His method of working at the poem is thus described by Donatus, 'Aeneida prosa prius oratione formatam digestamque in xii. libros particulatim componere instituit, prout liberet quidque et nihil in ordinem arripiens. Ut ne quid impetum moraretur, quaedam imperfecta transmisit, alia levissimis verbis veluti fulsit, quae per iocum pro tibicinibus interponi aiebat ad sustinendum opus donec solidae columnae advenirent.' In what order the Books were written it is impossible to decide; but Book vi. was not read to Augustus till after the death of the young Marcellus, B.C. 23. Donatus, 'Cui [Augusto] multo post perfectaque demum materia tres omnino libros recitavit, secundum quartum sextum, sed hunc notabili Octaviae adfectione, quae cum recitationi interesset ad illos de filio suo versus, "Tu Marcellus eris," defecisse fertur atque aegre focillata est.' Virgil, writing to the emperor, insists on the magnitude of the task he had rashly undertaken. Macrob. _Saturn._ i. 24, 11, 'Tanta incohata res est, ut paene vitio mentis tantum opus ingressus mihi videar, cum praesertim, ut scis, alia quoque studia ad id opus multoque potiora impertiar.' Although in his will Virgil left instructions to Varius (and Tucca) to destroy all his unpublished manuscripts, Varius was expressly desired by Augustus to revise and publish the _Aeneid_. Donatus, 'Egerat cum Vario, priusquam Italia decederet, ut si quid sibi accidisset Aeneida combureret; sed is facturum se pernegarat ... Edidit autem auctore Augusto Varius, sed summatim emendata, ut qui versus etiam imperfectos sicut erant reliquerit.' This account is corroborated by Pliny the elder, _N.H._ vii. 114, Gellius, and Macrobius. The rules laid down to the editors by the Emperor were, according to Servius, 'ut superflua demerent, nihil adderent tamen.' It seems probable that the _Aeneid_ was published B.C. 17, for it is in the _Carmen Saeculare_ of that year that Horace first alludes to the story of Aeneas (cf. l. 50, 'clarus Anchisae Venerisque sanguis'), and in the fourth Book of the _Odes_ (four years later) it is more than once introduced. The _choice of the subject_ was influenced (1) by the personal desire of the Emperor; (2) by the connexion of the Caesarian house with Venus, through Iulus;[51] cf. the invention of Atys (_Aen._ v. 568) by Virgil to please Augustus, whose mother was Atia; (3) by Virgil's design to write an epic on the greatness of Rome, in the manner of Homer.