The difference is that in the first example both people are clearly defined and are the only ones we are talking about. In the second sentence, Darwin is only one of many contributors to the Economist. If we change it to 'the early contributor' it would mean he was the only early contributor.
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How2dieAbout your second remark. True, Spencer is not the only contributor to the magazine, but so is Soury: he is not the only student in the world. No difference here either.in the first example, only two people are under discussion, not the whole world. out of those two, Soury is the only student, the student. if a third student had died, giv