328 Science of Plant Life 



opportunities for new sets of plants to take possession of the 

 land. They are perhaps also the cause of mutations in plants 

 that give jise to new forms which replace the species from 

 which they were derived. 



The land areas have changed many times in the earth's 

 history. Sometimes large parts of the continents have been 

 under water ; at other times the continents have been more 

 elevated than at present. These changes in the land areas 

 have resulted in the extinction of many species of plants. 

 Forms derived from them may have survived on the land not 

 submerged. So the great changes in climate and in land 

 surfaces, and the less pronounced changes in plant habitats, 

 act as selective agencies, determining the kinds of plants that 

 survive. 



Isolation. If a continuous land area, having a number of 

 elevations, were submerged so that the elevations became 

 islands, a plant that had been originally distributed over the 

 whole area might persist on a number of the islands. The 

 plants on the several islands w^ould then be isolated from one 

 another. As time went on these isolated individuals might 

 give rise to new forms that were quite distinct in their char- 

 acteristics. Different new forms might arise on different 

 islands, and since the selective agencies on the various islands 

 would not be the same, the varieties that persisted on one 

 island might come to be quite distinct from those on other 

 islands. The production of the new forms depends on vari- 

 ation, but isolation tends to preserve new forms by preventing 

 their spreading and interbreeding. 



The deciduous forest of eastern North America was for- 

 merly continuous with the forests of China through Alaska. 

 Since then the connecting land has been depressed, and the 



