The Evolution of Plants 329 



two continents, Asia and North America, have become 

 separated and their climates have changed. This separa- 

 tion resulted in the isolation of the deciduous forest plants 

 of eastern Asia from those of eastern America. Many of 

 the same species are still found in both areas. But in the 

 two widely separated regions there are many other quite 

 distinct species that have arisen by evolution from a com- 

 mon ancestor. The American sycamore, or example, is 

 distinct from the Cliinese sycamore ; but the resemblance 

 between the two is sufficient to suggest that they have a com- 

 mon ancestry. 



Variations; the eUmination of many individuals through 

 lack of room in which to grow ; changes in climates and in land 

 areas ; and isolation are beHeved to be important factors in 

 the evolution of plants. 



