196 BOTANY. [CHAP. vn. 



logical peculiarities. A comparison of arctic vegetation 

 growing but little above the sea-level with that growing 

 at an elevation of 17,000 feet in the Himalayas, where 

 15 inches of pressure were removed, showed no observ- 

 able difference in the habits and characters of such 

 plants as were common to the two regions. Again, the 

 common plants of the lowland portions of India that 

 bloom during the cold season are identical with the 

 same species that bloom at great elevations in the same 

 country during the alpine summer ; hence the variation 

 sometimes observable between lowland and alpine forms 

 of the same species does not depend on diminished 

 atmospheric pressure. 



Moisture exerts a marked influence on the distribution 

 of plants as existing at the present time. The very 

 existence of life depends on the presence of water. The 

 plants of moist regions are often of a loose, spongy 

 texture with large, soft, smooth leaves, whereas those 

 characteristic of regions where there is but little mois- 

 ture in the air have the leaves firm and of a hard, dry 

 texture, as seen in many Australian plants, whereas in 

 very arid regions the leaves are often small and scanty, 

 and the stem covered with prominent spines. 



Mountain ranges, deserts, and seas are unsurpassable 

 barriers to the migration and distribution of plants ; in 

 the case of mountains a two-fold difficulty is presented ; 

 the plants of low areas would be exposed to an increasing 

 degree of cold as they ascended, and at the same time 

 would have to struggle with the vegetation indigenous 

 to the regions on which they encroached, hence as a rule 

 the floras on the opposite sides of elevated mountain ranges 

 are often very distinct, not necessarily due, however, to 



