PART II. 



THE CLASSIFICATION AND SPECIAL MORPHOLOGY 



OF LICHENS. 



CHAPTER I. 



A SYSTEM OF CLASSIFICATION. 

 I. INTRODUCTORY CONSIDERATIONS. 



The student on beginning a consideration of the classification 

 of lichens is apt to adopt some one system with the impression that it 

 is something fixed and definite, and this impression is likely to be 

 strengthened on reading the laboriously worded descriptions of 

 species. Nothing is, however, further from the truth. 



There is at present no satisfactory natural or artificial system of 

 lichens. All arrangements must of necessity undergo various changes 

 as our knowledge of the life-history of the individuals becomes more 

 and more perfect. This, however, does not imply that the systems 

 proposed are without value. Any system is useful and in a sense 

 indispensable in so far as it is in harmony with our actual knowledge 

 of the subject under consideration. A system should therefore cor- 

 respond as nearly as possible with the conceptions of the leading in- 

 vestigators on the subject. In a certain sense artificial system and 

 natural system are only relative terms ; as our scientific knowledge 

 of a subject increases the corresponding system becomes more and 

 more " natural," from which we may safely conclude that an abso- 

 lutely natural system is practically and theoretically impossible since 

 our knowledge of any subject is far from ultimate and is continually 

 subject to change. It is, however, possible to set up useful systems 

 which are quite "artificial," that is in which only a comparatively 

 few of the known data are employed, but such systems are always 

 much inferior to the " natural " system. 



In the arrangement of lichens here proposed the endeavor has been 

 made to conform to the results obtained by the ablest students of 



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