Natural selection is the combination of two ideas: 1. Population characteristics change over time if members of the population are systematically disallowed reproduction. 2. Nature systematically disallows reproduction.

I'm willing to accept that I'm suffering from hindsight bias. But will you at least give me that his theory is much easier to understand than any of the others? And maybe a few guesses on the topic of why it was so hard to think of?

Extraordinarily so, yes -- it does astonish me that no one hit it before. Nonetheless, the empirical fact remains, so...

I suppose the sense of "mystery" people attached to life played into it somewhat.

People were breeding animals, people were selecting them, and...socially there was already some idea of genetic fitness. Men admired men who could father many children.The idea of heredity was there.

Honestly, the more I think of it, the more I share your confusion. It is deeply odd that we were blinded for so long. Perhaps we should work to figure out how this happened, and whether we can avoid it in the future.

I don't think luck can factor in quite as much as you imagine though. We're not attempting to award credit, so much as we are attempting to identify circumstances which tend to produce people who tend to produce important insights. Darwin's insight was incredibly important, and had gone unseen for centuries. To me, that qualifies him.

Even if you put it at a remove, even if you say, well, Darwin was uniquely inspired by his voyage, another biologist could have done the same, then the voyage becomes important. Why didn't another biologist wind up on a voyage like that? What can we do to ensure that inspiring experiences like that are available to future intellectuals? In this way, Darwin's life remains an important data point, even if -- especially if -- we deny that there was anything innately superior about the man.

Addendum: Here is an argument that ancient scientists and mathematicians don't deserve as much credit as we give them: they were prolific.

Agreed, completely -- they pulled the low-hanging fruit from the search space.