128 STEVENS: PREVALENCE OF ENDOTHIA GYROSA 
extended the range of Endothia gyrosa beyond that known twenty- 
five years ago. It may therefore be fairly assumed that this 
fungus has reached its natural limits of distribution in this country 
and that the present range is determined by environmental factors, 
in contrast to fungi which have been recently introduced and have 
therefore not yet spread throughout the region suitable for their 
development. 
In collaboration with Dr. Shear the writer has prepared a 
map (Fic. 1), which gives a somewhat more accurate idea of the 
AS 
Fic. 1. Known distribution of Endothia gyrosa. Cross hatching indicates 
area in which the fungus is known to be abundant. Dots indicate localities where 
it has been collected. Map prepared by the writer in collaboration with Dr. C. L. 
Shear. : 
distribution of Endothia gyrosa than that previously published. 
The cross hatching indicates the area in which the fungus is 
known to be abundant, the dots outside that region indicate 
localities in which the fungus has been collected. In considering 
the distribution of E. gyrosa the problem is double, for the fungus 
has been found at widely separated localities hundreds of miles 
outside the region in which it is common. Conditions under 
which it can exist are not then, necessarily, those under which it 
will become abundant. The conditions which limit the occurrence 
