North American species of Mycena.

PREFACE

A T THE present time generic concepts in the gill fungi may be
said to be in a state of transition. The genera of the Friesian
classification have been critically evaluated in the light of information obtained on microscopic characters and as a result of the discovery of many interesting species from other parts of the world,
and it has become evident that considerable regrouping throughout
the agarics is desirable. As yet the systems proposed since that of
Fries have not been subjected to the rigid test of extensive use, and
consequently their faults have not been brought to light in as detailed a manner. Few of the new genera or extensive regroupings
have been treated monographically, and until such treatments are
available any new system of classification or regrouping of species
among now accepted genera must be regarded as tentative.
At present the trend is toward natural as opposed to artificial
genera, and a so-called natural arrangement of species within genera.
Closeness of relationship, which is what we are dealing with, is based
on degree of similarity in characters accepted as fundamental.
Since the taxonomist is working almost entirely from circumstantial
evidence, there is not much to be gained by insisting that a classification be based on relationship when the point has been reached
where affinities are difficult to ascertain and must be determined on
secondary characters. Consequently, in my estimation, the most
satisfactory arrangement of species will always involve some compromise between artificiality and naturalness.
At the generic level, however, more emphasis can rightly be placed
on relationship, and this is the modern trend. This does not necessarily involve splitting up large genera into smaller ones, as some
investigators seem to think. At least in the gill fungi, the results
have been the creation of such large genera as Rhodophyllus and
Psathyrella as well as small ones such as Laccaria, Cystoderma, and
others. In Mycena it appeared logical to accept a broader rather
than a narrower concept than that of Fries. It is evident that in
this genus speciation has proceeded largely on microscopic characters, such as those of spores and cystidia, and because of this I have
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