OCCURRENCE AND DISTRIBUTION. 53 



rence as earth-lichens is quite evident, as scarcely any- 

 other substrata exist; trees, fences, and the like 

 being wanting. Jn the more temperate regions we 

 find earth-lichens, rock-lichens and a considerable num- 

 ber of tree-lichens. In the tropics tree-lichens pre- 

 dominate, owing to the greater competion for space. 

 The richness of vegetation of all sorts, trees among the 

 rest, makes it evident that vegetable substrata are, in a 

 sense, forced upon the lichens. In fact, the intimate 

 associations of plants with plants and of animal with 

 plants, etc., are much more prevalent in the tropics, 

 for similar reasons. The great struggle for existence 

 necessitates the establishment of mutual adaptations 

 as well as antagonism or parasitism. 



Some lichens have a limited range, wliile others are ' 

 truly cosmopolitan, such as Oladonia rangiferina, 

 Usnea barbata, Lecanora subfusca and others. As to 

 substrata, some species are limited to a given rock- 

 formation, while others occur upon different rock- 

 formations as well as upon trees, and even soil, such 

 as many of the Parmelias and Physcias. Trees (bark), 

 rocks and soil are the principal substrata for lichens; 

 they are also common upon dead (not rotten) wood, 

 such as fences, roofs of buildings, etc. Exceptionally 

 they have been found growing upon rotten wood, 

 old leather, bones, glass, iron railings, etc. Some are 

 truly parasitic upon the green leaves of plants in the 

 tropical and subtropical regions. 



Crustose lichens very frequently encroach upon each 

 other, the stronger and more rapidly growing species 

 finally spreading over and destroying the less resist- 



