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.
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