Fink: LICHENS IN THE Upper Mississipp1 VALLEY 287 
seen, are deposited in various herbaria. As the determinations were 
made by various persons, doubtless some of the names of species 
listed are affected by the personal element in determination, and 
specimens named differently would in some instances almost cer- 
tainly prove to be identical specifically could they be compared. 
However, the reports add much to our knowledge of the distribu- 
tion of North American lichens as given by Tuckerman,* as much 
of the work in all of the states considered except Missouri, has 
been done since his death in 1886. 
The papers referred to above have been too frequently mere 
lists, simply giving species and locality with no notes as to sub- 
strata or frequency of occurrence, and much less as to the causes 
which have led to the various peculiar features of distribution and 
habitat. So far as I know, the first paper to treat the subject of 
distribution in anything like a satisfactory manner was that by Mr. 
T. A. Williams} upon the lichens of the Black Hills. Since that 
time Professor C. Macmillanf has touched upon certain features, 
and the writer$ has attempted in several papers to consider lichen 
distribution in a somewhat adequate manner, presenting, to some 
extent, different phases of the subject in different papers as various 
localities and the progress of the work brought special problems 
into prominence. 
Some of the interesting questions which may well claim the at- 
tention of lichen collectors in the future are as follows: Why are 
some regions richer in species and individuals than others? Why 
do certain areas produce a relatively large proportion of epiphytic, 
lithophytic, or epigeal lichens? Why are lichens commonly 
epiphytic, lithophytic or epigeal found on other substrata in certain 
localities? Where are, and what constitute the dividing lines 
between licheno-biological areas? What particular lichen-floral 
elements predominate in certain regions and why? Why do we 
find rapid changes in the lichen-flora in passing a certain distance 
or in a certain direction from one place and not in going the same 
distance from some other locality or in a different direction from the 
* Tuckerman, E., 21 and 22. 
T Williams, T. A., 28. 
t Macmillan, C , 18. 
$ Fink, B., 11, I2, 13, I4 and 15. 
