Another difficulty presented by the Qur'an is the fact that Muhammad sometimes revoked whole verses and de clared them to be "abrogated" (II., loo, "Whatever verse We ab rogate or cause to be forgotten, We bring a better or its like"; see also XVI., 103, 104). Thus the Qur'an contains different direc tions as to the treatment which idolators are to receive at the hands of believers. The number of abrogated verses has been vari ously estimated by commentators at from five to five hundred.
At the head of 29 of the chapters stand certain initial letters (e.g. ALM, tIM, etc.), for which various interpretations have been offered, e.g. that they are abbreviations for the names of God, for the signature of owners of mss. etc. (See Noldeke-Schwally, II., 68-78; E. Goossens in Der Islam, XIII., 191 sqq.), but no expla nation has yet gained general acceptance.
When Muhammad died, separate pieces of the Qur'an appear to have been already written down by individuals, but many portions had also been committed to memory. The first complete written version is attributed to Zayd ibn Thabit, who had been Muhammad's secretary, and was instructed in the reign of Abu Bakr to collect the scattered portions into one volume. This copy passed into the possession of `Umar, and at his death to his daughter, Hafsa, one of the widows of the Prophet. When in the reign of `Uthman quarrels arose in the army as to the true form of the revealed text, Zayd was again appointed by the Caliph, to gether with three members of the tribe of the Quraysh, to prepare an authoritative version. Copies of this were sent to the chief cities of the empire, and all earlier codices, except that of liafsa, were ordered to be burnt. This recension of `Uthman thus became the only standard text for the whole Muslim world up to the present day.
The vast exegetical literature of Islam begins with the explanations of individual verses attributed in the Hadith to the Prophet himself and his companions. Much of the work of the early commentators is embodied in the vast Tafsir of Tabari (ob. 922). The Kashshaf of the Muctazilite theologian, Zamakh shari (ob. 1143) enjoyed a high reputation, in spite of its heretical tendencies, and was made by Bay.;lawi (ob. 1286) the basis of his own work, the most widely read commentary in the Muslim world. Thousands of other commentaries have been written, many of which still exist in manuscript form only. One of the most recent is that by Muhammad 'Ali, The Holy Qur-dn, with English trans lation and commentary (1917). For the history of Muslim exege sis, in Arabic, see I. Goldziher, Die Richtusgen der islamischen Koranamlegung (1920) ; in Persian, C. A. Storey, Persian Litera ture. Section I., Qur'anic Literature. (1927).
most comprehensive work is Theodor Noldeke, Geschichte des Qorans, and ed. by F. Schwally and G. Bergstrasser (1909, 1919, 1926) ; H. Hirschfeld, New researches into the composi tion and exegesis of the Qoran (19o2) ; H. U. W. Stanton, The Teaching of the Qur'an, with an account of its growth and a subject index (1919). Translations: G. Sale (1734, etc.), J. M. Rodwell (1876, etc., arranged chronologically), E. H. Palmer (188o, etc.), Muhammad 'Ali (1917). Sources of the Qur'an: A. Geiger, Was hat Mohammed aus dem Judenthume aufgenommen? (1833, 1902); J. Horovitz, Koranische Untersuchungen (1926) ; Tor Andrae, Der Ursprung des Islams and das Christentum (1926) ; A. Mingana, Syriac influence on the style of the Kur'dn (1927).