It's one of the funny things I know about Charles Darwin.
Dearly detested D married his first cousin, I suppose it was more common back in those days. After most of his children died and the rest turned out sickly, he looked at the inbred tomatoes in his garden and began to wonder. This was before genetics was fully understood so nobody really knew why one should not marry relatives.
Of course, people also didn't know just how complex biology really is. They thought cells were just identical lumps of protein that did nothing other than divide. Now that we know about nuclei, DNA, and all the parts of a cell, we know that macroevolution is impossible.
I consider it a blessing from the Almighty that nobody thought up evolution sooner. The thinkers of the Enlightenment period were already trying to ignore God as much as possible. The only reason they couldn't was that they didn't know how else to explain how life, the universe, and everything got here. That's why they came up with the Watchmaker notion. They decided that God had made the world, set it running, and left it to its own devices.
If Darwin had come around with his theories during the Enlightenment, that would have been a theological, philosophical, and scientific disaster.