with Geographic Botany. — Humidity. 331 



supply of moisture, would not support a vegetation : much of 

 the coast of Chili and Peru has not a plant on it ; the soil is 

 further rendered ungenial by a copious admixture of nitre and 

 muriate of soda; a few valleys alone intersect the surface, 

 carrying a stream or a river to the ocean, but here often is a 

 lively vegetation. 



The activity of man has in some measure compensated for 

 the sterility of nature ; large districts have been subjected to 

 a methodical and well-regulated irrigation, and rendered 

 capable of bearing crops and fruits ; in Egypt irrigation was 

 in former times carried to a considerable extent, as it is in 

 many parts of the world at the present day. Some plants re- 

 quire a certain submersion for their growth : rice, which of 

 the various grains supports the greatest portion of mankind, 

 requires this either by natural or artificial means ; and with 

 Caladium esculentum, the root of which is the taro and staple 

 food of the Pacific islanders, it is the same. 



Estimates have been made as to the various ways in which 

 the moisture which falls on the land has been appropriated ; 

 these are sometimes very vague, and disagree among them- 

 selves. In this manner it has been stated that evaporation 

 again removes a fifth part, and that vegetation disposes of 

 another fifth, the three remaining being carried off in a fluid 

 form by streams and rivers. These proportions do not agree 

 with what has been advanced respecting the relative amount 

 of rain, dew, and evaporation in different latitudes, and we 

 are further assured of their slender claims to correctness on 

 finding one person stating the quantity passing off by rivers in 

 England to be equal to four inches, and another authority fixing 

 it at thirteen. An important part of the water which finds its 

 way into rivers is still destined to administer to the wants of 

 vegetation. In those hot climates where most of the large 

 rivers are found, they periodically overflow their banks and 

 inundate the adjoining plains ; no rain may fall anywhere near 

 these parts of the rivers, which are usually in the neighbour- 

 hood of the mouth, but at great distances, frequently among 

 the mountainous countries whence they take their origin, as 

 is the case with the Nile in a remarkable degree, and with the 

 Ganges. The natives residing near their banks are eminently 

 alive to the great benefit conferred on their cultivated grounds 

 by the rising of the waters, and frequently regard the river, 

 especially at the period of its swelling, with much reverence 

 and religious awe. Large quantities of vegetable substances 

 in different stages of decomposition are swept down in the in- 

 creasing current, and, spreading over the surface of the sub- 

 mersed country , are left behind on the subsiding of the river. 



Z 2 



