Krempelhuber’s periods are as follows: I. From the earliest times to the end of the seventeenth century. II. Dating from the arrangement of plants into classes called genera by Tournefort in 1694 to 1729. III. From Micheli’s division of lichens into different orders in 1729 to 1780. IV. The definite and reasoned establishment of lichen genera based on the structure of thallus and fruit by Weber in 1780 to 1803. V. The arrangement of all known lichens under their respective genera by Acharius in 1803 to 1846. VI. The recognition of spore characters in classification by De Notaris in 1846 to 1867. A seventh period which includes modern lichenology, and which dates after the publication of Krempelhuber’s _History_, was ushered in by Schwendener’s announcement in 1867 of the hypothesis as to the dual nature of the lichen thallus. Schwendener’s theory gave a new impulse to the study of lichens and strongly influenced all succeeding investigations. B. PERIOD I. PREVIOUS TO 1694 Our examination of lichen literature takes us back to Theophrastus, the disciple of Plato and Aristotle, who lived from 371 to 284 B.C., and who wrote a _History of Plants_, one of the earliest known treatises on Botany. Among the plants described by Theophrastus, there are evidently two lichens, one of which is either an _Usnea_ or an _Alectoria_, and the other certainly _Roccella tinctoria_, the last-named an important economic plant likely to be well known for its valuable dyeing properties. The same or somewhat similar lichens are also probably alluded to by the Greek physician Dioscorides, in his work on _Materia Medica_, A.D. 68. About the same time Pliny the elder, who was a soldier and traveller as well as a voluminous writer, mentions them in his _Natural History_ which was completed in 77 A.D. During the centuries that followed, there was little study of Natural History, and, in any case, lichens were then and for a long time after considered to be of too little economic value to receive much attention. In the sixteenth century there was a great awakening of scientific interest all over Europe, and, after the printing-press had come into general use, a number of books bearing on Botany were published. It will be necessary to chronicle only those that made distinct contributions to the knowledge of lichens. The study of plants was at first entirely from a medical standpoint and one of the first works, and the first book on Natural History, printed in England, was the _Grete Herball_[27]. It was translated from a French work, _Hortus sanitatis_, and published by Peter Treveris in Southwark. One of the herbs recommended for various ailments is “Muscus arborum,” the tree-moss (_Usnea_). A somewhat crude figure accompanies the text.