62 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Vol.Yl. 



to the botaoy of the northern hemisphere generally. When they come 

 to be applied to the theoretical elucidation of the great difference be- 

 tween the Atlantic and the Pacific floras it will need to be noted that 

 the two sides of the continent, at the time when they received the pro- 

 genitors of the present vegetation, were more completely separated than 

 now; that they seem to have been, as it were, two long peninsulas 

 stretching southward from a mainland at the north, the great plains 

 between our eastern district and the Rocky Mountains being then under 

 water. 



It may be inferred that the Atlantic side of the continent was more 

 open than the western to the reception of the ancestral flora from the 

 north, and so received it in larger measure and variety, or that it has 

 been since t hat time more free from disturbance and catastrophe. Proba- 

 bly the two causes may have conspired in the production of the result. 

 There Is, moreover, reason to suspect that the recession of the glaciation 

 was earlier on the Atlantic side of the continent than in the more ele- 

 vated central and Pacific regions; and that, from all these causes, its 

 preglacial flora was more completely restored to it than to that of the 

 Pacific side. 



And, finally, we infer that the Pacific region, while preserving through 

 all vicissitudes a moderate number of boreal types, and receiving a few 

 Eastern Asiatic ones probably at a later date, has been mainly replen- 

 ished from the Mexican plateau, and at a comparatively late period. 

 .V large part of the botany of California, still more of Nevada, Utah, and 

 Western Texas, and, yet more, that of Arizona and New Mexico, may be 

 regarded as a northward extension of the botany of the Mexican plateau. 



This may, at least, be said : that two types have left their impress 

 upon the North American flora, and that its peculiarities are divided 

 between these two elements. One we may call the boreal-oriental ele- 

 ment ; this prevails at the north, and is especially well represented in 

 the Atlantic flora aud in that of Japan and Manchuria ; the other is the 

 Mexican -plateau element, and this gives its peculiar character to the flora 

 of the whole southwestern part of North America, that of the higher 

 mountains excepted. 



[Prom the American Journal of Science and Arts, Vol. XVI, 1878.] 



FOREST GEOGRAPHY AND ARCHAEOLOGY. 



By Asa Gray. 



[A lecture delivered before the Harvard University Natural History Society, April 18, 1878.] 



* * * It is the forests of the northern temperate zone which we are to traverse. 

 After taking some not" of them in their present condition and relations, we may in- 

 qnire Into their pedigree; and, from a consideration of what and whore the compo- 

 nent trees have been In days of old, derive some probable explanation of peculiarities 

 which otherwise seem Inexplicable. 



In speaking of our forests in their present condition, I mean not exactly as they are 

 to-day, hut as they were before civilized man had materially interfered with them. 



