218 The Plant World. 



the Colorado deserts. About the western borders of this region 

 have been discovered a large number of lichens adapted to these 

 very arid conditions and many of them are spread northward 

 throughout a large part of the state. Undoubtedly further in- 

 vestigation in western Nevada and Arizona will reveal that many 

 of these species are scattered all over the interior Basin and the 

 valley of the Colorado, but beyond these regions they will hardly 

 be found. 



Along the more elevated peaks of the Sierras, especially in 

 the Yosemite region, also occur a number of lichens not known 

 elsewhere, but there is no question in my mind but that all or 

 nearly all of them will be found on the highest ranges of Nevada 

 and Oregon, if not elsewhere in the west. 



Passing to the coast, we find on the rocky cliffs, bathed 

 by the chill waters of the Pacific, and in the forests and foothills 

 of the Coast Range, California's most conspicuous, characteristic, 

 and unique lichens. The climatic conditions of this region, suf- 

 ficiently set forth in a former paper on lichen distribution in 

 the Santa Cruz peninsula, are evidently highly favorable to a 

 luxuriant or unusual growth of lichens which elsewhere are not 

 of such rankness or high stage of development. 



Though sexual processes in the life history of most lichens 

 are a doubtful if not indeed a minus quantity, it still seems 

 evident to me that the combination of geographic isolation, to- 

 gether with an equable climate, accompanied by peculiar mois- 

 ture and growth conditions, has been the agent which has ef- 

 fected the evolution of such an unusual number of endemic 

 species. That they have undergone many marked evolutionary 

 phases all will agree, and in fact the phylogeny of several fam- 

 ilies is pretty well worked out, while no one can study the Cla- 

 donias for long without assenting to the proposition that here 

 we have a group of lichens still in an unstable stage of evolution- 

 ary development. 



It will perhaps be impossible to work out satisfactorily 

 all the factors in the evolution of such composite organisms as 

 lichens, a composite of alga and fungus, without the sexual 

 methods of breeding upon which most organisms depend. Here 

 there are apparently no Mendelian factors, no karyokinetic 

 problems, no mutation or germinal selection, no germ plasm 



