From a phylogenetic point of view we should regard the sorediate lichens as the more highly evolved, and those which have no soredia as phylogenetically young, though, as Lindau[502] has pointed out, soredia are all comparatively recent. They probably did not appear until lichens had reached a more or less advanced stage of development, and, considering the polyphyletic origin of lichens, they must have arisen at more than one point, and probably at first in circumstances where the formation of apothecia was hindered by prolonged conditions of shade and moisture. That soredia are ontogenetic in character, and not, as Nilson[503] has asserted, accidental products of excessively moist conditions is further proved by the non-sorediate character of those species of crustaceous lichens belonging to _Lecanora_, _Verrucaria_, etc. that are frequently immersed in water. Bitter[504] found that the soredia occurring on _Peltigera spuria_ were not formed on the lobes which were more constantly moist, nor at the edges where the cortex was thinnest: they always emerged on young parts of the thallus a short way back from the edge. Bitter[504] points out that in extremely unfavourable circumstances—in the polluted atmosphere near towns, or in persistent shade—lichens, that would otherwise form a normal thallus, remain in a backward sorediose state. He considers, however, that many of these formless crusts are autonomous growths with specific morphological and chemical peculiarities. They hold these outposts of lichen vegetation and are not found growing in any other localities. The proof would be to transport them to more favourable conditions, and watch development. 4. ISIDIA A. FORM AND STRUCTURE OF ISIDIA Many lichens are rough and scabrous on the surface, with minute simple or divided coral-like outgrowths of the same texture as the underlying thallus, though sometimes they are darker in colour as in _Evernia furfuracea_. They always contain gonidia and are covered by a cortex continuous with that of the thallus. This very marked condition was considered by Acharius[505] as of generic importance and the genus, _Isidium_, was established by him, with the diagnostic characters: “branchlets produced on the surface, or coralloid, simple and branched.” In the genus were included the more densely isidioid states of various crustaceous species such as _Isidium corallinum_ and _I. Westringii_, both of which are varieties of _Pertusariae_. Fries[506], with his accustomed insight, recognized them as only growth forms. The genus was however still accepted in English Floras[507] as late as 1833, though we find it dropped by Taylor[508] in the _Flora Hibernica_ a few years later.