I have just read an essay by Martin Hengel, "The Four Gospels and the One Gospel of Jesus Christ," in the collected volume, The Earliest Gospels (ed. Charles Horton; London/New York: T & T Clark, 2004), 13-26. Hengel asks himself: "How is it that we have the narrative of Jesus' activity in a fourfold and often contradictory form in the New Testament and what is the origin of these texts? A single Gospel about Jesus would have already spared the church – to the present day – much soul-searching" (p. 14). Then he refers to Irenaeus, ca 180 CE, who not only indicates the existence of the (older) Four-Gospels-Collection, but has to defend it with a variety of arguments. Likewise Justin, around 150 CE, who said that the reminiscences "were composed by apostles and their successors," which Hengel takes as a reference to Matthew and John (apostles) and Mark and Luke (successors). Further on, Hengel expresses the unexpected fact that "the early church resisted the temptation to replace the four Gospels with a Gospel Harmony, which would have done away with all these problems" (p. 15). Admittedly, as Hengel points out, Tatian composed his Diatessaron around 170 CE, and it became established in the Syriac-speaking East, but later it was again displaced by the four 'separate' Gospels of the mainstream church.
All of these things are of course well-known, but when Hengel comes to the implications for textual criticism he only mentions the "widespread freedom" in dealing with the Gospel writings in the second century; "in this early period the texts of the Gospels could still be changed." At the same time, however, he is optimistic in that "nearly all the alterations and interploations can be picked up in the multiple textual tradition" (p.19).
When I read this part of the essay, I wondered if there was not another important implication for textual criticism: In the second century there was not only a harmonistic tendency, but also a strong anti-harmonistic tendency (cf. Irenaeus) and I wonder to what degree such a tendency is reflected among early scribes. In a review of David Parker's The Living Text of the Gospels, co-blogger Peter Head criticizes Parker for over-emphasising the freedom of the manuscript tradition. PH points out: "We might say that it is precisely because not all scribes acted in a free manner that we are now able to study those which did, and to rank readings, when possible, in an order of historical development."