60 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [YolYI. 



The natural and obvious line of communication between the botany of 

 the northern and southern temperate zones has been along the central 

 part of North America and Mexico, and along the western part of South 

 America. When our cool temperate flora flourished only along or near 

 the southern borders of the United States, the warm-temperate (to 

 which most of the above- enumerated forms belong) were still further 

 south. When the climate became again warmer, a portion of these 

 plants were as well placed for southward as for northward retreat. 



IV. 



NOTES ON THE SOURCES OF THE NOKTH AMERICAN FLORA. 



Before yielding the pen to his associate, who will develop the rela- 

 tions of the whole North American flora to those of other parts of the 

 northern hemisphere, the present writer may sum up, without develop- 

 ing them, one or two of the probable or plausible inferences or theo- 

 retical deductions which the present state of our knowledge, gathered 

 from a great variety of data, appear to enable us to draw. They are 

 conclusions the acceptance of which affords at least a clue to the expla- 

 nation of the condition, constitution, and seeming anomalies of the actual 

 geographical distribution of the genera and species of our part of the 

 world. The non-professional reader may best apprehend the ground of 

 these deductions by a perusal of the discourse already referred to, which 

 is appended to this report. 



The present vegetation of the world is a continuation with successive 

 modification of that of preceding geological times, and the plants indige- 

 nous to anj^ country are completely adapted to its climate, and there- 

 fore are capable of enduring its extremes. 



Accordingly the explanation of the ijresent assignment of species and 

 genera is to be sought partly in the geological past, partly in the actual 

 climate. Questions of the latter kind are comparatively simple. There 

 is no difficulty in understanding why our Atlantic region was naturally 

 covered with forest, why the great plains toward the Rocky Mountains 

 are woodless, and why plains with a saline soil are abandoned to a vege- 

 tation resembling that of sea-coasts. There is no insup'erable difficulty 

 in comprehending how high mountains may nourish forests, even when 

 favored with little absolute rainfall. The difficulty is in ascertaining 

 how a particular species of tree or other plant came to be a constituent 

 of a certain flora, at stations widely separated from its nearest relatives, 

 or even from other members of the same species. This is not a difficulty, 

 but only a sterile wonder, to those who suppose that facts of this order 

 have no scientific explanation, or none which they can hope to reach. 

 It is one only to those who assume that all the members of a species, 

 and even all the species of a natural genus, were derived at some time 

 or other from a common stock; but this is the assumx)tion now generally 

 made in natural history. A reference to the existing state of things will 



