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 dififerent 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 

 refreshincr mists of the Tablecloth. The drouo;htv 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 Euvenzori 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. It is 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 difi'erent 

 species adapted to the peculiar conditions in which 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. 



