True cactuses (in the native state) are confined to America; but the unhappy naturalist who ventures to say so in mixed society is sure to get sat upon (without due cause) by numberless people who have seen ’the cactus’ wild all the world over.  For one thing, the prickly pear and a few other common American species, have been naturalised and run wild throughout North Africa, the Mediterranean shores, and a great part of India, Arabia, and Persia.  But what is more interesting and more confusing still, other desert plants which are not cactuses, living in South Africa, Sind, Rajputana, 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 awkward 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, due to mere stress of local conditions, have no more weight as indications of real relationship than the wings of the bat or the nippers of the seal, which don’t make the one into a skylark, or the other 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 ‘salsolaceous,’ 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 differ 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 always utterly misconceived in Europe.  Most people at home picture the desert to themselves as wholly dead, flat, and sandy.  To talk about the fauna and flora of Sahara sounds in their ears like self-contradictory nonsense.  But, as a matter of fact, that uniform and lifeless desert of the popular fancy exists only in those sister arts that George II.—­good, practical man—­so heartily despised, ‘boetry and bainting.’  The desert of real life, though less impressive, is far more varied.  It has its ups and downs, its hills and valleys.  It has its sandy plains and its rocky ridges.  It has its lakes and ponds, and even its rivers.  It has its plants and animals, its oases and palm-groves.  In short, like everything else on earth, it’s a good deal more complex than people imagine.