DISTRIBUTION 345 



but, as yet, has been found only in the eastern parts of North America; and 

 CL pycnoclada, a plant which braves the climate of Cape Horn and the 

 Falkland Islands, but has not travelled northward beyorrd temperate North 

 America: the southern origin of that species is thus plainly indicated. Wainio 

 also finds that evidence of the primitive locality of a very widely spread 

 species may be obtained by observing the locality of species derived from 

 it, which are as yet of limited distribution ; presumably these arose in the 

 ancestral place of origin, though this indication is not always to be relied 

 on. If, however, the ancestral plant has given rise to several of these rarer 

 related species, those of them that are most closely allied to the primitive 

 plant would be found near to it in the original locality. 



A detailed account of species distribution according to these indications 

 is given by Wainio and is full of interest. No such attempt has been made 

 to deal with any other group, and the distribution of genera and species can 

 only be suggested. An exhaustive comparison of the lichens of different 

 regions is beyond the purpose of our study and is indeed impossible as, 

 except in some limited areas, or for certain species, the occurrence and dis- 

 tribution are not fully known. It is in any case only tentatively that genera 

 or species can be described as local or rare, until diligent search has been 

 made for them over a wider field. The study of lichens from a floristic point 

 of view lags behind that of most other groups of plants. The larger lichen 

 forms have received more attention, as they are more evident and more 

 easily collected ; but the more minute species are not easily detected, and, 

 as they are largely inseparable from their substratum of rocks, or trees, etc., 

 on which they grow, they are often difficult to collect. They are also in 

 many instances so indefinite, or so alike in outward form, that they are 

 liable to be overlooked, only a microscopic examination revealing the differ- 

 ences in fruit and vegetative structure. 



Though much remains to be done, still enough is known to make the 

 geographical distribution of lichens a subject of extreme interest. It will be 

 found most instructive to follow the usual lines of treatment, which give the 

 three great divisions : the Polar, the Temperate and the Tropical regions 

 of the globe. 



B. LICHENS OF POLAR REGIONS 



Strictly speaking, this section should include only lichens growing within 

 the Polar Circles; but in practice the lichens of the whole of Greenland and 

 those of Iceland are included in the Arctic series, as are those of Alaska: 

 the latitudinal line of demarcation is not closely adhered to. With the 

 northern lichens may also be considered those of the Antarctic continent, 

 as well as those of the islands just outside the Antarctic Circle, the South 



