282 ESSAYS. 
Asclepiadee, the very numerous Polemoniacew, Hydrophyl- 
lace, Eriogonee, 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. 
