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132 ACACIAS IN VARIOUS PLACES. (Ses ox. 
near deserts, and, so far as My own experience goes, they are 
almost always the scouts or pioneers which extend farthest 
into the desert. 
Any attempt to make separate associations of every 
transitional stage between desert and wood must result in 
confusion, because all sorts of changes take place Climates 
do not, as is usually supposed, remaim invariable. Im such a 
transitional zone, if a climate becomes, ever so little, drier, 
then the pioneer acacias will be killed out, woods will break 
up into thickets, and piomeers are formed from the thickets. 
li, on the contrary, the climate becomes a degree more humid, 
then the acacias break new ground im the desert amd the 
whole army of plants behind them colonises a little mere 
of it. 
Moreover, changes in the vegetation may occur without 
any change im climate. The acacia fringe is, or used to be, 
the favourite pasture of hordes of grazing animals, antelope, 
zebra, guanaco, kangaroo, etc. Now these amimals are not 
necessarily mere vegetable demons. If ome watches a flock 
of mermo kids devastating am acacia without paying the 
slichtest attention to its horrible-looking spimes, one is apt 
to think that this mild, mmocent-looking little ammmal is a 
mere Apollyon of planta. The camel and the goat also are 
supposed, and probably with some reason, to have utterly 
devastated the flora of Koypt. But m all such eases the 
battle is uniair- grazing animals im big battalions are supplied 
with water and protected by man, so that enormous numbers 
act on one particular spot Under matural conditions I 
think that grazing amumals, although they may do harm, do 
an enormous amount of good. 
Desert soil consists of sand, gravel, or rock; it 1s unworked 
soil, “urboden” in the German sense; it has no leaf-mould, 
no black earth full of valuable salts and decaying animal 
matter; there are no worms and probably no nitrogenous 
bacteria except those of the acacia and other leguminous 
roots. Im such places the manure of grazing animals is 
seattered by birds and imsects over a square foot or so of the 
neighbouring soil and improves it enormously. On this 
square foot young herbaceous seedlings will develop 
vigorously ; they will form a close green carpet during the 
rainy season and will protect each other, remalming green 
