TUB WORK OF ORGANISMS IN HOCKS. 7 



On land we find that grass and trees prevent the 

 mechanical removal of soil, and starr grass is planted on 

 dunes to stop the onward march of sand. Mosses and 

 lichens, by lessening the radiation from the rocks they 

 cover, are also agents in preservation. In our own 

 country we can scarcely realise how rocks may be broken 

 up by being subject to extremes of heat and cold. In the 

 tropics, however, the sun's heat and its withdrawal are 

 most important factors, and any covering which tends to 

 preserve an equable temperature renders the sun's rays 

 less powerful as an agent of change. 



Organisms in their Active Capacity. 



Turning now to organisms as active agents, let us 

 first consider their work in the unmaking of rocks. 



Some have the power of excavating crypts or 

 burrowing in search of food or protection, others browse 

 on coral or other material which affords them sustenance. 

 Among excavators we have Pholas, Saxicara, Cliona, 

 Worms, Limpets, Helix and other Grasteropods ; Ants, 

 Ant Bears, Puffins, Moles, Habbits and Man. Among 

 plants we have parasitic Algse, Mosses and Lichens, the 

 roots of trees and Bacteria. Among browsing animals I 

 may mention Holothuria, Starfish and Fish. 



In traversing the deserts of South Africa we occa- 

 sionally come across shallow hollows or " vleis " in which 

 water may be found in the time of rain, and sometimes 

 through the dry season. They are frequently half a mile 

 or more in diameter. What has excavated them? 

 Passarge* has suggested that they are due to the animals 

 which come there to drink. A curious calculation as to 

 how many beasis visit the place in a certain time, the 



* Passarge, Die Kalahari, Nature, vol. 71, p. 481. 



