or
butterflies, and to its juicy fruit, of which the common prickly pear is
a familiar instance, but it has the special property of springing afresh
from any stray bit or fragment of the stem that happens to fall upon the
dry ground anywhere.

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