I. THE LICHEN FLORA OF ICELAND. 



THERE is hardly any other group of plants in which the boundary 

 line between the species is so indefinite as it is in the Lichens. 

 Several types are easy to describe, and readily recognizable after 

 description, but between such readily recognizable types there fre- 

 quently occur so many intermediate forms, that we are quite per- 

 plexed in deciding to which type or species - - or whatever we now 

 choose to call it the plant in question should be referred, when 

 it is to be included in a list of species. No doubt the majority of 

 botanists have occasionally tried to determine, for instance, some 

 or other Cladonia-species and have thereby experienced for them- 

 selves the difficulties which thus arise. But as with Cladonid, so 

 is it with the majority of the genera, only, in many cases, the dif- 

 ficulties are even more considerable. To the less skilful investiga- 

 tors any sure determination is usually impossible, but even for the 

 best-trained lichenologist, it is often extremely difficult to identify 

 a species which he has before him, with one already known and 

 described by others, a circumstance which has caused much contro- 

 versy, to a great extent unnecessary, between the "patres" of lichen- 

 systematology. 



The reason of this richness of forms, this abundance of forms 

 intermediate between the most easily distinguishable types, is not 

 known. We may naturally form our surmises on the subject. It 

 may be assumed that the lichen-group, taken as a whole, is a group 

 in process of rapid development, that is to say, in the act of forming 

 numerous new species which, in the course of lime, will separate 

 themselves into a smaller number of easily distinguishable species, 

 through many of the intermediate forms dying out. Or we may 

 suppose that the types in themselves are few, but possess a wide, 



