﻿ON THE BOTANY OF JAPAN. 449 



up to its present level, and down to its present cool temperature," * — giving the arctic 

 flora its present range, and again separating the temperate floras of the New and of 

 the Old World to the extent they are now separated. 



Under the light which these geological considerations throw upon the question, I 

 cannot resist the conclusion, that the extant vegetable kingdom has a long and eventful 

 history, and that the explanation of apparent anomalies in the geographical distribution 

 of species may be found in the various and prolonged climatic or other physical vicis- 

 situdes to which they have been subject in earlier times ; — that the occurrence of 

 certain species, formerly supposed to be peculiar to North America, in a remote or an- 

 tipodal region afibrds of itself no presumption that they were originated there ; — and 

 that the interchange of plants between Eastern North America and Eastern Asia is 

 explicable upon the most natural and generally received hypothesis, (or at least ofl"ers 

 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 now do, are points which go far towards explaining 

 why Eastern North America, rather than Oregon and California, has been mainly 

 concerned in this interchange, and why the temperate interchange, even with Europe, 

 has principally taken place through x\sia. 



* For the collocation and communication of tlie geological data here presented, I am indebted to the kind- 

 ness of my friend, Professor Dana. 



I 



*^t* On page 425, after line 6, insert Brasenia peltata in the Asiatic, Japanese, Western North American, 

 and Eastern North American columns. 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 Exploring Expedition, in a stream which foils into Gray's Harbor, lat. 47°. 

 It must be local on the western side of the continent, or it would have been met with before. Wlien this re- 

 markable plant was known to occur only in Eastern North America and Eastern Australia, it made the strong- 

 est 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 Northwestern America also, 

 the case seems to crown the conclusions to which this memoir arrives. 



Page 428, line 11, in the Asiatic column, add Osmorrkiza hrevistylis. 



