Deserts 
water, though it may be at great depths below the sur- 
face. The only real guide to the existence or absence 
of water is the character of the plant-world. The study 
of plant associations will assuredly lead to a definite 
knowledge of the capacities of an unknown and desert 
country when, that is to say, a sufficiently definite and 
practical classification of those associations has been 
attained. 
The desert of Egypt, which, thanks to our own energy 
and enterprise, is now being made valuable agricultural 
land, seems to have been once a land of marshes and 
lagoons, where primitive Egyptians lived, like the hunting 
and fishing tribes of the Sudd region. The silt of the 
Nile has filled up those lagoons to such a level that it is 
only by irrigation that they can be forced to bear a 
harvest. 
In other places, also, it is quite clear that it is man 
himself and his attendant animals (vegetable fiends like 
the goat, the camel, and the ass) that have destroyed the 
natural vegetation and turned what were once forest- 
clad hillsides and well-watered slopes into glaring stony 
hills and arid sandy plains. 
One cannot conclude, therefore, that the world is dry- 
ing up from such facts as the deposition of silt which 
has made dry alluvium of what was once a lagoon 
country, nor from the desiccation due to the destruction 
of forests, and the havoc of the goat, camel, and ass. 
But every sort of desert is always being invaded by 
vegetation. 
A country so dry as to be absolutely destitute of 
plants is exceedingly unusual. In most deserts, scanty, 
scattered little plants of the most miserable character 
manage, somehow, to exist in spite of fierce sunshine 
by day and severe cold at night, not to speak of the 
friction of the gritty dust carried by gales blowing with- 
170 
