28 Proceedings 
The same state of things was found in the case of the 
rust of wheat, barley, brome grasses, &c. 
We may state it thus: We have two forms or species 
(a) and (d), which morphologically are indistinguishable, 
showing even under the microscope no differences. Yet 
these two forms are essentially distinct physiologically or 
biologically, since we find that (a) can attack and live on 
only a certain host or hosts x, while (4) can only attack 
and live on other host-plants y, Such a form is called a 
biologic species. 
Eriksson found in the case of these rusts that the seven 
species of rusts attacking cereals, described by morpholo- 
gists, consisted really of no less than thirty “biologic 
species.” 
Some systematists are inclined to treat these ‘biologic 
species” with contempt. This is not surprising perhaps, 
considering what systematists are! and that the acceptance 
of the fact of the existence of biologic species prevents 
the systematist, pure and simple, from being that absolute 
king over his own dominion which he usually considers he 
has a right to be. In the old days, ten years ago, before 
Eriksson’s time, if a parasitic fungus occurring on a host- 
plant—which we will call (@)—had been taken to a sys- 
tematist he could have said with immense confidence : 
“This fungus is such and such a species !—notwithstanding 
the fact that this species has not hitherto been recorded 
on this host, because I have examined it microscopically, 
and it is identical.” 
Now-a-days, if he is a conscientious systematist, he can 
only say: “I have examined the fungus, and it agrees mor- 
phologically with such and such a species, but it may be 
quite distinct dzologically.” 
Now, it is obvious what great economic importance is 
attached to the problem: Do biologic species exist in the 
Erysiphacee ? 
Take the case of the Hop Mildew, Spherotheca Humu- 
Ui. This name was given by De Candolle in 1805 to the 
mildew growing on the Hop. Since then a form morpho- 
logically the same has been noted on twenty or thirty spe- 
