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 



