288 Fink: LICHENS IN THE UPPER MIsSISSIPPI VALLEY 
same place? To what extent can one predict as to what species 
will inhabit certain substrata in one region from knowledge of 
species found upon like substrata in other localities? To what 
extent do fruticulose and foliaceous lichens seek different substrata 
than those occupied by crustaceous species? What evidence does. 
the present composition of a lichen-flora furnish concerning the 
former composition, the migrations which have taken place and 
their direction, and the struggle which has brought about the 
present floral composition ? 
In answering these questions, the various ecologic factors which 
influence the distribution of lichens must be kept in mind. Those 
which have especially attracted notice in my studies are the par- 
ticular kinds of substrata in a region; changes in these substrata 
through the destruction of old and the introduction of new ones 
naturally, or through some animate and commonly human agency ; 
climatic conditions of heat and moisture at present, and, in some 
instances, in the past as well; amount of illumination ; the effects 
of bodies of water in bringing moisture; direction of prevailing 
winds influencing temperature ; permanency and age of substrata 
and elevation of land. Other factors will present themselves to the 
careful observer. What I have outlined here is intended to be 
merely suggestive, and, for more detailed account, I must give refer- 
ence to some 250 pages in the papers cited above from the three 
authors named. It isto be hoped that the important factors sug- 
gested above as influencing lichen distribution, and any others which 
may be brought out in actual work in the field in the future may be 
kept in mind by lichen collectors; and, if this brief paper shall in 
any degree aid in bringing about this result, the improvement in 
results in the lichenological field of work will be ample reward. 
Notes regarding new or critical species might be added here, 
but I prefer to leave such work for future papers, adding here only 
a list of the 527 lichens hitherto found in the area under consider- 
ation and indicating the distribution by states. I have followed 
the arrangement, and, in general, the synonymy of Tuckerman, 
the latter of which is faulty in many instances, but which must serve 
till time is afforded for correcting it by careful research. Regard- 
ing the collections made by Dr. C. C. Parry and determined by the 
writer,* I have not been able to ascertain which were made in Wis- 
* Fink, B., 8. UMEN ERI aT 
