GEOGRAPHICAL DISTRIBUTION 



157 



Among the plants may be mentioned ladies' tresses (Spi- 

 ranthes Romanzoffiana) and the seven-angled pipewort 

 (Eriocaulon septangular e); and among animals, the land 

 snail, Helix hortensis. 



FIG. 73. Asa Gray (1810-1888). Noted American botanist and student 

 of plant geography. 



Of the various theories which have been advanced to 

 explain the occurrence of identical species on opposite 

 shores of the North Atlantic, Scharff enumerates the 

 following three. 1 



i. Migration from Europe across Asia and a Bering 

 Strait land-bridge to America, or vice versa. 



1 Scharff, Robert Francis. On the evidences of a former land bridge be- 

 tween northern Europe and North America. Proc. Royal Irish Acad. 

 288:1-28. Nov., 1909. 



