THE FLORA OF JAPAN. 141 



pothesis (or at least offers no greater difficulty than does the 

 arctic flora, the general homogeneousness of which round the 

 world has always been thought compatible with local origin 

 of the species), and is perhaps not more extensive than might 

 be expected under the circumstances. That the interchange 

 has mainly taken place in high northern latitudes, and that 

 the isothermal lines have in earlier times turned northward 

 on our eastern, and southward on our northwest coast, as they 

 do now, are points which go far towards explaining why east- 

 ern North America, rather than Oregon and California, lias 

 been mainly concerned in this interchange, and why the tem- 

 perate interchange, even with Europe, has principally taken 

 place through Asia. 



Brasenia peltata. — To the remarks upon the known range 

 of this species, I have now to add the interesting fact, that 

 it exists upon the northwestern coast of America, having 

 been gathered by Dr. Pickering, in Wilkes's South Sea Ex- 

 ploring Expedition, in a stream which falls into Gray's Har- 

 bor, lat. 47°. It must be local on the western side of the con- 

 tinent, or it would have been met with before. When this 

 remarkable plant was known to occur only in eastern North 

 America and eastern Australia, it made the strongest case in 

 favor of double creation that perhaps has ever been adduced. 

 But since it has been found to occur throughout the eastern 

 Himalayas and in Japan, and has now been detected in north- 

 western America also, the case seems to crown the conclu- 

 sions to which this memoir arrives. (Note to reprint in 

 " American Journal of Science and Arts," 3 ser., xviii. 199.) 



