Periodic Mutations 695 



species has never been denied. It is observed 

 throughout extended localities, and during long 

 series of years. Other proofs are afforded by 

 those plants which have been transported to dis- 

 tant localities some time since, but do not ex- 

 hibit any change as a result of this migration. 

 Widely dispersed plants remain the same 

 throughout their range, provided that they be- 

 long to a single elementary species. Many 

 species have been introduced from America into 

 Europe and have spread rapidly and widely. 

 The Canadian horsetail {Erigeron canadensis) , 

 the evening-primrose and many other instances 

 could be given. They have not developed any 

 special European features after their introduc- 

 tion. Though exposed to other environmental 

 conditions and to competition with other spe- 

 cies, they have not succeeded in developing 

 a new character. Such species as proved ade- 

 quate to the new environment have succeeded, 

 while those which did not have succumbed. 



Much farther back is the separation of the 

 species which now live both in arctic regions 

 and on the summits of our highest mountain- 

 tops. If we compare the alpine flora with the 

 arctic plants, a high degree of similarity at once 

 strikes us. Some forms are quite identical; 

 others are slightly different, manifestly repre- 

 senting elementary species of the same sys- 



