NOVEMBEE 9, 1900.] 



SCIENCE. 



711 



brought the Great Basin and the arid 

 Southwest from sea level or submergence 

 to their present elevation, also witnessed 

 the development of a vigorous flora which 

 has continued to occupy these regions, con- 

 taining many of the peculiarly West-Amer- 

 ican groups. The same sequence of up- 

 heavals may have opened up similar areas 

 southward at the east of the Andes upon 

 which this flora could also extend, though 

 subsequently excluded by tropical condi- 

 tions more like the present. 



A vegetation developing under such con- 

 ditions as those cited above, would have had 

 a most varied, not to say precarious history, 

 now reaching far northward in luxuriance, 

 now driven back by the encroaching ice- 

 sheet ; now, a species distributed over a 

 wide area, and again only the remnants of 

 it in widely separated areas. Here, a ter- 

 rain covered with a varied vegetation 

 which with the next change of conditions 

 becomes a sea or an arid basin. Not only 

 were these tremendous changes going on 

 in the make-up of the two continents and 

 their relations to each other, but conditions 

 existed which related each to other land 

 masses, whereby floral elements were re- 

 ceived which were to play a part in the 

 subsequent development of the floral his- 

 tory. Such was the contact of the North 

 American region with Europe and Asia, 

 and of South America through the Ant- 

 arctic Continent with Australia, New Zea- 

 land and probably South Africa. 



In taking up a more specific analysis of 

 the floral elements common to both Amer- 

 icas, we must therefore bear in mind certain 

 physical conditions involving not only those 

 which prevail at the present time, but also 

 the varying conditions, which haveprevailed 

 since at least the middle Tertiary period. 



First, the north-south zones of elevation 

 have interruptions of distances great enough 

 to offer a very eflicient check to north-south 

 distribution, greater in the case of Arctic 



Alpine conditions, less in the transition 

 zone and greatest of all in the case of ex- 

 treme xerophilous elements of enclosed des- 

 ert basins and valleys. 



Second, we must allow for fluctuations 

 in elevation and depression of the conti- 

 nental axis, especially in the region of junc- 

 ture of the two continents, and consequent 

 changes in relation of the two land masses. 

 These fluctuations would extend back over 

 a period in which the flora of the earth was 

 undergoing tremendous changes, migrations 

 and adjustments, all of which would be in- 

 fluential in the final setting. 



Third, as to the sources of elements 

 which might be brought into the field of in- 

 fluence, we must allow for the intimate re- 

 lation of North America to the Eur-Asian 

 continent whereby floral elements were 

 shared in common, and for the early iso- 

 lation of the South American continent 

 from Antarctic land masses, although the 

 Antarctic flora of South America does show 

 a community of elements with South Africa, 

 Australia, New Zealand and Antarctic 

 islands. 



Fourth, the prevalent southward pres- 

 sure of elements is to be associated with 

 glacial influences which may well have been 

 most powerful in driving so great a boreal 

 element southward. This would be all the 

 more notable in the case of the warm tem- 

 perate xerophilous elements, which have 

 shown such vigor of development and en- 

 croachment, constituting the most charac- 

 teristic and unique elements of the New 

 World flora. 



The relationship of the floras of North 

 and South America will be discussed under 

 the following heads : 1st, The Gulf Zone 

 Neo Tropical, 2nd, The Alpine and Arctic- 

 Alpine, and 3rd, the Warm-temperate and 

 Semi- tropical xerophilous elements embrac- 

 ing (a) high plateau and mountain forms of 

 the Transition and Upper Sonoran Zones ; 

 (h) enclosed basin and valley forms of the 



