THE VEGETABLE CLOTHING OF THE WORLD. 81 



3. Each species of plant has its center of distribu- 

 tion at the spot from which it originally sprang. It 

 is not easy, however, to determine these centers, be- 

 cause of plant migration. All plants are not 

 equally capable of migration, or the strongest would 

 have replaced the rest and occupied all the ground. 

 Migration is also hindered by seas, deserts, mountain- 

 chains, and climate, as well as by the existence of 

 other plants and animals. 



4. The transitions from one species to another met 

 with in gradually ascending mountain regions are 

 not such as Darwin's theory of natural selection might 

 lead us to expect, nor do they favor any theory of 

 transmutation of one kind into another. Such a 

 mountain-side is the most appropriate place in the 

 world for practically testing such theories. If any 

 transitional forms ever existed between species we 

 may reasonably expect to find them here. But the 

 Alpine species make their appearance, and those of 

 the plains disappear suddenly at particular elevations, 

 and we find no transitional varieties. 



5. Climate has much to do with the similarity in 

 the floras of different regions. This holds good in 

 widely separated regions, as seen in the resemblances 

 of the beeches of Japan and the Straits of Magellan, 

 and of the heaths of the Cape of Good Hope and of 

 Western Europe. 



6. Griesbach divides the surface of the earth into 



6 



