LIFE. 605 



tropical regions, plants growing at the water's edge, like the man- 

 grove, drop new roots from the branches into the shallow water, 

 which act like a thicket of brush-wood to retain the floating leaves, 

 stems and detritus; and, as the water shallows, other roots are 

 dropped farther out, which are attended with the same effect ; and 

 thus they keep moving outward, and subserve the double purpose 

 of protecting and making land. 



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, wherever existing, are usually 

 guarded from destruction with great care. 



2. Transporting Effects. 



1. Seeds are often caught in the hair or fur of animals, and thus 

 transported from place to place. 



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

 food, which sometimes pass out undigested, and become planted in 

 a new region ; and, in the case of birds on their migration, they 

 may be carried far from the place where gathered. 



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. Snails and fresh-water shells are often floated off on logs or 

 floating plants, and sometimes are carried into estuaries or the sea, 

 and so 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, cock- 

 roaches, and smaller vermin. The origin of tribes may often be 

 inferred from the species of plants and of domesticated and other 

 animals found to have accompanied them. 



3. Destructive Effects. 



The destructive effects proceed either from living plants or ani- 

 mals, or from the products of decomposition. 



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

 vice of a rock, as they increase in size, act like a wedge in tending 

 to press the rock apart ; and, when the roots are of large size, 

 masses tons in weight may be torn in sunder ; and, if on the edge 

 of a precipice, the detached blocks may be pushed off, to fall to its 

 base. 



