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PHYLOGENY
genera which correspond to these lichen genera. Only two of them
Patinella and Patellaria are saprophytic ; in all the other genera of the
family, the species with very few exceptions are parasitic on lichens : they
are parasymbionts sharing the algal food supply ; in any case, they thrive
on a symbiotic thallus.
Rehm unhesitatingly derives the corresponding lichen genera from these
fungi. He takes no account of the difficulty that if these parasitic (or saprophytic)
fungi are primitive, they have yet appeared either later in time than
the lichens on which they exist, or else in the course of ages they have
entirely changed their substratum.
He has traced, for instance, the lichen, Buellia, to a saprophytic fungus
species, Karschia lignyota, to a genus therefore in which most of the species
are parasitic on lichens and have generally been classified as parasitic lichens.
There is no advance in apothecial characters from the fungus, Karschia, to
Buellia, merely the change to symbiosis. It therefore seems more in accordance
with facts to regard Buellia as a genus evolved within the lichen series
from Patinella through Lecidea, and to accept these species of Karschia on
the border line as parasitic, or even as saprophytic, reversions from the
lichen status. We may add that while these brown-spored lichens are fairly
abundant, the corresponding athalline or fungus forms are comparatively
few in number, which is exactly what might be expected from plants with
a reversionary history.
Occasionally in biatorine or lecideine species with a slight thalline
development all traces of the thallus disappear after the fructification has
reached maturity. The apothecia, if on wood or humus, appear to be
saprophytic and would at first sight be classified as fungi. They have un-
or in certain con-
doubtedly retained the capacity to live at certain stages,
ditions, as saprophytes.
The thallus disappears also in some species of the crustaceous genera
that possess apothecia with a thalline margin, and the fruits may be left
stranded and solitary on the normal substratum, or on some neighbouring
lichen thallus where they are more or less parasitic ; but as the thalline
margin persists, there has been no question as to their nature and affinity.
Rehm suggests that many species now included among lichens may be
ultimately proved to be fungi ; but it is equally possible that the reverse may
be the case, as for instance Bacidiaflavovirescens, held by Rehm and others to
be a parasitic fungus species, but since proved by Tobler 1
2
A note by Lightfoot ,
to be a true lichen.
one of our old-time botanists who gave lichens a
considerable place in his Flora, foreshadows the theory of evolution by
gradual advance, and his views offer a suggestive commentary on the subject
under discussion. He was debating the systematic position of the maritime
1 Tobler 191 1 2 , p. 407.
2
Lightfoot 1777, p. 965.