394 Wisconsin Academy of Sciences , Arts , and Letters. 
shown by the spores which measure about 3—4x5—7/x in 
each species. My plants do not exactly agree with any of 
these descriptions. The authors of the species do not help 
for the descriptions are written independently and agree¬ 
ments and differences are not noted. 
As a step in the right direction I have endeavored to 
place the forms in their natural groups regardless of the 
characters used to distinguish genera or make artificial 
keys, to name the groups after their most common and wide 
spread representatives and to compare the groups with cor¬ 
responding groups in Europe and other regions. 
1. Emphasis must be laid on plylogenetic rather than 
fluctuating differences in recognizing the natural groups. 
Spore characters are generally considered among the most 
diagnostic features and in all the groups here considered 
the spores in the different forms are alike and characteristic. 
This does not, however, result in uniting forms unlike in 
general appearance. It will be long before phylogenetic 
relationship can be proved in all cases among the agarics 
but it is probable that natural groups which agree in the 
fruiting bodies, the character of the hyphae or tissue and the 
general structure are in most cases phylogenetic. The differ¬ 
ences in shape, size, color and surface can often be seen to 
be due to accidental causes. The sudden changes or 
mutations which plants undergo the causes of which are as 
yet little understood make the grouping of forms still more 
essential. In some cases the supposed boundaries of a species 
are overstepped in a single generation. The groups should 
be broad enough to include the mutants. 
The recent careful study of. the structure and develop¬ 
ment of the agarics by Fayod, Atkinson, Lavine, Zeller and 
others will undoubtedly lead to a better understanding of the 
natural groups. In several of the groups here illustrated I 
have given a description of the structure as far as it is shown 
in the photographs. 
2. Naming the groups after their most common repre¬ 
sentative helps in the determination of the forms and in 
understanding the relation between them. It is also the logi¬ 
cal carrying out of the natural system of classification. It 
substitutes a plant fof a verbal description and aims at a 
comparison of varieties. The so-called identity of species is 
