1898 | FLORA OF LOWER SONORAN AND ARID ZONES 145 
likelihood mean an elevation of the Andes much higher above 
their present summits than we are warranted in ascribing to 
them. Professor Engler* says that the Andes of Venezuela, 
Colombia, and Ecuador could not have been completely glaci- 
ated during the glacial period, for in this zone we find peculiar 
tropical genera which, without doubt, date from the oldest times. 
In this event these mountains could not, within the era of pres- 
ent vegetation (dating presumably from somewhere in the Ter- 
tiary period) have had a much greater elevation than at present. 
On the other hand, there is conclusive evidence of a state of 
submergence during the Tertiary period and of subsequent 
upheaval, a fact significant in the present discussion. He says 
in effect: If the geological conclusions be correct, we have in the 
Tertiary period the Andes representing an island separated from 
the Guiana-Brazilian triangle of land by an arm of the sea, nar- 
row at the north, wider at the south, and from Central America 
by a strait. Central America was united with western North 
America, which latter was separated from eastern North America 
by inland seas. In these conditions an exchange of tropical 
elements between Central America, West Indies, Guiana-Brazil, 
and the Andes could occur. With the progress of upheaval of 
the Andes and consequent changes of climatic conditions the 
tropical nature of these mountains was modified, only those 
forms remaining which could adapt themselves to the extremes 
of greater altitude. With this elevation, in particular, the flora 
of the north pushed southward over a newly opened territory, 
peopling the Andes in such numbers that the present high andean 
vegetation is to be reckoned with the boreal. This southward 
wandering of North American species was at first of the hygro- 
Philous elements, embracing many forms coming from the Hima- 
layas to North America, and so explains the presence of 
Himalayan types in the Andes. But, as the Andes began to 
attain their present elevation, the moisture of the trade winds was 
withdrawn, and a pathway for more xerophytic elements was 
*% Entwicklungsgeschichte der Pflanzenwelt x: 198. 
“ENGLER: Entwicklungsgeschichte 1: 196-198. 
