Who originally played Darrin on Bewitched?

Running from 1964-1972, Bewitched is one of those shows that proved the more absurd the plot in a sitcom, the better.

The plot for this one is that a beautiful, immortal witch (Samantha) falls in love with and marries a dorky mortal (Darrin). Just the clear fact that Samantha is way out of Darrin's league is absurd enough, then you throw in the whole magical witch thing, and boom — a classic TV show that crushed the ratings during its run.

But in a fairly rare move for a sitcom, the lead actor playing Darrin Stephens was replaced about halfway through the show's run. Who played Darrin first?

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