The object of this book is to place before English readers this Common Tradition, as being a tradition earlier than any of our existing Gospels, and consequently exhibiting the closest approximation we possess to some parts of the original narrative from which our Gospels are derived. . . .
It is possible that for some time the Evangelistic records were handed down not in writing, but by means of oral tradition, like the Mishna of the Jews; which is said to have been editorially arranged about the end of the second century, but not committed to writing till afterwards. A tradition intended to be handed down orally might naturally aim at brevity; and the following extract from a condensed and interesting essay on the Mishna will shew that, at all events in Jewish tradition, brevity was occasionally accompanied by its proverbial danger, obscurity.
--from the Introduction