DESERT SANDS 353 



South Africa, Siiid, Rajputaim, and elsewhere unspecified, 

 have been driven by the nature of their circumstances and 

 the dryness of the soil to adopt precisely the same tactics, 

 and therefore unconsciously to mimic or imitate the cactus 

 tribe in the minutest details of their personal appearance. 

 Most of these fallacious pseudo-cactuses are really spurges 

 or euphorbias by family. They resemble the true Mexican 

 type in externals only ; that is to say, their stems are thick, 

 jointed, and leaf-like, and they grow with clumsy and awk- 

 ward angularity ; but in the flower, fruit, seed, and in short 

 in all structural peculiarities whatsoever, they differ utterly 

 from the genuine cactus, and closely resemble all their 

 spurge relations. Adaptive likenesses of this sort, duo to 

 mere stress of local conditions, have no more weight as 

 indications of real relationship than the wings of the bat 

 or the flippers of the seal, which don't make the one into 

 a skylark, or the oth.'^r into a mackerel. 



In Sahara, on the other hand, the prevailing type of 

 vegetation (wherever there is any) belongs to the kind 

 playfully described by Sir Lambert Playfair as ' salso- 

 laceous,' that is to say, in plainer English, it consists of 

 plants like the glass-wort and the kali-weed, which are 

 commonly burnt to make soda. These fleshy weeds 

 resemble the cactuses in being succulent and thick-skinned 

 but they difter from them in their curious ability to live 

 upon very salt and soda-laden water. All through the 

 great African desert region, in fact, most of the water is 

 more or less brackish; 'bitter lakes' are common, and 

 gypsum often covers the ground over immense areas. 

 These districts occupy the beds of vast ancient lakes, now 

 almost dry, of which the existing chotts, or very salt pools, 

 are the last shrunken and evanescent relics. 



And this point about the water brings me at last to a 

 cardinal fact in the constitution of deserts which is almost 



