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LIMITS TO THE RANGE OF PLANT-SPECIES. 215 
example is from South-west Madagascar. There is a very distinct 
desert region here, beginning a little west of Fort Dauphin, in 
which the flora is totally different both from the highland 
Malagash plants and the tropical jungle plants of the eastern 
mountain slopes. I found a few of these desert forms, whose 
presence puzzled me until I discovered the existence of the 
barren, waterless country to the east, which I had neither time, 
money, nor health to explore. 
Moreover, wherever any mountain raises its head above the 
low country, the climate, and therefore the plants, change 
completely. At the Cape, if one climbs the side of Table 
Mountain, there is a sudden definite and distinct change in the 
flora when one reaches the top, which is moistened by the 
refreshing mists of the Tablecloth. The droughty heathers and 
withered-looking little shrubs of the slopes give place to a 
profusion of beautiful orchids such as Disa, Irids of all kinds, tall 
and handsome composites, and so on. 
On Ruvenzori the forest is followed by Bamboos, then Tree 
Heathers, finally reduced Alpine plants. 
In the Pyrenees the Walnut trees in the valleys are followed 
by hardwoods, then come the fir woods, then Alpine meadows, 
finally the stunted Dryas and Salix reticulata, Linn., formation. 
The same sequence exists in Scotland. 
The conclusion to which I have come is that all floras are in 
a state of migration, and this migration is by no means easy to 
trace. Itis something like an army on the march into an enemy’s 
country. There is a sort of reconnoitring cavalry screen, then a 
broad border of piquets, then an advanced guard, behind which 
follows the main body. Each of these is composed of different 
species adapted to the peculiar conditions in whieh they function, 
For example, if the British plants are trying to occupy a sand- 
dune, Agropyrum begins on the barren sand exposed to the salt 
foam; then comes Psamma; in the ground occupied and half 
sheltered by Psamma comes Eryngium and the dune plants; 
then are found the wild grasses and sandy plants; and, finally, 
the main body of useful, if uninteresting, domesticated plants and 
their attendant weeds. 
A similar series of fringes may be found round every loch, even 
on bare rocks or the sides of a heap of blaes, 
