ARTHUR: PROBLEMS IN THE Stupy oF Pranr Rusts 11 
they inhabited. The aecidial hosts inthis case are exceedingly di- 
verse and represent about the whole gamut of the spermatophytes. 
A similar case is that of the American Carex rusts, having aecidia 
on the three large intergrading genera Solidago, Aster and Evrigeron, 
Although they have not yet been studied thoroughly enough to 
speak with entire confidence, still it is evident that they have very 
slight morphological distinctions, and possibly have no constant 
differential characters except the association of the aecidial host. 
The practical question that arises is in regard to the systematic 
treatment of such forms. Shall they be classed as species, sub- 
species, varieties or races? This is not so much a question of 
fact, as of convenience and intelligent interpretation. No one, 
nowadays, believes that species are circumscribed in any definite 
manner, but that usually they exhibit developmental mobility 
which is recognizable in all manner of intergradations and differen- 
tiations. What is wanted of systematic botany is a nomenclature 
that expresses in a fair way the relation that exists between forms 
as found in nature. It will at the best be an imperfect and un- 
equal interpretation of the facts, but it surely ought to be employed 
with understandable uniformity. 
Botanists, I believe, have not often gone to the extreme of 
some of their co-workers in allied sciences. From a communi- 
cation in a recent number of Science it appears that ‘‘in ornithology, 
and especially in mammalogy, perfectly good species are often so 
similar in size and color that even the expert can not satisfactorily 
identify them from descriptions. * * * They present to the eye 
differences that are sufficiently impressive but which, owing to the 
imperfection of descriptive terms, can not be adequately expressed 
in keys or in diagnoses.’’ If one may venture an opinion on 
matters outside of his own domain, forms which differ by such 
intangible characters as here indicated, so that they can not be 
expressed on paper, are not species in the generally accepted sense. 
They may be deserving of names, and be of great interest to the 
specialist, and may serve most important ends as basis for par- 
ticular inquiries, but their rank is not that of true species in the 
general sense; at least, I think this may be safely asserted if 
plants are in question. 
I need not speak of the validity of physiological species as 
