Variation under domestication -- Variation under nature -- Struggle for existence -- Natural selection -- Laws of variation -- Difficulties on theory -- Instinct -- Hybridism -- On the imperfection of the geological record -- On the geological succession of organic beings -- Geographical distribution -- Geographical distribution (continued) -- Mutual affinities of organic beings: morphology, embryology, rudimentary organs -- Interlude: almost like a whale? -- Recapitulation and conclusion.