Fink: rock lichens of taylors falls. 11 



the north and south of it, questions of extreme interest have 

 been suggested to me which can onl}^ be solved by a study of 

 this flora. 



The principal conclusions are as follows : 



(i) The region considered in this paper is an important one 

 for the study of lichen-flora because of position, and 

 geological relations past and present. 



(2) The flora considered is one of great interest as to origin 



and present composition and as to evidence of struggle 

 between flora elements. 



(3) The present lichen-flora is composed of arctic, sub-arctic 



and temperate florae elements in which the last have long 

 since gained the advantage and are killing out the 

 others. 



(4) It is not supposed that the northern species migrated south 



in post-glacial times, but rather that this flora is one that 

 followed the last retreat of the glaciers and was for a 

 time essentially arctic, having since changed to its pres- 

 ent composition. 



(5) Reasons for the above conclusions are as follows : 



{a) Southward migration would more naturally result 

 from the decrease of temperature to the south 

 inaugurated by the on-coming of a glacial cli- 

 mate and would thus go on even though suit- 

 able substrata might be somewhat scarce. 



{b) But during the glacial advances rocky substrata 

 were doubtless more numerous than now, a 

 condition adding to the ease of migration. 



(c) Under the influence of increasingly favorable cli- 

 matic conditions to the south, the plants would 

 take more easily to unfavorable substrata and 

 migrate more readily on this account also. 



{d) The 8 northern lichens are all but one essentially 

 rock lichens and are, therefore, the ones that 

 would be expected to persist longest. 



(6) The northern floral elements considered may have been 



driven south during a late glacial advance not extending 

 quite to the region, but more probably have been forced 

 to migrate further south several times and migrate north 

 as many times. 



