The preservation of the scriptures was a joint effort by the community of monks and nuns. They would meet together at regular intervals and chant parts or all of the Tipitaka. This made it virtually impossible for anything to be added or changed. Think of it like this. If a group of a hundred people know a song by heart and while they are all singing it one gets a verse wrong or tries to insert a new verse, what will happen? The sheer number of those who know the song correctly will prevent the odd one making any changes. It is also important to remember that in those days there were no T.V.’s, newspapers or advertising to distract and clutter the mind which together with the fact that monks and nuns meditated, meant that they had extremely good memories. Even today, long after books have come into use, there are still monks who can recite the whole Tipitaka by heart. Mengong Sayadaw of Burma is able to do this and he is mentioned in the Guinness Book of Records as having the world’s best memory.