The results obtained by internal analysis are supplemented and verified by collecting all the external evidence relative to the document under criticism which can be found scattered over the documents of the same or later epochs--quotations, biographical details about the author, and so on. Sometimes there is a significant absence of any such information: the fact that an alleged Merovingian charter has not been quoted by anybody before the seventeenth century, and has only been seen by a seventeenth-century scholar who has been convicted of fraud, suggests the thought that it is modern. II. Hitherto we have considered only the simplest case, in which the document under examination is the work of a single author. But many documents have, at different times, received additions which it is important to distinguish from the original text, in order that we may not attribute to X, the author of the text, what really belongs to Y or Z, his unforeseen collaborators.[83] There are two kinds of additions--interpolations and continuations. To interpolate is to insert into the text words or sentences which were not in the author's manuscript.[84] Usually interpolations are accidental, due to the negligence of the copyist, and explicable as the introduction into the text of interlinear glosses or marginal notes; but there are cases where some one has deliberately added to (or substituted for) the author's text words or sentences out of his own head, for the sake of completeness, ornament, or emphasis. If we had before us the manuscript in which the deliberate interpolation was made, the appearance of the added matter and the traces of erasure would make the case clear at once. But the first interpolated copy has nearly always been lost, and in the copies derived from it every trace of addition or substitution has disappeared. There is no need to define "continuations." It is well known that many chronicles of the middle ages have been "continued" by various writers, none of whom took the trouble to indicate where his own work began or ended.