The Long Argument About Drifting Continents

[A] The idea that the continents had once been joined was put forward with a great deal of evidence behind it. The coastlines fitted, the same fossil plants and reptiles turned up on land masses now separated by oceans, and sequences of rock ended at one shore and resumed at another. [B] The proposal was rejected for half a century all the same. [C] The reason usually given is that geologists were conservative, and that is too easy. [D]

What the proposal lacked was a mechanism. Continents were understood to be resting on rock that would not flow, and the forces its author suggested were calculated by his critics and shown to be far too weak to push a continent through anything. A claim with abundant evidence and no possible cause is in an awkward position, and the critics were doing what critics are for. What changed matters was not new evidence of the old kind but the discovery that the ocean floor was itself spreading, which supplied the engine that had been missing. The lesson is not that the doubters were fools. It is that evidence and explanation are different things, and that a field can be right to wait for the second even when the first has been piling up for years.