THE WAT IN WHICH NEW PLANTS ARISE 131 



is fonnd only on tlie granite slopes of Stone Moun- 

 tain in Georgia. 



Every species originated in some one locality, and 

 it spreads its seeds or spores by various methods. 

 The spores or seeds find a foothold wherever they 

 may in suitable places. Probably no species has 

 succeeded in senduig seeds or spores to all of the 

 places which would be suitable for its growth. As 

 the species spreads across a continent, it may meet 

 barriers in the form of seas, mountains, or other 

 obstacles which stop its progress. 



179. Methods of reproduction. — The life of each 

 individual is limited, and no matter whether the 

 number is great or small, if each does not constantly 

 give rise to new individuals the species would soon 

 become exterminated. Furthermore new individuals 

 must be produced as fast as the older ones die, or 

 extermination will result. Numerous species of ani- 

 mals have disappeared in the memory of man, but 

 modern examples of the extinction of plants are not 

 common, although the earth's crust is rich with the 

 remains of species existent in former geologic periods. 

 A tree belonging to the sunflower family, once found 

 on the island of Saint Helena, is known to be ex- 



