ON THE RELATION OF THE FLORA OF THE LOWER 
SONORAN ZONE IN NORTH AMERICA TO THE 
FLORA OF THE ARID ZONES OF CHILI AND 
ARGENTINE.* 
WILLIAM L. BRAY. 
Tue relation of the flora of extra-tropical North America to 
that of extra-tropical South America was first discussed by Gray 
and Hooker in their report on “The vegetation of the Rocky 
mountain region and a comparison with that of other parts of the 
world,” under the title “ North American types in South Amer- 
ica.” In this report there was printed a list of some eighty 
genera which contained identical or more or less closely related 
species in the southwestern states and south of the equator, con- 
cerning which they remark: 
Clear if not very numerous indications exist that there has at some time 
been greater opportunity for extension of North American types into the 
Southern hemisphere. It appears that this has taken place along the western 
side of the American continent, along the central part of North America and 
Mexico, and the western part of South America. When our cool temperate 
flora flourished only along or near the southern border of the United States, 
the warm temperate, to which most of the plants enumerated belong, was 
still farther south. When the climate became again warmer, a portion of 
these were as well placed for southward as for northward retreat. 
Two years later Professor Engler’ defined this relation of 
the two floras more precisely, presenting a revised list of about 
eighty genera, involving many species, characterized chiefly by 
the fact that they were elements of the Mexican plateau and 
*For assistance in the preparation of this paper very cordial thanks are due to 
the management of the Berlin Herbarium, and in particular to Geheimrath Professor 
Adolph Engler, at whose suggestion the subject was taken up, and by whose friendly 
courtesy I was enabled to have free access to all of the resources of the herbarium. 
‘Bulletin U. S. Geol. and Geog. Surv. of the Territories 6 :—- 1880. 
‘ * Versuch einer Entwicklungs-geschichte der Pflanzenwelt 2 : 224. 1882. 
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