The following is an excerpt from a GRE Verbal exercise:
The hypothesis of an expanding Earth has never attracted notable support, and if it were not for the historical example of continental drift, such indifference might be a legitimate response to an apparently improbable concept. It should be remembered, however, that drift too was once regarded as illusory, but the idea was kept alive until evidence from physicists compelled geologists to reinterpret their data.
I don't understand what "the idea" in the sentence refers to. Does it mean the "drift theory" or the opinion that "drift is illusory"?