“But then with me the horrid doubt always arises whether the convictions of man's mind, which has been developed from the mind of the lower animals, are of any value or at all trustworthy. Would any one trust in the convictions of a monkey's mind, if there are any convictions in such a mind?”

Basically because he's a monkey.., just like you and I

In which of Darwin's books or letters did he write that?

DB Roy wrote:In which of Darwin's books or letters did he write that?

You exposed me! I made it up!

You are good!

That looks like Platinga's argument against supernaturalism.

Darwin didn't say it. The Duke of Argyll wrote that Darwin said this to him. This was in the 1880s so there was no way to record the man's words so the good duke was going off memory and we all know how reliable that is. What Darwin actually said we will never know but we know it wasn't what the duke set down because he would have had to memorize it as soon as Darwin spoke it which seems unlikely. At best, it's simply anecdotal evidence. Of course, that such is not evidence has never stopped people like ant from using it that way.

Interbane wrote:That looks like Platinga's argument against supernaturalism.

That's precisely right.
Do you think that's where he got it from or was the argument in circulation prior to Plantinga?

I have no idea where the idea might have originated. I could see it popping up independently, but I don't know. It's interesting that we see it from that long ago, during the birth of the idea of evolution. I agree with it, because it makes sense to me.