LIFE. 607 



form the same kinds of geological work, being very effectual agents in 

 entangliDg detritus, and in protecting from erosion. 



5. Patches of forest-trees, on the declivities in Alpine valleys, serve 

 to turn the course of the descending avalanche, and entangle snows 

 that, but for the presence of the trees, would only add to its extent ; 

 and in the Alps, such groves are usually guarded from destruction, 

 with great care. Forests also retard the melting of snow and ice in 

 spring, and thus lessen the devastations of floods. Earthworms trans- 

 fer earth to the surface and loosen soils. 



6. The calcareous Algae, called Nullipores (p. 135), served to pro- 

 tect the margins of coral reefs from wear ; and ordinary seaweeds 

 often cover and protect the rocks of a coast nearly to high-tide level. 



2. Transporting Effects. 



1. Seeds caught in the hair or fur of animals, or contained in the 

 mud adhering to their feet, are transported from place to place. 



2. Seeds are eaten by animals as food, or in connection with their 

 food, and are dropped in another region undigested. 



3. Ova of fish, reptiles, and inferior animals are supposed to be 

 transferred from one region to another by birds and other animals. 

 Authenticated instances of this are wanting. 



4. Floating logs and seaweeds carry Mollusks, Crustaceans, Worms, 

 and other species from one region to another along the courses of ma- 

 rine currents. Sometimes land and fresh-water shells, etc., are borne 

 from rivers into estuaries or the sea, there to become mingled with 

 marine shells. 



5. Migrating tribes of men carry, in their grain, or otherwise, the 

 seeds of various weeds, and also, involuntarily, rats, mice, cockroaches, 

 and smaller vermin ; also insects injurious to vegetation, and other 

 kinds. The origin of tribes may often be inferred from the species of 

 plants and of domesticated and other animals found to have accom- 

 panied them. 



3. Destructive Effects. 



The destructive effects proceed either from living plants or animals, 

 or from the products of decomposition. 



1. The roots which come from the sprouting of a seed in the crevice 

 of a rock, as they increase in size, act like wedges, in tending to press 

 the rock apart ; and, when the roots are of large size, masses tons in 

 weight may be torn asunder ; and, if on the edge of a precipice, the 

 detached blocks may be pushed off, to fall to its base. This is one of 

 the most effective causes of the destruction of rocks. Many regions 



