Bonus Spells: Dragon disciples gain bonus spells as they gain levels in this prestige class, as if from having a high ability score, as given on Table: The Dragon Disciple. A bonus spell can be added to any level of spells the disciple already has the ability to cast.
If a character has more than one spellcasting class, he must decide to which class he adds each bonus spell as it is gained. Once a bonus spell has been applied, it cannot be shifted.
是后者啦,后者。楼主想得太多了。