Er, beg to differ. Here's a quote from Darwin's letter to Joseph Hooker in 1871. It is used in numerous lecture notes on the internet:
Yes. (ARRRGH, ya got me!) He speculated in private. Don't we all? But his formal works don't deal with the issue. And "classical" evolution (if I may use that expression) studies the development of life -- however that life first came into existence. [I'll give you a hug anyway.]