134 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
arid regions are usually more pubescent than those of cool, moist 
habitats. Pubescence is extremely variable in the same species and 
except in a few cases of no moment whatever. 
It is in the flower structures that the greatest and most constant 
differences are tobe found. The relative length of the sepals, laming, 
and spurs is very uniform for plants of the same species, as is also 
the position of the sepals, and it is on these characters that most of 
the present classification is based. Color is quite suggestive, although 
yellow forms of red flowers and white forms of blue flowers are not 
uncommon. Neither the stamens nor the pistils offer many dif- 
ferences, although the length of the style is at times a valuable 
character. The size of the follicles and their shape vary but little. 
That indefinable resultant of several characters—these mostly 
unimportant in themselves—described by the overworked word 
“aspect” is often asafeand sure guide to species, butit must necessarily 
be learned by each for himself, since it can not be stated except in 
suggestive terms. Finally, the range of a species must be regarded 
as a check of great importance. Plants differing by comparatively 
slight characters yet consistently separated in range must be consid- 
ered distinct species, while plants exhibiting what seem to be no 
greater differences, yet having no distinct ranges, must be consid- 
ered conspecific. Since hybridism is so easy and so prevalent, it is 
difficult to see how very closely related species could remain distinct 
when growing together. In this connection it must be remembered 
that in the western and particularly in the southwestern portions of 
this continent habitats vary immensely within a few miles, and 
that, although the ranges may seem to overlap on the map, the 
species commonly do not really intermingle, being separated by 
many hundred feet of altitude and living in very different surround- 
ings. 
The genus Aquilegia has for several years been of unusual interest 
. to the author, and it is with great pleasure that, through the kind- 
ness of Prof. Aven Nelson, he has been able to bring to completion 
what he hopes will prove a conservative survey of the North Ameri- 
can species. The work was done at the Rocky Mountain Herbarium 
under the direction of Professor Nelson, to whom the writer is greatly 
indebted for suggestions and constant encouragement and for making 
possible certain field work which has been of great advantage. To 
Mr. J. Francis Macbride, of the Gray Herbarium, the author wishes 
also to express his thanks for bibliographic assistance. 
The success of any taxonomic work must depend largely upon the 
availability of adequate exsiccatse. In this matter the author feels 
himself particularly fortunate and wishes to acknowledge his indebt- 
edness to the individuals and institutions who have so kindly lent 
material. He is under special obligation to the authorities of the 
