Given the fact that rudimentary theories of evolution go back at least as far as the early Greeks, why did it take until the 19th century for adequate theories of evolution to develop? Why did Darwin...

Given the fact that rudimentary theories of evolution go back at least as far as the early Greeks, why did it take until the 19th century for adequate theories of evolution to develop?

Why did Darwin delay publication of his theory for so long? What finally prompted him to publish it?