other part of India; as, a species of Cinna-