The Roman language served as the first model for answering the question. Even to someone with no knowledge of Latin, the similarities among Roman languages would have made it natural to suggest that they were derived from a common ancestor. On the assumption that the shared characteristic of these languages came from the common ancestor, it would have been possible to reconstruct many of the characteristics of the original common language. In much the same way it became clear that the branches of the Indo-European family could be studied and a hypothetical family tree constructed, reading back to a common ancestor. This is the tree approach. The basic process represented by the tree model is one of divergence: when languages become isolated from one another, they differ increasingly, and dialects gradually become different until they become separate languages.
    Divergence is by no means the only possible tendency in language evolution. Johannes Schmidt introduced a 'wave' model, in which linguistic changes were like waves, eventually leading to convergence; that is, growing similarity among languages that were initially quite different.
    Today, however, most linguists think primarily in terms of familytrees. It is necessary to construct some models of how language change might occur according to a process-based view. There are four main classes of models.
    The first is the process of initial colonization, by which an uninhabited territory becomes populated; its language naturally becomes that of the colonizers. Second are processes of divergence, such as the linguistic divergence arising from separation or isolation mentioned above in relation to early models of the Indo-European languages. The third group of models is based on processes of linguistic convergence. The wave model, formulated by Schmidt in the 1870's, is an example, but convergence methods have not generally found favor among linguists.
    Now, the slow and rather static operation of these processes is complicated by another factor: linguistic replacement. That factor provides the basis for a fourth class of models, in many areas of the world the languages initially spoken by the indigenous people have come to be replaced, fully or partially, by languages spoken by people coming from outside. Were it not for this large complicating factor, the world's linguistic history could be faithfully described by the initial distribution of Homo Sapiens, followed by the gradual workings of divergence and convergence. So linguistic replacement also has a key role to play in explaining the origins of the Indo-European languages.

单选题     The characteristics of the original common language can be described on the basis of ______.

单选题     According to Johannes Schmidt, ______.

单选题     It can be inferred from paragraph 4 that ______.

单选题     Which of the following is TRUE according to the passage?

单选题     This passage is primarily written to ______.