98 : Muhlenbergia, Volume 4 
on Solidago and one on Erigeron, which can not be made to in- 
terchange, although they possess no evident morphological dif- 
ferences. This result depends, as I assume and as seems to be 
pretty well authenticated by all the researches, upon a question 
of nutrition.” 
He states that he has described these three forms as dis- 
tinct species, because it was more convenient to designate them 
as species “until a time when the matter could be more fully 
considered.” 
“They are not three species in the generally accepted sense 
because they can not be distinguished morphologically. What 
then is, or should be, our conception of a species? Is it some- 
thing to be distinguished physiologically or morphologically, or 
in both ways?” 
“Now, just another instance that arose during an extensive 
series of culture studies; that is the case of the Helianthus rusts. 
They have been described according to the host plants on which 
they grow. Each species of Helianthus and ite close relatives 
appear to bear a distinct kind of rust, which acts in cultures as 
if it were an independent species. Yet any and all of these will 
grow on Helianthus annuus, a so-ealled bridging host. Are 
these different species different biological or physiological spe- 
cies, or simply forms or races? Possibly it would be well to re- 
fer them to some sub-category, as Dr. Britton has suggested.” 
“Finally, I would say, at least it is a rule which I have 
formulated for my own guidance—it was a necessity that I 
should formulate some rule—that species, which are concepts, 
as I take it, for our convenience in discussing the various ques- 
tions pertaining to plants, should be distinguished by sufficient 
morphological characters, the distinctions based upon physio- 
logical differences having subspecific rank. What constitute 
sufficient morphological characters must be left to the individ- 
ual judgment. 
I shall quote only the last two paragraphs of Dr. MacDou- 
gal’s short but interesting article. 
