The dual hypothesis, as it was termed, was not however universally accepted. It was indeed bitterly and scornfully rejected by some of the most prominent lichenologists of the time, including Nylander[147], J. Müller and Crombie[148]. Schwendener held that the lichen was a fungus parasitic on an alga, and his opponents judged, indeed quite rightly, that such a view was wholly inadequate to explain the biology of lichens. It was not till a later date that the truer conception of the “consortium” or “symbiosis” was proposed. The researches undertaken to prove or disprove the new theories come under review in Chapter II. Stahl’s work on the development of the carpogonium in lichens gave a new direction to study, and notable work has been done during the last forty years in that as in other branches of lichenology. Exploration of old and new fields furnished the lichen-flora of the world with many new plants which have been described by various systematists—by Nylander, Babington, Arnold, Müller, Th. Fries, Stizenberger, Leighton, Crombie and many others, and their contributions are scattered through contemporary scientific journals. The number of recorded species is now somewhere about 40,000, though, in all probability, many of these will be found to be growth forms. Still, at the lowest computation, the number of different species is very large. Systematic literature has been enriched by a series of important monographs, too numerous to mention here. While treating definite groups, they have helped to elucidate some of the peculiar biological problems of the symbiotic growth. Morphology, since Schwendener’s time, has been well represented by Zukal, Reinke, Lindau, Fünfstück, Darbishire, Hue, and by an increasing number of modern writers whose work is duly acknowledged under each subject of study. Hesse and Zopf, and more recently Lettau, have been engaged in the examination of those unique products, the lichen acids, while other workers have investigated lichen derivatives such as fats. Ecology of lichens has also been receiving increased attention. Problems of physiology, symbiosis, etc., are not yet considered to be solved and are being attacked from various sides.