Evolve This: Getting Language from Darwin's "Horrid Doubt"

In a letter to William Graham in 1881, Charles Darwin -- a year before he died -- expressed a grave misgiving about his theory:

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?

Conviction: it's a word implying semantics. The dictionary defines conviction...