Darwin was not at all unsure about the value of his orchid book. He viewed it as presenting the most convincing case for natural selection that he could make.
Even Darwin would be aghast at what the world has made of a mere abstract that he was almost pathologically ambivalent about ever publishing.
Did the admirably candid Charles Darwin himself really believe all that he wrote about evolution?
All the evidence points to Darwin as a generally healthy man with a strong, active libido, normal sexual function, and prodigious fertility.
We may dispute what Darwin felt or thought in the privacy of his study — but the bulk of his writings fall clearly into advocating for one perspective: naturalism.