74 GUIDE TO THE STUDY OF LICHENS. 



to show the natural relationship of the organisms, 

 while in the latter an attempt is made to show the 

 affinities of families, genera and species. If, for in- 

 stance, all flowering plants are classified according to 

 the differences in the leaves, we have a purely artificial 

 system ; in such a system it would be necessary to 

 place together plants which have no close relationship ; 

 and, again, it would be necessary to separate others 

 which are undoubtedly closely related. In the so-called 

 natural system all the known characters are employed. 

 In the case, for example, of the flowering plants, not 

 only the leaf characters, but also those of the flowers, 

 the seed, the stem, the histological and physiological 

 differences, etc. 



If we stop to consider, we find that the natural sys- 

 tem is, in a degree, also artificial, for the simple reason 

 that all of the facts regarding morphology and physiol- 

 ogy, evolution, etc, are not yet known, and, so far as 

 we know at present, never will all be known. Human 

 knowledge is finite ; it is, so to speak, hemmed in on 

 all sides ; what little we do recognize as "facts" has 

 for its basis something which is taken for granted, and 

 of which we therefore know nothing. 



As regards the classification of lichens, it may be 

 stated that a natural system is practically non-existent. 

 Only within a few years have any efforts been made to 

 show the natural affinities of the different lichens and 

 lichen-groups. Heretofore, nearly every specialist on 

 lichenology had proposed a new system, thus adding to 

 the existing confusion. We now know that a lichen 

 is a morphological unit, which resulted from the sym- 



