2 Included as of the third edition, 1861. In this "historical sketch", Darwin dealt with the many "precursors" to his theory of natural selection which commentators after 1859 had noted.
3 Included as of the sixth edition, 1872. This chapter continues the previous one on "difficulties of the theory" and responds in particular to St. George Mivart, whose critique of natural selection appeared in 1872 in Genesis of Species.
4 Included as of the sixth edition, 1872, compiled by W. S. Dallas.