the upper Rio Negro valley in Colombia and Brazil, 
carrying out investigations of Hevea rubber and allied 
plants for the Bureau of Plant Industry of the United 
States Department of Agriculture in collaboration with 
the Instituto Agronémico do Norte. In large measure, 
I followed the trail of Richard Spruce in search of rare 
species of Hevea; it was possible also to make a small 
general collection, especially from the caatingas. It is 
amazing to learn how many of the collections represent 
plants which seem not to have been collected since 
Spruce’s time. The same is true of the collections of 
other naturalists (Ducke, Frées, Baldwin, Mur¢a Pires 
and Black) who have recently penetrated the Rio Negro. 
With deep respect and in a spirit of humbleness, I 
dedicate this paper to Richard Spruce on the occasion 
of the Centennial of his arrival on the continent of South 
America for his self-sacrificing exploration which, with- 
out respite, he carried out from 1849 to 1864 in Brazil, 
Colombia, Ecuador, Peru and Venezuela. A pioneer 
phytogeographer, he was animated by a deep love of all 
nature and an insatiable thirst for knowledge in pure 
science. In dedicating this paper, I cannot refrain from 
using two touching passages from Spruce’s writings 
which reveal the inner soul of the man. In a letter writ- 
ten at Sio Gabriel on the Rio Negro on February 17, 
1851 to Mr. Baines and preserved in the Yorkshire Philo- 
sophical Society (Report 1907 Yorksh. Philos. Soe. 
(1907) 18), we read what has appealed to me as the sim- 
plest and most honest presentation of the underlying 
motives of phytogeographical endeavors. 
‘“Then there is the greatest of all pleasures to the naturalist, how- 
ever some utilitarians may affect to undervaluate it, that of discovering 
new species, of dotting in (as it were) new islands on the map of na- 
ture, and, in some cases, of even peopling continents that appear to 
be deserts. ’’ 
And, in another letter, directed to Mr. George Ben- 
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