In later editions he expanded his thoughts on this, and in the final chapter of the 6th edition concluded that species have been modified "chiefly through the natural selection of numerous successive, slight, favourable variations; aided in an important manner by the inherited effects of the use and disuse of parts".
Nas últimas edicións ampliou os seus pensamentos sobre isto, e no capítulo final da 6ª edición concluíu que as especies foran modificadas "principalmente pola selección natural de variacións numerosas sucesivas lixeiramente favorables; axudado de maneira importante polos efectos herdados do uso e desuso das partes".