_c._ COURSE OF EVOLUTION IN ASCOLICHENS. It is in the association with Ascomycetes that evolution and adaptation have had full scope. In that sub-class there are four constantly recurring and well-marked stages of thalline development. (1) The earliest, most primitive stage, is the crustaceous: at first an accretion of separate granules which may finally be united into a continuous crust with a protective covering of thick-walled amorphous hyphae forming a “decomposed” cortex. The extension of a granule by growth in one direction upwards and outwards gives detachment from the substratum, and originates (2) the squamule which is, however, often of primitive structure and attached to the support, like the granule, by the medullary hyphae. Further growth of the squamule results in (3) the foliose thallus with all the adaptations of structure peculiar to that form. In all of these, the principal area of growth is round the free edges of the thallus. A greater change takes place in the advance to (4) the fruticose type in which the more active growing tissue is restricted to the apex, and in which the frond or filament adheres at one point only to the support, a new series of strengthening and other structures being evolved at the same time. The lichen fungi associate, as has been already stated, with two different types of algae: those combined with the Myxophyceae have been designated _Phycolichenes_, those with Chlorophyceae as _Archilichenes_. The latter predominate, not only in the number of lichens, but also in the more varied advance of the thallus, although, in many instances, genera and species of both series may be closely related. B. COMPARATIVE ANTIQUITY OF ALGAL SYMBIONTS One of the first questions of inheritance concerns the comparative antiquity of the two gonidial series: with which kind of alga did the fungus first form the symbiotic relationship? No assistance in solving the problem is afforded by the type of fructification. The fungus in Archilichens is frequently one of the more primitive Pyrenomycetes, though more often a Discomycete, while in Phycolichens Pyrenomycetes are very rare. There is, as already stated, no correlation of advance between the fruit and the thallus, as the most highly evolved apothecia with well-formed thalline margins are constantly combined with thalli of low type. Forssell[997] gave considerable attention to the question of antiquity in his study of gelatinous crustaceous lichens in the family Pyrenopsidaceae, termed by him Gloeolichens, and he came to the conclusion that Archilichens represented the older combination, Phycolichens being comparatively young.