A note by Lightfoot[995], 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 lichen genus _Lichina_, considered then a kind of _Fucus_, and had observed its similarity with true lichens. “The cavity,” he writes, “at the top of the fructification (in _Lichina_) is a proof how nearly this species of _Fucus_ is related to the scutellated lichens. Nature disdains to be limited to the systematic rules of human invention. She never makes any sudden starts from one class or genus to another, but is regularly progressive in all her works, uniting the various links in the chain of beings by insensible connexions.” III. THE THALLUS A. GENERAL OUTLINE OF DEVELOPMENT _a._ PRELIMINARY CONSIDERATIONS. The evolution of lichens, as such, has reference mainly to the thallus. Certain developments of the fructification are evident, but the changes in the reproductive organs have not kept pace with those of the vegetative structures: the highest type of fruit, for instance, the apothecium with a thalline margin, occurs in genera and species with a very primitive vegetative structure as well as in those that have attained higher development. Lichens are polyphyletic as regards their algal, as well as their fungal, ancestors, so that it is impossible to indicate a straight line of progression, but there is a general process of thalline development which appears once and again in the different phyla. That process, from simpler to more complicated forms, follows on two lines: on the one there is the endeavour to increase the assimilating surface, on the other the tendency to free the plant from the substratum. In both, the aim has been the same, to secure more favourable conditions for assimilation and aeration. Changes in structure have been already described[996], and it is only needful to indicate here the main lines of evolution. _b._ COURSE OF EVOLUTION IN HYMENOLICHENS. There is but little trace of development in these lichens. The fungus has retained more or less the form of the ancestral _Thelephora_ which has a wide-spreading superficial basidiosporous hymenium. Three genera have been recognized, the differences between them being due to the position within the thallus, and the form of the _Scytonema_ that constitutes the gonidium. The highest stage of development and of outward form is reached in _Cora_, in which the gonidial zone is central in the tissue and is bounded above and below by strata of hyphae.