Darwin himself arrived at his conclusions through comparison of species he had seen in agronomy and in the wild.
dˈɑːɹwɪn hɪmsˈɛlf ɚɹˈaɪvd æt hɪz kəŋklˈuːʒənz θɹuː kəmpˈæɹɪsən ʌv spˈiːsiːz hiː hæd sˈiːn ɪn ɐɡɹˈɑːnəmi ænd ɪnðə wˈaɪld.