4 MINNESOTA BOTANICAL STUDIES. 



questions involved are, it seems to me, none the less worthy of 

 consideration when we notice that it is one of a series of similar 

 areas where certain floral elements have become isolated and 

 gradually killed out by others. 



The 8 species commonly found farther north have evidently 

 migrated southward, and there are at least two views as to time 

 and method or cause of migration worthy of consideration. 

 First, the northern species might perhaps have migrated from 

 Lake Superior along the exposures of igneous rocks extend- 

 ing from the lake to four or five miles below Taylors Falls in 

 quite recent times, long after the retreat of the last glacier. 

 Second, they seem undoubtedly to be the remnant of a flora 

 driven south, doubtless from some region far north of Lake 

 Superior, by the advancing glaciers and left stranded on favor- 

 able substrata at Taylors Falls as the southern extremity of a 

 flora migrating south before the glacier or more probably mi- 

 grating north on the return of post-glacial climate in the north 

 temperate zone. 



The outcrops of the igneous rocks between Lake Superior 

 and Taylors Falls are not frequent enough to make either theory 

 seem very plausible ; but the second is reasonable since, under 

 the influence of slow decrease in temperature to the southward, 

 migration would naturally follow increasingly favorable cli- 

 matic conditions in that direction even if the outcrops were not 

 more frequent than now. However, it seems probable that at 

 the time of the first glacial advance the outcrops were much 

 more numerous than now. Also, the rock lichens, now found 

 on the igneous rocks only, doubtless found a foothold on the 

 sandstones on the line of retreat under the more favorable cli- 

 matic conditions of glacial times. The numerous boulders of 

 the same igneous rocks, scattered over the ground by the gla- 

 ciers could help in the advance southward of some of the 

 species since glacial times, but hardly of those seldom or never 

 found on boulders, as the Umbilicaria listed. On the whole, it 

 seems doubtful whether a single one of the 8 northern species 

 has migrated southward in post-glacial times under increasingly 

 unfavorable conditions as to climate and substrata. 



As the remnant of a lichen-flora driven south by glaciers, 

 these plants must either have been stranded during a late gla- 

 cial advance, as during a slight advance during, or more prob- 

 ably after, the Wisconsin stage ; or more probably have been 



