158 HEKEDITY AND EVOLUTION IN PLANTS 



2. Occasional transport by birds across the Atlantic 

 Ocean. 



3. Migration across a direct Atlantic land-connection. 

 Human agency is generally rejected, except in cases where 

 it can positively be demonstrated. 



In interpreting the above facts Scharff argues that "the 

 interchange between the fauna and flora of north-western 

 Europe and north-eastern America was effected across 

 the northern land bridges," which th6 facts of distribu- 

 tion and other evidence indicate existed in pre- Glacial 

 times, and probably in late Phocene and early Pleistocene. 



Numerous alpine species have a present discontinuous 

 distribution in the lowlands of arctic and sub-arctic lati- 

 tudes, and on lofty mountain peaks, widely separated, 

 in more southern latitudes. Darwin called attention to 

 the fact that " a Ust of the genera of plants collected on the 

 loftier peaks of Java raised a picture of a collection made 

 on a hillock of Europe;" and again that "certain plants 

 growing on the more lofty mountains of the tropics in 

 all parts of the world, and on the temperate plains of the 

 north and south, are the same species or varieties of the 

 same species." A striking illustration of this latter fact 

 is the small white water-lily {Casialia tetragona), which is 

 found along the Misinaibi and Severn rivers in Ontario 

 (Canada), and at Granite Station, in northern Idaho 

 (U. S. A.), but is not known elsewhere except in Siberia 

 China, Japan, and the Himalaya mountains (Kashmir). 



The flora near the summit of Mt. Washington and other 

 peaks of the White Mountains, in New Hampshire, has 

 elements in common with that of Labrador. "In ap- 

 proaching these mountain summits," says Flint, ^ "one 



• Flint, William F. The distribution of plants in New Hampshire. 

 In Hitchcock, C. H. The Geology of New Hampshire, i : 393. 1874. 



