beginning with a common noun followed by of may take the article, as in the Isle of Wight or the Isle of Portland (compare Christmas Island), same applies to names of institutions: Cambridge University, but the University of Cambridge.
In Middle English, these had all merged into þe, the ancestor of the Modern English word the. Old English had a definite article se (in the masculine gender), sēo (feminine), and þæt (neuter). The and that are common developments from the same Old English system. The, as in phrases like "the more the better", has a distinct origin and etymology and by chance has evolved to be identical to the definite article. Definite article principles in English are described under " Use of articles".