AND THE ADJOINING TEBRITORY. 207 



is a further point to be noted— that on the west coast the rainfall 

 is almost exclusively confined to winter, whereas in Patagonia it 

 is spread pretty uniformly over the seven warmer months of the 

 year from October- to April, not more than five inches falling in the 

 five colder months, from May to September, a condition evidently 

 favourable to tree-vegetation. 



The true explanation, in my opinion, of the exceptional 

 poverty of the Patagonian flora is to be sought in the direction 

 long ago indicated by Charles Darwin, when, in discussing the 

 absence of tree-vegetation from the pampas, he remarks that in 

 that region, recently raised from the sea, trees are absent, not 

 because they cannot grow and thrive, but because the only 

 country from which they could have been derived — tropical and 

 subtropical South America — could not supply species organized 

 to suit the soil aud climate. So it happened in Patagonia — 

 raised from the sea during the latest geological period, and 

 bounded to the west by a great mountain-range mainly clothed 

 with an Alpine flora requiring the protection of snow in winter, 

 and to the north by a warm temperate region whose flora is 

 mainly of modified subtropical origin — the only plants that could 

 occupy the newly formed region were the comparatively few 

 species which, though developed under very different conditions, 

 were sufficiently tolerant of change to adapt themselves to the 

 new environment. The flora is poor, not because the land cannot 

 support a richer one, but because the only regions from which a 

 large population could be derived are inhabited by races unfit 

 for emigration. The rapidity with which many introduced species 

 have spread in this part of South America is perhaps to be 

 accounted for less by any special fitness of the immigrant species, 

 than by the fact that the ground is to a great extent unoccupied. 

 Doubtless, if no such interference had taken place, and the 

 operation were left to the slow action of natural causes, a gradual 

 increase in the vegetable population would come about. Fresh 

 species of Andean plants would gradually become modified to 

 suit the climate of the plain (perhaps one such recent instance 

 is supplied in Boopis laciniata of the following list) ; still more 

 slowly new varieties would have been developed among the indi- 

 genous plants, from which, by natural selection, new species 

 would have been formed. No doubt these causes have been in 

 action during the short time that has elapsed since Patagonia 

 has existed as part of the continent; but the time has been far 



