NUMBER OP LICHENS 343 



or described by Nylander, and now in the Paris herbarium. There are 135 

 genera with 3686 species. Of these, about 829 belong to the larger foliose 

 and fruticose lichens (including Cladoniae); the remaining 2857 belong to 

 the smaller kinds, most of them crustaceous. 



2. GEOGRAPHICAL DISTRIBUTION 



A. GENERAL SURVEY 



The larger foliose and fruticose lichens are now fairly well known and 

 described for Europe, and the knowledge of lichens in other continents is 

 gradually increasing. It is the smaller crustaceous forms that baffle the in- 

 vestigator. The distribution of all lichens over the surface of the earth is 

 controlled by two principal factors, climate and substratum ; for although 

 lichens as a rule require only support, they are most of them restricted to one 

 or another particular substratum, either organic or inorganic. As organisms 

 which develop slowly, they require an unchanging substratum, and as sun- 

 plants they avoid deeply shaded woodlands: their occurrence thus depends 

 to a large extent on the configuration and general vegetation of the country. 



Though so numerous and so widely distributed, lichens have not evolved 

 that great variety of families and genera characteristic of the allied fungi 

 and algae. They conform to a few leading types of structure, and thus the 

 Orders and Families are comparatively few, and more or less universal. 

 They are most of them undoubtedly very old plants and were probably 

 wide-spread before continents and climates had attained their present 

 stability. Arnold 1 indeed considers that a large part of the present-day 

 lichens were almost certainly already evolved at the end of the Tertiary 

 period, and that they originated in a warm or probably subtropical climate. 

 As proof of this he cites such genera as Graphis, Thelotrema and Arthonia z 

 which are numerous in the tropics though rare in the colder European 

 countries ; and he sees further proof in the fact that many fruticose and 

 gelatinous lichens do not occur further north than the forest belt, though 

 they are adapted to cold conditions. Several genera that are abundant in 

 the tropics are represented outside these regions by only one or few species, 

 as for instance Conotrema urceolatum and Bombyliospora incana. 



During the Ice age of the Quaternary period, not many new species can 

 have arisen, and such forms as were not killed off must have been driven 

 towards the south. As the ice retreated the valleys were again stocked with 

 southern forms, and northern species were left behind on mountain tops all 

 over the globe. 



1 Arnold 1890. 



2 These genera are associated with Trentepohlia algae which are numerous and abundant in 

 tropical climates, and their presence there may possibly account for these particular lichens. 



