(4<>5) 



area which provides the necessary host conditions. Even with 

 the numerous cases which appear to be exceptions the argument 

 in favor of this tendency is still strong when the full data con- 

 cerning the apparent exceptions are considered. 



In the first place, we are not yet in a position to say just what 

 the geographic distribution of some of the species of fungi really 

 is because of insufficient field work. The four species (under A, 

 b) which are said to be distributed in the central part of the 

 area common to both sets of hosts are comparatively little known 

 species. When they are thoroughly collected they will doubtless 

 be found to have a broader distribution than that known at present. 

 The six species (under B) cited as bearing no apparent relation 

 to the area over which they might be distributed also need further 



clavariaeforme, for example, is known from Maine and Delaware 



and Wyoming. The host conditions are furnished in the area 

 between these regions through Minnesota, North Dakota and 

 Montana but so far as the writer knows the fungus has not been 

 reported from any of these states. The proper association of 

 hosts for this species also prevails clear across southern Canada, 

 but the fungus has apparently not been collected in this whole 

 area. Whether the lack of collections from these regions can be 

 taken to indicate that the fungus does not occur there seems very 

 doubtful. It is much more probable that it indicates lack of well 

 directed efforts toward collecting. G. juniperinum appears to 

 furnish a much better example of a possible exception to the 

 tendency of distribution over the common host area than do any 

 of the other species listed above. The hosts of this species, 

 Sorbus, and Juniperus § Oxycedrus are associated together from 

 Labrador across southern Canada and the great lake region to the 

 Rocky Mountains but the fungus is only known in the Rocky 

 Mountain region from Alberta and British Columbia south to 

 Colorado. 



Taken as a whole the evidence regarding the distribution of the 

 heteroecious species in this group indicates that the association 

 of the two sets of hosts furnishes the necessary conditions for the 

 growth of the fungi and that wherever this association prevails 

 we may expect, either now or eventually, to find the fungi. There 

 is practically nothing to indicate that other factors such as temper- 



