Mr. Turner's Descriptions of four nezv 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 more 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 muscorum 
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. scruposus, 
occupying the leaves of L.pyxidatus. 
various 
