he relevant cluster; another is to enumerate each member; yet another orders the members of each cluster in some way then gives the first and last separated by an ellipsis. Hopefully, whichever approach prevails will not lead to a profusion of different names for the same clusters. In what follows, I will identify each cluster with its first and last members as ordered in the dendrogram.
 The dendrogram has a self-similar character whereby each subtree has a similar appearance to the whole. A subtree can itself be partitioned by chopping at some height. Thus, the great cloud of witnesses which make up the fourth cluster of the six-way partition (i.e. K, L, P, and the rest) can be divided into seven by cutting at a height of 0.4:
K ... 1319
104 ... 263
365 ... slav
P ... 044
 Although some details of this partition would probably change if a larger sample was analysed, what is seen here serves as a starting point for discussion. The fourth, fifth, and sixth clusters are solitary, having only one or two members among the sampled witnesses. Nevertheless, they are kindred to the other witnesses in this great cloud. With one possible exception, all of the remaining clusters have been identified before: the K ... 1319 cluster is associated with the Byzantine standard text which became dominant after the fourth century; Waltz lists five members of the 104 ... 263 cluster under his “family 2127,” noting that some belong to von Soden's Ia3 group in Paul; as for the P ... 044 cluster, MDS maps produced during my doctoral research reveal that P, 044, and 0150 are neighbours. Concerning the possible exception, I don't know whether anyone has previously noticed a similarity between 365, 1912, and the Greek text presumed to stand behind the Old Church Slavonic version.
 It is natural to ask what each cluster represents and how the overall picture came to have its present form. If it was possible to produce a sequence of maps showing the distribution of witnesses in textual space at the end of each year for the first few centuries of textual transmission, I imagine that animating the sequence would result in what looks like an exploding firework whose initial burst gives rise to a series of other bursts which in turn gives rise to yet more bursts, and so on, expanding ever outwards. If this is a good analogy, what agents are to blame for driving the fragments apart? The culprits are the copyists.
 Whether by chance or design, copyists introduced novel readings. Once there was a choice of readings, copyists began to choose among those that were already known in their sphere. It seems that typical copyists preferred to choose among existing readings rather than to create novel ones. This is not to say that a copyist would never amend the exemplar's text, especially when faced with an apparent difficulty or corruption. However, there is evidence to suggest that normal practice was to choose among existing readings. For example, the UBS4 apparatus of Hebrews shows that the number of readings in a variation unit follows a certain pattern:
 Here, the number of readings in a variation unit is usually two, perhaps three or four, but seldom more. If copyists were creating new readings all the time then one would expect the frequency of variation units with more than a few readings to be greater.
 Given that the main outcome of the copying process was to reproduce known readings, what determined which ones would be chosen at a particular place and time? If readings had a high mobility then copyists in far flung places would be aware of them and so be able to select them in preference to others. These “viral” readings would then spread throughout the population of texts with ease. If, on the other hand, readings were not so mobile then the preponderance of those available for selection would be local ones. If this was the case then one would expect geographically localised species of text to arise. Regardless of the mobility of readings, the particular reading chosen when the copyist knew there were alternatives was a matter of scribal psychology