380 BIOLOGY 



animals, but it is not true of plants, which, as a rule, have no 

 independent power of motion. 2. By winds. Many plants 

 produce seeds or spores which can be blown for long distances 

 by the wind, until they land in a favorable locality, where 

 they can develop into new plants. This dispersal by the wind 

 is not so common among animals, although some of the lighter 

 animals which fly, like the insects and bats, may be blown 

 for long distances by the wind. 3. By water currents. The 

 ocean currents and fresh-water streams carry many animals 

 and plants long distances. The Gulf Stream carries living 

 organisms across the Atlantic Ocean, and a river flowing through 

 a country may distribute seeds for hundreds, and even thou- 

 sands, of miles. 4. Incidental means. There are various inci- 

 dental methods by which seeds or eggs, or even living animals, 

 may be distributed. -Wood-boring insects may be carried on 

 drifting logs; seeds may be carried in particles of mud clinging 

 to the feet of flying birds; living animals may be carried for 

 long distances on floating ice; ships carry living animals and 

 plants all over the world; migrating animals not infrequently 

 distribute seeds of plants as they move about from place to 

 place, and they may even carry living eggs and some living 

 animals in the same way. 



By some of these means all organisms have an efficient 

 method of distribution, and tend to scatter themselves in all 

 directions from the centers, where they are produced in large 

 numbers. Although the dispersal may be slow, in the end 

 even the most slowly migrating animal or plant might be 

 distributed over the face of the earth. All organisms tend 

 to disperse themselves until further migration is checked. 

 The factors which check their migration are spoken of as 

 barriers. 



Barriers.^ The ocean. Bodies of salt water are effectual 

 barriers against the distribution of land animals. Flying ani- 

 mals cross small bodies of salt water, and animals and plants 

 that are blown by winds may be distributed over the ocean 



