A. GENERAL OUTLINE OF DEVELOPMENT OF THALLUS 281 _a._ Preliminary considerations _b._ Course of evolution in Hymenolichens _c._ Course of evolution in Ascolichens B. COMPARATIVE ANTIQUITY OF ALGAL SYMBIONTS 282 C. EVOLUTION OF PHYCOLICHENS 283 _a._ Gloeolichens _b._ Ephebaceae and Collemaceae _c._ Pyrenidiaceae _d._ Heppiaceae and Pannariaceae _e._ Peltigeraceae and Stictaceae D. EVOLUTION OF ARCHILICHENS 287 _a._ Thallus of Pyrenocarpineae _b._ Thallus of Coniocarpineae _c._ Thallus of Graphidineae _d._ Thallus of Cyclocarpineae _AA._ LECIDEALES _aa._ Coenogoniaceae _bb._ Lecideaceae and Gyrophoraceae _cc._ Cladoniaceae 1. Origin of Cladonia 2. Evolution of the primary thallus 3. Evolution of the secondary thallus 4. Course of podetial development 5. Variation in Cladonia 6. Causes of variation 7. Podetial development and spore-dissemination 8. Pilophorus, Stereocaulon and Argopsis _BB._ LECANORALES _aa._ Course of Development _bb._ Lecanoraceae _cc._ Parmeliaceae _dd._ Usneaceae _ee._ Physciaceae CHAPTER VIII SYSTEMATIC I. CLASSIFICATION A. WORK OF SUCCESSIVE SYSTEMATISTS 304 _a._ Dillenius and Linnaeus _b._ Acharius _c._ Schaerer _d._ Massalongo and Koerber _e._ Nylander _f._ Müller-Argau _g._ Reinke _h._ Zahlbruckner B. FAMILIES AND GENERA OF ASCOLICHENS 311 C. HYMENOLICHENS 342 II. NUMBER AND DISTRIBUTION 1. ESTIMATES OF NUMBER 2. GEOGRAPHICAL DISTRIBUTION A. GENERAL SURVEY 343 B. LICHENS OF POLAR REGIONS 345 C. LICHENS OF THE TEMPERATE ZONES 348 D. LICHENS OF TROPICAL REGIONS 352 III. FOSSIL LICHENS CHAPTER IX ECOLOGY A. GENERAL INTRODUCTION 356 B. EXTERNAL INFLUENCES 357 _a._ Temperature _b._ Humidity _c._ Wind _d._ Human Agency C. LICHEN COMMUNITIES 362