8 MINNESOTA BOTANICAL STUDIES. 



sion a large number of individuals cannot become established 

 on them and remain long enough to constitute a flora rich in in- 

 dividuals, yet because of the porous character of the sandstone 

 more of the species characteristic of temperate regions have 

 doubtless already become established on them in one place or 

 another than on the igneous rocks. 



While the 8 species and varieties so fully treated above are of 

 special interest there are some thoughts concerning the other 70 

 (excluding the last one listed) that must not be lost sight of. 

 As to distribution they are a heterogeneous group, 30 of them 

 being pretty generally distributed over the United States and 

 Canada, 24 being limited so far as their distribution is known 

 to the territory east of the Rocky mountains, 7 being thus far 

 found only in the northern United States and Canada and 5 oc- 

 curring throughout the United States. The North American dis- 

 tribution of the last 4 here considered is so little known that 

 nothing can be definitely stated of it. Of these 70 all but 4 or 

 5 occur on rocks in some other portion of North America, but 

 only 15 are strictly rupicoline. Of the other 55, some, though 

 more characteristic of rocks, are more or less frequently found 

 growing on other substrata ; and others actually prefer other 

 substrata and are growing on rocks here under unusually favor- 

 able conditions. These lichens, like the others, are of course the 

 descendants of a race that has migrated several times. Nearly 

 all of them being species also occurring in Europe, it is certain 

 that they were represented by like species during early Tertiary 

 times, far to the north where our continent was then connected 

 with the Eastern continent on both sides. The coming of a 

 cooler climate and finally of glacial conditions inaugurated the 

 series of migrations. Finally after the last retreat of the glaciers 

 began, the 55 species, because of their adaptation to more than 

 one substratum, would follow the retreat more surely and more 

 rapidly and thus more certainly and sooner reach a given 

 locality and begin to replace a flora growing under unfavorable 

 conditions. To just what extent the arctic flora would become 

 established before these species would come in and begin to re- 

 place it can not be stated since the rate of retreat of the glaciers 

 relative to the rate of migration of essentially stationary organ- 

 isms is not known. 



In the second paper of this series, I accounted for the com- 

 parative scarcity of lichens about Minneapolis by dryness of 



