the latter ; of Verbenacece, the Teak in India, and the Vervein in Europe; of Rubiacece, the Cinchona in South America, and the weak Rubia in Europe; and of the Euphorbiacece, the lowly spurges in European countries, the shrubby Euphorbias in the hot parts of Africa and India, and the arboreous species of Etnblica, Rottlera, Elceococca, Stillingiq, and Siphonia, in tropical parts of the world.
As the diminution of temperature in the atmosphere is very gradual, according to the elevation ; so is the disappearance of tropical forms as we ascend mountains : hence we find such plants diminishing in number and size as we climb either the Andes or the Himalayas. Their existence at considerable heights may probably be favoured by the range of the thermometer being less on mountain tops than on plains, even where the mean temperature is the same ; and, perhaps, the effect of the extremes of temper- ature may be less injurious when transmitted through a more rarefied medium. But in mountains under the influ- ence of tropical rains, a peculiarity of atmosphere occurs analogous to that so well characterised by Baron Humboldt in the Andes, as "the region of clouds." So in the Himalayas, at seven and eight thousand feet of elevation, the ther- mometer does not vary ten degrees during three months : and even when rain does not fall, there is constant humidity, from the air charged with moisture in the heated valleys rising and depositing it on the mountains, when it reaches an elevation where it is cooled below the point of saturation. The cloudiness, at the same time, preventing the full influence of the sun's rays; and at night the radiation from a mountain ridge bearing but a small proportion to the mass of the atmosphere, comparative little cooling takes place; and the thermometer is but a few degrees lower in the morning than it was on the previous evening : so that the same equability which we have observed at the base of
