282 ESSAYS. 



Asclepiadece, the very numerous Polemoniacece, HydrophyU 

 lacece, Eriogonece, and the like. 



I had formerly recognized this element in our North Amer- 

 ican flora ; but I have only recently come to apprehend its full 

 significance. With increasing knowledge we may in a good 

 measure discriminate between the descendants of the ancient 

 northern flora and those which come from the highlands of 

 the southwest. 



