14 
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 
