1. Literal quotations, whether the author cited is named or not. Such quotations serve the double purpose of proving the existence of the work quoted in the time of the writer who makes the reference, and of identifying, and sometimes even of correcting, the extant text. If, for example, in subsequent writers, we find only a dozen or twenty sentences, taken from different parts of an earlier work, the verbal coincidence is sufficient to prove that the work, such as we now find it, is the same as that quoted. When such quotations are numerous and exact, they afford the best means, either of restoring the genuine reading of authors, or of judging of the comparative purity of different manuscripts. For frequently these quotations seem to have suffered less in the course of transcription than either the other parts of the work in which they are found, or than that from which they are taken. The reason of this difference may readily be imagined:--either the author himself quoted from a copy purer than any that are now extant; or the transcriber, meeting with a passage which he remembered to belong to a well-known work, consulted the original, of which he had a good copy, and the very circumstance of doing so would naturally induce somewhat more of care than in ordinary transcription. 2. Incidental allusions are often met with, either to the words or to the sense of an author, sufficiently obvious to prove that the one writer was known to the other; and yet they are too incidental and remote to be regarded as an interpolation. In questions of apparent difficulty, such accidental references may be conclusive in proof of the existence of a work at a certain time. Among the ancient historians, there are instances in which two writers, who do not mention each other, narrate the same facts with so many coincidences of method, or of details, embellishments, or reflections, as to make it certain either that both narratives were derived from the same source; or that the one was copied from the other. And if the one narrative has altogether the air of originality, and is in accordance with the writer’s style and spirit, the other writer must be held to be the quoting party, and therefore he establishes the prior existence of the work from which he has borrowed.