Wettstein on the Shorter Reading

if it was far more often the case (because there were many more compelling factors) that something was added than that something was omitted, then it follows that it is much more plausible that something has been added in one ms than that something has been omitted in the other. I can see really only two reasons for omission: carelessness, which occurs particularly in cases of homoioteleuton; and ignorance, when the scribe, because he did not understand an unfamiliar word, believed that it could be completely omitted without even bothering to weigh up arguments on either side. And it ought to be the case that the effects of both of these factors are restricted to a few mss and to a few passages. For additions, on the other hand, there were far more frequent opportunities: seeing that marginal glosses were at a later stage introduced into the text; that certain words which according to the usage of the church lectionary stood at the beginning or end of the reading in order to express the meaning more fully were added to the text; seeing that (this is by far the most common tendency) one Gospel was supplemented from another Gospel which described the same event a little more fully, or that one passage of Paul was supplemented by the addition of material from a parallel passage.