344 SYSTEMATIC 



In examining therefore the distribution of lichens, it will be found that 

 the distinction between different countries is relative, certain families being 

 more or less abundant in some regions than others, but, in general, nearly 

 all being represented. Certain species are universal, where similar conditions 

 prevail. This is especially true of those species adapted to extreme cold, as 

 that condition, normal in polar regions, recurs even on the equator if the 

 mountains reach the limit of perpetual snow ; the vertical distribution thus 

 follows on the lines of the horizontal. 



In all the temperate countries we find practically the same families, with 

 some few exceptions; there is naturally more diversity of genera and species. 

 Genera that are limited in locality consist, as a rule, of one or few species. 

 In this category, however, are not included the tropical families or genera 

 which may be very rich in species: these are adapted to extreme conditions 

 of heat and often of moisture, and cannot exist outside tropical or subtropical 

 regions, extreme heat being more restricted as to geographical position than 

 extreme cold. 



In the study of distribution the question which arises as to the place of 

 origin of such widely distributed plants is one that is difficult to solve. 

 Wainio^ has attempted the task in regard to Cladonia, one of the most 

 unstable genera, the variations of form, which are dependent on external 

 circumstances, being numerous and often bewildering. In his fine mono- 

 graph of the genus, 132 species are described and 25 of these are cosmo- 

 politan. 



The distribution of Phanerogams is connected, as Wainio points out, 

 with causes anterior to the present geological era, but this cannot be the 

 case in a genus so labile and probably so recent as Cladonia, though some 

 of the species have existed long enough to spread and establish themselves 

 from pole to pole. Endemic species, or those that are confined to a com- 

 paratively limited area, are easily traced to their place of origin, that being 

 generally the locality where they are found in most abundance, and as 

 a general rule in the centre of that area, though there may be exceptions: 

 a plant for instance that originated on a mountain would migrate only in 

 one direction — ^towards the regions of greater cold. 



The difficulty of determining the primitive stations of cosmopolitan, or 

 of widely spread, species is much greater, but generally they also may be 

 referred to their area of greatest abundance. Thus a species may occur 

 frequently in one continent and but rarely in another, even where the con- 

 ditions of climate, etc., are largely comparable. It may therefore be inferred 

 that the plant has not yet reached the full extent of possible distribution in 

 the less frequented area. As examples of this, Wainio cites, among other 

 instances, Cladonia papillaria, which has a very wide distribution in Europe, 



^ Wainio 1897. 



