This thesis addresses some issues of parametric changes in the history of English. On the hypothesis that functional categories are responsible for parametric variations, I will discuss the problem of whether there is a valid generalization of diachronic changes and if there is one, how it should be formulated. The topics to be discussed are the development of the complementizer system, the loss of verb movement, and the transitions of infinitival constructions. After reviewing some basic issues in Chapter 1, I willinvestigate the inventory of functional categories in Chapter 2. Specifically, I will propose a universal split-CP hypothesis, in which complementizers morphologically realized as single words are syntactically decomposed into two distinct functional heads. In Chapter 3, I will demonstrate that parameter resetting can be described in terms of diachronic feature shift among functional categories and that the process of feature shift exhibits the property of locality. This generalization will enable us to solve some puzzling phenomena concerning the development of complementizers. In Chapter 4, I will derive the locality property of diachronic feature shift from independent principles of UG by introducing a viinovel notion of Attraction Domain. It will be shown that parametric changes are caused by the decline of verbal inflections and that there is no necessity of postulating principles of language change. In Chapter 5, I will shed light on diachronic feature shift within infinitives and thereby demonstrate that the same mechanism that accounts for parametric changes in finite clauses is also valid for explaining changes in non-finite clauses. Thus, this chapter provides us with more evidence for our feature shift hypothesis. Conclusion and a comment on further application of the proposed system will come in Chapter 6.