86 PLANT GEOGRAPHY 



epiphytic Orchids, and Palms ; and these last are repre- 

 sented by Rhopalostylis even in New Zealand. Torres 

 Strait is, even now, but a shallow and narrow separation. 

 The evidence seems to favour a belt of more or less 

 continuous " Melanesian " land from Papua to the Fiji 

 Islands, and through the New Hebrides and New 

 Caledonia to New Zealand ; but the outer, or " Micro- 

 nesian," chain, extending from the Philippines by the 

 Pelew, Caroline, Marshall, Ellice, Samoan, and Cook 

 groups to Tahiti and the Austral Islands, would seem to 

 have been stocked by the ordinary transmarine agencies, 

 currents, winds, and birds. 



CHAPTER IX 



MOUNTAIN FLORAS 



ALTHOUGH on low hills the direct and indirect effects of 

 altitude are not noticeable, in the ascent of mountains, 

 especially in the tropics, the existence of successive 

 altitudinal zones of climate and vegetation is very 

 marked. The mistakes have been made of attempting 

 to delimit these zones too precisely for the whole world, 

 and of explaining them exclusively by the fall of tem- 

 perature. The various changes that occur in the 

 surroundings of plants in the course of ascent to succes- 

 sively greater altitudes may be briefly recapitulated: 



1. Slope increases drainage and produces a marked 

 contrast between periods of light ("insolation") and 

 shade. 



2. Up to heights at which the temperature does not 

 become markedly cold, the amount of precipitation 

 (rain) increases with altitude, and the air becomes 

 consequently more humid. 



3. Temperature falls at a rate which has been roughly 

 averaged at i F. for every 300 feet until the snow-line 

 is reached, above which the snow never completely 

 disappears a line the altitude of which rises from 

 sea-level near the poles to about 2000 feet at North Cape, 

 4000 feet in Southern Norway, 8000 feet in the Alps, and 

 18,000 feet in the Himalayas. 



4. Winds become stronger. 



