20 ON FARLETON FELL 
as broad glaciers into the sea. But the life of these 
high northern plants is slow and difficult, as is 
evidenced by their paucity and their stunted stature. 
Later on we shall have to consider how they adapt 
themselves to the adverse conditions under which 
they exist (Chapter VIII.); and we shall find their life 
problems are reproduced in many respects by those of 
the interesting alpine plants which may be found 
nestling in the rock crevices of the higher mountains 
of our own country. 
But the more familiar deserts of the world, those to 
which the mind turns when we use the term, are 
mainly due, not to absence of light as in the ocean 
depths, nor to want of heat as in the polar regions, 
but to failure of the water-supply. A vast desert 
region of this kind stretches across Northern Africa 
from west to east, and onward through Arabia, 
Southern Persia, and Baluchistan. Another, almost 
continuous with it, extends from the Caspian Sea 
across great plains into Central Asia, and on over 
vast mountain areas into Western China. Other 
similar deserts, familiar to us in word and picture, are 
situated in the south-western United States, Mexico, 
and South Africa. In all these tracts, with their 
diverse characters and diverse sparse floras, the 
scarcity of rain is the primary cause of their peculiar 
features. The dryness prevents a protecting cover- 
ing of vegetation, and allows heat and cold—both 
sharply accentuated by the scarcity of the moderating 
influence of water in either soil or air—to pursue their 
work of disintegrating the surface, reducing the rocks 
to sand and dust, which the winds sweep hither and 
thither. In such circumstances plants exist under 
