DESERTS AND THEIR INHABITANTS 129 
and pans of water lie upon every hand. Another month 
and all is drought; the pans are dry again, and travel is 
full of difficulty.” During the grassy season herds of 
springbok used to migrate in the old days to the Kala- 
hari, in the northern part of which giraffes live the whole 
year, although they must exist without tasting water for 
months. 
Although such a district can scarcely be termed a 
desert in the proper sense of the word, yet its sands have 
precisely the same origin as those of deserts of the typical 
description. 
For sand to accumulate to the depths in which it occurs 
in many parts of the Sahara and the Gobi by the slow 
disintegration of the solid rocks under the action of 
atmospheric agencies must require an enormous amount 
of time, to be reckoned certainly by thousands, and, for all 
we know, possibly by millions of years. And we accord- 
ingly arrive at the conclusion that the larger desert tracts 
must not only have existed as land for an incalculable period, 
but also as desert. Hence we can readily understand why 
the animals of Algeria and the rest of Northern Africa 
differ for the most part from that portion of the continent 
lying to the south of the northern tropic, the Sahara 
having for ages acted as an impassable barrier to most, if 
not all. 
But if other evidence were requisite, there is another 
reason which would alone suffice to compel us to regard 
deserts as areas of great antiquity. The habitable parts 
of all deserts—and it is difficult for the inexperienced 
to realise that barren tracts will suffice for the mainten- 
ance of animal life—are the dwelling-places of many 
animals whose colour has become specially modified to the 
needs of their environment. And it will be quite obvious 
9 
