710 



SCIENCE. 



[N. S. Vol. XII. No. 306. 



any of the various agencies operating now. 

 In other words, we should expect endemism 

 to increase with latitude and a consequent 

 minimum of forms common to two corre- 

 sponding zones. As a matter of fact, how- 

 ever, these simple conditions are wholly 

 changed because of the existence of north- 

 south zones of elevation, shading off vert- 

 ically from tropical to Arctic Alpine, and 

 cutting through the tropical and sub-trop- 

 ical latitudinal zones at right angles, and 

 so approximating a couuectiag bridge be- 

 tween boreal and austral zones. Here, 

 again, if we take any given era in the 

 history of vegetation and assume the north- 

 south zones to be continuous, the results of 

 distribution could be fairly predicted. 



But in considering the relations of the 

 floras of the two continents, no fact stands 

 forth more prominently than this, namely, 

 that we have to deal largely with the geo- 

 logical and climatic changes which have 

 taken place during the time since the flora 

 of the earth began to assume its present as- 

 pect, and to possess its present specific con- 

 tent. For many elements of vegetation 

 which still persist we could reasonably go 

 back as far as the Eocene Tertiary, although 

 much of what are called Peeuliar West Amer- 

 ican Elements must have developed at a very 

 much later period. Assuming, however, 

 the Eocene Tertiary as the starting point, 

 some very important conditions and changes 

 in the relations of the two continents may 

 be pointed out, which would influence the 

 development and the distribution of plant 

 life most profoundly. These are to be borne 

 in mind when we seek to explain the floral 

 relations of JSTorth and South America. 

 Such, for example, are the following : 



1. At the beginningoftheTertiary period, 

 a large part of "Western North America 

 was at sea level. The Kocky and the Sierra 

 Nevada Mountains ranged from 3,000 to 

 5,000 feet in elevation. West America was 

 separated from Atlantic America by a wide 



sea. By the end of the Tertiary Period the 

 elevation of West America was tripled, 

 bringing the mountains to at least their 

 present height and elevating the plains and 

 Great Basin region. 



2. During the Eocene and Miocene eras. 

 Central America was submerged, thus sep- 

 arating the two continents. 



3. With the beginning of the Tertiary 

 Period, the Andes stood but little above sea 

 level. A Cretaceous sea had extended along 

 their eastern front from Venezuela to Ar- 

 gentine, separating the Brazilian region 

 from the Andean. By the close of the Ter- 

 tiary, the Andes had emerged as much as 

 20,000 feet on their east front, and the re- 

 gion at their eastern base stood emerged 

 from the sea. 



4. During some portion of the late Ter- 

 tiary upheaval, or subsequently. South 

 America was joined to Cuba and probably 

 to Florida. There is reason to believe that 

 at a similar period the land masses of 

 Mexico and the Californian region included 

 the now isolated islands lying to the west- 

 ward, thus making a broad highway for 

 distribution between the American conti- 

 nents. 



5. The climate of the Eocene and Mio- 

 cene eras in North America was mild, and 

 permitted an extension of warm-temperate 

 flora as far north as Alaska. 



6. In the Pleiocene era the climate be- 

 came cooler. Subsequently in the Glacial 

 Pleistocene, the encroaching ice-sheet drove 

 all plant life far southward. As the Andes 

 were at their present height approximately, 

 and as the Central American highlands 

 in common with the Mexican Cordilleras 

 and the Eocky Mountains were in a period 

 of upheaval, probably greater than the pres- 

 ent, a highway was opened to the south for 

 Alpine and Arctic- Alpine elements, as well 

 as for the southward migrating warm tem- 

 perate flora. 



7. The sequence of upheavals which 



