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algal symbiont during its phylogenetic history ; for example, Sticta 

 and Stictina doubtless originated from the same fungal symbiont 

 but after a time Stictina formed a new natural branch by adapting 

 itself to a different alga. Similar conditions are met with in other 

 groups. 



Just when, how and why lichens took their origin is not definitely 

 known. The paleeontologic record leaves us practically without data, 

 so we are obliged to content ourselves with theoretical and hypo- 

 thetical assumptions ; these assumptions bear, however, the stamp 

 of certainty, though they are incapable of direct proof. In a certain 

 sense the probable conditions were very peculiar ; it is now gener- 

 ally admitted that the various groups of fungi have had a polyphy- 

 letic origin from various algal ancestors ; a change in the environ- 

 ment has doubtless influenced the chlorophyllian function in certain 

 algae which adapted them to lead a parasitic life, and subsequently 

 these same organisms again found themselves placed in a position 

 favorable to or requiring carbon-assimilation, but having lost their 

 chlorophyll they entered into a mutualistic relationship with algae 

 which could perform the required function. 



Concerning the oldest types of lichens as well as the relative ages 

 of the various phyletic groups, nothing definite is known. Reinke 

 assumes that Collema perhaps represents the prototype ; there are, 

 however, no data to show that Collcma is older than Parmclia or 

 Cladonia. No authentic observations have as yet been made on the 

 variability of lichen-species nor has it been possible to indicate the 

 relative rate of phylogenetic evolution of the different groups. The 

 question of variability is of great importance : that the species 

 are very variable is generally known, but no one has as yet deter- 

 mined the limitations of such variation. It is very necessary that 

 these limitations should be known before any reliable system of clas- 

 sification can be established, and observations on the variability of 

 lichens will also throw light on the question of their phyletic evolu- 

 tion. 



II. THE FUNGAL TYPES. 



Upon one point all scientific lichenologists are agreed, and that is 

 that the fungal symbionts of the Ascolichenes are derived from the 

 Ascomycetcs. It is also quite universally agreed that various lichen 

 groups are represented by different groups of fungi, but for reasons 

 already stated it is impossible to ascertain the exact fungal ancestors. 



