Darwin wrote extensively to other scientists while he was collecting evidence and developing his theory of evolution. Review Darwin's letters to understand the evidence he collected and the processes he used to develop his theory.
Even Darwin had bad days. In a letter to his cousin, he expressed his frustration:
My Dear Fox: I am rather low today about all my experiments -- everything has been going wrong -- the fan-tails have picked the feathers out of the Pouters in their Journey home -- the fish at the Zoological Gardens after eating seeds would spit them all out again -- Seeds will sink in saltwater -- all nature is perverse & will not do as I wish it, & just at present I wish I had the old Barnacles to work at & nothing new.
(From Burkhardt, Frederick, ed. Charles Darwin's Letters: A Selection 1825-1859. [Cambridge, England: Cambridge University Press, 1998] p.139)