76 THE FITNESS OF THE ENVIRONMENT 



cipal constituent of active living organisms. 1 ' 

 Water is ingested in greater amounts than all 



measure it falls back again directly into the ocean. A con- 

 siderable proportion, however, descends upon the land, and 

 it is this part of the condensed vapor which we have now 

 to follow. Upon the higher elevations it falls as snow, and 

 gathers there into snow fields, which, by means of glaciers, 

 send their drainage towards the valleys and plains. Else- 

 where it falls chiefly as rain, some of which sinks underground 

 to gush forth again in springs, while the rest pours down the 

 slopes of the land, swelling the brooks and torrents which, 

 fed both by springs and rains, gather into broader and yet 

 broader rivers that bear the accumulated drainage of the 

 land out to sea. Thence once more the vapor rises, con- 

 densing into clouds and rain to feed the innumerable water 

 channels by which the land is furrowed from mountain top 

 to seashore. 



"In this vast system of circulation, ceaselessly renewed, 

 there is not a drop of water that is not busy with its allotted 

 task of changing the face of the earth. When the vapor 

 ascends into the air, it is, comparatively speaking, chemi- 

 cally pure. But when, after being condensed into visible 

 form, and working its way over or under the surface of the 

 land, it once more enters the sea, it is no longer pure, but more 

 or less loaded with material taken by it out of the air, rocks, 

 or soils through which it has traveled. Day by day the 

 process is advancing. So far as we can tell, it has never ceased 

 since the first shower of rain fell upon the earth. We may 

 well believe, therefore, that it must have worked marvels 

 upon the surface of our planet in past time, and that it may 

 effect transformation in the future." — Geikie, "Textbook 

 of Geology." London, 1903, 4th ed., Vol. I, pp. 447, 448. 



1 Thus water makes up from 70 to 85 per cent of fishes, 

 about 87 per cent of oysters, 85 per cent of apples, 78 per 

 cent of potatoes, 95 per cent of the edible portion of lettuce, 

 etc. 



