Mr. Turner's Descriptions of four new British Lichens. 87 



to the obtaining a knowledge of them: and I cannot but think 

 that there is in botany no greater desideratum than a work on the 

 Lichens, conducted on these principles, and at the same time 

 carefully collecting- the synonyms of the different authors. I must 

 be indulged in one mom remark, arising from this subject, which 

 is that while some botanists, anxious to create new species, have 

 not made among these the same* allowances as among other 

 vegetables, for differences caused by the several periods of their 

 age, by their situation, by the substances on which they grow, or 

 by the aspects to which they are exposed, it appears to me that 

 others have run still more hastily into the opposite extreme, and 

 united plants which are most truly and specifically distinct, 

 merely because in some particulars they approach each other in 

 different stages of their growth; not considering that among other 

 genera of the class Cryptogamia, instances are occasionally found 

 of plants bearing in age a stronger resemblance to some other 

 species than to the appearance they had themselves when young: 

 but that similitude between one individual, while verging upon 

 decay, and another in its highest perfection, is very far indeed 

 from constituting a proof of identity. Great difficulties are un- 

 questionably opposed to our researches among the Lichens; but 

 these difficulties are increased tenfold, if we examine them with- 

 out at the same time endeavouring to trace them through their 



* Very strong proofs in favour of this observation are afforded by Lichen miiscorum 

 and L. impressus of Acharius, both which are almost universally considered as distinct 

 species ; though any botanist who will be at the trouble of examining the former may 

 soon be convinced that it is nothing more than the common L. parasemus, which, in 

 passing from a wall or moss, necessarily acquires a less compact crust ; a thing I have 

 myself repeatedly traced: and as for the latter, I am as fully convinced, not so much 

 from my own observations as from the suggestions and specimens of my acute friend 

 Dr. Scott, Professor of Botany in Dublin, that it is only the scutclla of L. scrupostts, 

 occupying the leaves of L. pyxldatus. 



various 



