iz AMERICAN FERN JOURNAL 
mutual reactions of the effects of several causes. Some 
of these are: 
(1) The greater portion of the state is too arid for a 
rich development of fern growth; much of the surface is 
unsuitable for any ferns, the suitable areas being few and 
restricted, thus limiting the possible number of species. 
(2) Entire absence of endemic forms; there are no 
species peculiar to Nebraska. . 
(3) Lack of development of features favorable to 
distributional adaptation; ferns are rather rigid in 
their requirements and do not readily overcome new 
environmental difficulties. 
(4) Ferns are comparatively old from the standpoint 
of evolution, being far beyond their period of culmina- 
tion, while the soil of Nebraska is comparatively new. 
(5) The centers of migration for the ferns into this 
area are the Appalachian and the Rocky Mountains. 
The broad prairies of northern Missouri, of Iowa, and 0 
southern Minnesota prevent migration from the east. 
A number of species of ferns that are absent from similar 
localities in eastern Nebraska are found along the east- 
ern border of Iowa. The aridity of the western portion 
of the state prevents migration from the Rocky Moun- 
tains. 
(6) The line of easiest migration into Nebraska is by 
way of the Missouri river valley, a route suitable to 
only a limited number of ferns. 
(7) Lack of development of a mountain range within 
the state or near by with a humid climate, the submon- 
tane region of western Nebraska being too arid. 
(8) The area of fern distribution tends to lessen, many 
species are now quite limited in range, and ferns as & 
whole have little migrating tendency. 
ACKNOWLEDGMENT 
The material on which this paper is based is in the 
herbarium of the department of botany of the University 
