exhibiting an exceptional purity of text, that is, readings which the fullest consideration of Internal Evidence pronounces to be right in opposition to formidable arrays of Documentary Evidence, the cause must be that, as far at least as these readings are concerned, some one exceptionally pure MS was the common ancestor of all the members of the group; and that accordingly a recurrence of this consent marks a recurrence of joint derivation from that particular origin, and accordingly a strong presumption that exceptional purity is to be looked for here again. The inference holds equally good whether the transmission has been wholly divergent, or partly divergent and partly mixed; and any characteristic, favourable or unfavourable, may be the subject of it.
78. The value of Internal Evidence of Groups in cases of mixture depends, it will be seen, on the fact that by its very nature it enables us to deal separately with the different elements of a document of mixed ancestry. In drawing general conclusions from the characteristics of the text of a document for the appreciation of its individual readings successively, we assume the general homogeneousness of its text; but this assumption is legitimate only if unity of line of ancestry is presupposed. The addition of a second line of ancestry by mixture introduces a second homogeneousness, which is as likely as not to conflict with that of the first, and thus to falsify inferences drawn from the first, unless there be means of discriminating from the rest of the text the portions taken from the second original. But each well marked group of which the mixed document is a member implies at least the contingency of a distinct origin; and thus, in readings in which the document is associated with the rest of the group, its authority need not be that which
