 the oral tradition. This explanation would also provide an explanation for the relatively late orgins of the legend of the Holy Grail.
Sources and Further Reading
http://www.geocities.com/Paris/5339/graal.html - looks at the literary tradition of the Grail.
www.newadvent.org - The Catholic Encyclopedia. Gives a thorough coverage of all the issues covered here and more. The coverage maybe slightly biased at some points though
www.greatdreams.com/arthur.htm - a look at the Arthuruan legends involving the Grail. Something not well covered in this node.
historymedren.about.com/od/holygrail - links to sites that cover the literary tradition surrounding the Holy Grail
www.rosslyntemplars.org.uk/holy_grail.htm and www.timelessmyths.com/arthurian/grail - more literary tradition and folklore surrounding the Grail
Other Highly dubious (and untrue) but interesting "sources"
The Da Vinci Code by Dan Brown
Monty Python and the Holy Grail by Monty Python