Nor should what we say appear any more strange than to see a young person grown up, whom we do not see grow up; for what moves gradually is not at all to be recognized by us, and the longer something needs for its change to be recognized the more stable we think it is. So we are not surprised if the opinion of men, who are little distant from brutes, is that a given city has existed always with the same language, since the change in language of a city happens gradually only over a very long succession of time, and the life of men is also, by its very nature, very short. Therefore if over one people the language changes, as has been said, successively over time, and can in no way stand still, it is necessary that it should vary in various ways quite separately from what remains constant, just as customs and dress vary in various ways... (p. 321, Empires of the Word; original available elsewhere)
It's easy to forget just how difficult even the basics of historical linguistics must once have seemed, but texts like these help.
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