between carnivore and prey, only those species of plant can survive in
such exposed situations which happen to develop spines, thorns, or
prickles as a means of defence against the mouths of hungry and
desperate assailants.

Nor is this so difficult a bit of evolution as it looks at first sight.
Almost all plants are more or less covered with hairs, and it needs but
a slight thickening at the base, a slight woody deposit at the point,
to turn them forthwith into the stout prickles of the rose or the
bramble. Most leaves are more or less pointed at the end or at the
summits of the lobes; and it needs but a slight intensification of this
pointed tendency to produce forthwith the sharp defensive foliage of
gorse, thistles, and holly. Often one can see all the intermediate
stages still surviving under one's very eyes. The thistles, themselves,
for example, vary from soft and unarmed species which haunt
out-of-the-way spots beyond the reach of browsing herbivores, to such
trebly-mailed types as that enemy of the agricultural interest, the
creeping thistle, in which the leaves continue themselves as prickly
wings down every side of the stem, so that the whole plant is amply
clad from head to foot in a defensive coat of fierce and bristling
spearheads. There is a common little English meadow weed, the
rest-harrow, which in rich and uncropped fields produces no defensive
armour of any sort; but on the much-browsed-over suburban commons and
in similar exposed spots, where only gorse and blackthorn stand a
chance for their lives against the cows and donkeys, it has developed a
protected variety in which some of the branches grow abortive, and end
abruptly in stout spines like a hawthorn's. Only those rest-harrows
have there survived in the sharp struggle for existence which happened
most to baffle their relentless pursuers.

Desert plants naturally carry this tendency to its highest point of
development. Nowhere else is the struggle for life so fierce; nowhere
else is the enemy so goaded by hunger and thirst to desperate measures.
It is a place for internecine warfare Hence, all desert plants are
quite absurdly prickly. The starving herbivores will attack and devour
under such circumstances even thorny weeds, which tear or sting their
tender tongues and palates, but which supply them at least with a
little food and moisture: so the plants are compelled in turn to take
almost extravagant precautions. Sometimes the leaves end in