544 Wisconsin Academy of Sciences, Arts , and Letters. 
relations, being able to live under the same conditions, and do 
not differ in their morphological characters any more than do 
the squashes and the cucumbers. 
It is perhaps best at present to regard it as an open ques¬ 
tion whether any particular fungus which has acquired the 
parasitic habit first infected one species or a large number of 
more or less closely related plants. It is possible that the con¬ 
dition found in the mildew of the Brome grasses is the more 
primitive one, and that the mildew of the cucurbits represents 
a further step where a particular parasite has become adapted 
to live on a much wider range of hosts. Or the line of develop¬ 
ment may have proceeded in the other direction, the fungus at 
first infecting a large number of hosts and later becoming re¬ 
stricted and specialized to certain definite ones. 
Further investigations are needed which shall show whether 
there are cases where the mildew is even less specialized than 
it is on the cucurbits, the same form occurring perhaps on plants 
belonging to different families. It is possible also that we 
may find other cases where the specialization has proceeded so 
far that varieties of the same species of host plant may each 
be infected only by its own particular mildew. Such investiga¬ 
tions may also be expected to throw light on the nature and the 
origin of the parasitic habit. It is interesting to note in this 
connection that considerable doubt has been cast upon Masisee’s 
(17) theory of the chemotropic origin of parasitism by the 
recent work of Fulton (9). In view of the work of Miyoshi 
(18, 19) and others, however, we want much more conclusive 
evidence than has yet been brought forward before abandoning 
the idea that chemotronism is a factor of very considerable im¬ 
portance in relation to the parasitic habit of fungi. 
This work has been done under the direction of Professor 
R. A. Harper and I am greatly indebted to him for his kindly 
criticisms and helpful suggestions. 
Madison, Wisconsin, May 1906. 
