The greatest literary achievements, however, of the Mongol period belong beyond question to the theatre (or, rather, opera). The emperors were great theatre-goers, and the wealthy private families were also enthusiasts, so that gradually people of education devoted themselves to writing librettos for the operas, where in the past this work had been left to others. Most of the authors of these librettos remained unknown: they used pseudonyms, partly because playwriting was not an occupation that befitted a scholar, and partly because in these works they criticized the conditions of their day. These works are divided in regard to style into two groups, those of the "southern" and the "northern" drama; these are distinguished from each other in musical construction and in their intellectual attitude: in general the northern works are more heroic and the southern more sentimental, though there are exceptions. The most famous northern works of the Mongol epoch are _P'i-p'a-chi_ ("The Story of a Lute"), written about 1356, probably by Kao Ming, and _Chao-shih ku-erh-chi_ ("The Story of the Orphan of Chao"), a work that enthralled Voltaire, who made a paraphrase of it; its author was the otherwise unknown Chi Chün-hsiang. One of the most famous of the southern dramas is _Hsi-hsiang-chi_ ("The Romance of the Western Chamber"), by Wang Shih-fu and Kuan Han-ch'ing. Kuan lived under the Juchên dynasty as a physician, and then among the Mongol. He is said to have written fifty-eight dramas, many of which became famous. In the fine arts, foreign influence made itself felt during the Mongol epoch much more than in literature. This was due in part to the Mongol rulers' predilection for the Lamaism that was widespread in their homeland. Lamaism is a special form of Buddhism which developed in Tibet, where remnants of the old national Tibetan cult (_Bon_) were fused with Buddhism into a distinctive religion. During the rise of the Mongols this religion, which closely resembled the shamanism of the ancient Mongols, spread in Mongolia, and through the Mongols it made great progress in China, where it had been insignificant until their time. Religious sculpture especially came entirely under Tibetan influence (particularly that of the sculptor Aniko, who came from Nepal, where he was born in 1244). This influence was noticeable in the Chinese sculptor Liu Yüan; after him it became stronger and stronger, lasting until the Manchu epoch. In architecture, too, Indian and Tibetan influence was felt in this period. The Tibetan pagodas came into special prominence alongside the previously known form of pagoda, which has many storeys, growing smaller as they go upward; these towers originally contained relics of Buddha and his disciples. The Tibetan pagoda has not this division into storeys, and its lower part is much larger in circumference, and often round. To this day Peking is rich in pagodas in the Tibetan style.