LICHENS. 
31 
those of the Ascomycetes belonging to the different groups proves 
that Lichens consist of a polyphyletic group , characterized by 
analogous biological phenomena , and illustrated by the analogous 
development or “ lichenization ” of a certain number of Gymno- 
carpous and Angiocarpous genera of fungi. It follows that the 
Lichens do not constitute a distinct systematic group, but belong in 
part to the Gymnocarps (Discomycetes, in the broader sense), and 
in part to the Pyrenocarps (Pyrenomycetes, in the broader sense) 
among the Ascomycetes. 
Analogous evolution of the Thallus in different groups . — In the 
Lecanoreae, Thelochisteae, Buellieae, Pannarieae, Collemeae, Lecideae, 
also in the Coniocarpeae and in Pyrenolichens, types of thallus 
occur passing from crustaceous and squamulose through foliaceous 
to fruticulose ; intermediate forms also usually occur in the same 
groups, thus indicating that the last-mentioned stage of develop- 
ment is only a more perfect condition of the first-mentioned form, 
and is produced in an analogous form in the different groups. A 
classification of Lichens, the principal groups of which are 
characterized solely by the usual aspect of the thallus, cannot be 
natural. 
Analogous evolution of spores in different groups. — The remarks 
on the evolution of the thallus and its consequent absence of value 
as a primary factor in the formation of important sections apply 
with equal force to the spores. The evolution of the spores is, on 
the whole, too capricious and of unequal ratio of development in 
proportion to the thallus in the various groups to be of more than 
secondary value in classification. In the Thelochisteae, Buellieae, 
Peltigereae, Sticteae, Pannarieae, Heppieae, and several other tribes, 
the spores are very constant ; also in the Gyrophoreae and the 
Parmeliaeae, highly developed from the point of view of the thallus, 
the spores are colourless and simple with the exception of a certain 
number of species, where the spores are brown and parenchymatous 
(=muriform), as in Umbilicaria pustulata (L.), Hoffin., and Atestia 
Loxensis (Fee.), Trev., which from their external aspect are dis- 
tinguished with difficulty from nearly allied genera, Gyrophora and 
Alectoria. 
The spores are singularly variable in the genera Lecidea and 
Graphis, which on this account have been divided by several 
authors into a greater or less number of distinct genera. 
In this case, as usual, the classification cannot be other than 
artificial if the divisions are based on one preconceived idea which 
can be shown to have different values in different groups. 
Groups characterized by the gonidia. — Several forms of gonidia 
occur in most tribes. In the Peltigereae, Sticteae, Pannarieae 
(Lecideae), also in the Pyrenolichens, gonidia occur belonging to 
both the Cyanophyceae and to the Chlorophyceae, and in some cases 
groups exist that cannot be distinguished from each other by any 
other character except that in one of the groups the gonidia belong 
to the Cyanophyceae and in the other to the Chlorophyceae. As 
