36 
lichens may-also utilize the protonemas of'various mosses. There 
is no doubt that many of the lower lichens will develop to some 
extent (in artificial media) without the algae,* but no lichen-fungus: 
will develop to maturity in nature. From this we may conclude 
that all fungi which have entered into the formation of lichens no 
longer exist. 
Phylogenetic morphology is inthesately related to classification. 
It supplies science with the necessary building material for the 
natural system of the future. Conversely systematists in their 
various and varied attempts at classification have wereonseloosy 
added to our knowledge of phylogenetic morphology. 
In a natural system the oldest morphological characters serve 
to limit the main divisions; the next oldest limit the families. The 
more recently acquired characters serve to define the genera and: 
species. It is often difficult to determine what are in reality the old 
and newcharacters. In this difficulty lichens prove very interest- 
ing because itis to a certain degree possible to determine the relative 
ages of the phylogenetically acquired characters. The more recent 
characters occur in the thallus; the oldest in ‘the apothecium and 
related structures, since these are as old as the lichen plus the age 
of the fungal ancestor. Accordingly ‘the main divisions of the 
lichen-system should be based upon the characters of the apothecia, 
and not upon the thallus-characters. This, however, does not 
imply that the apothecia have undergone no variation ; the varia- 
tion is even considerable, as becomes evident upon comparing the. 
lecanorine type with the lecideine. The same may be said of the 
spores. In general, however, the variations in the thallus are much 
more considerable and represent the direct characters of adapta-_ 
tion. 
The future system of lichens will also consider the relation of 
these plants to their probable fungal ancestors. In the limitation 
of the genera it will be necessary to determine whether or not a 
given character was transmitted from the fungal ancestor or 
_ whether it was newly acquired. 
The following is a brief summary of this paper: 
*Miller. Ueber die Kultur flechtenbildender Ascomyceten ohne Algen. Miinster- _ 
1887. ; 
