One argument Darwin used is the anthropomorphic warning which you can read about here. Another argument Darwin used is that there really isn't any problem for evolution so long as evolution cannot be disproved.
Can you imagine a scientist proposing a dubious theory and then claiming it is true because an impossible falsification criterion has not been met? This is precisely what Darwin did. He wrote:
If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find out no such case.
This was hardly a concession. Darwin may sound generous here, allowing that his theory would “absolutely break down,” but his requirement for such a failure is no less than impossible. For no one can show that an organ “could not possibly” have been formed in such a way. So in short order Darwin reduced what seemed to be a dilemma for his theory into a logical truism.
Evolution was protected from criticism and all that was needed to explain complexity was a clever thought experiment. Darwin so lowered the requirements that anyone with a pen and a vivid imagination can now claim to have solved the problem of complexity. It is now common to see in the evolution literature vague explanations, which rely on such dubious mechanisms as “chance” or “opportunism,” put forth as though they are solutions to the problem of complexity.