States the evidence for a theory of evolution, explains how evolution takes place, and discusses instinct, hybrids, fossils, distribution, and classification.

Variation under domestication -- Variation under nature -- Struggle for existence -- Natural selection or the survival of the fittest -- Laws of variation -- Difficulties of the theory -- Instinct -- Hybridism -- On the imperfection of the geological record -- On the geological succession of organic beings -- Geographical distribution -- Mutual affinities or organic beings : Morphology : Embryology : Rudimentary organs -- Recapitulation and conclusions.