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, has 
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.) 
