28o PHYLOGENY 



genera which correspond to these lichen genera. Only two of them — 

 Patinella and Patellarid — 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 sapro- 

 phytic) fungi are primitive, they have yet appeared either later in timethan 

 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 accord- 

 ance 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- 

 doubtedly retained the capacity to live at certain stages, or in certain con- 

 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 Bacidia flavovirescens, held by Rehm and others to 

 be a parasitic fungus species, but since proved by Tobler^ to be a true lichen. 



A note by Lightfoot^ 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 



^Tobler ipn^ p. 407. , 2 Lightfoot 1777, p. 965. 



