"On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection" is the title of a journal article, comprising and resulting from the joint presentation of two scientific papers to the Linnean Society of London on 1 July 1858: On The Tendency of Varieties to Depart Indefinitely from the Original Type by Alfred Russel Wallace and an Extract from an unpublished Work on Species from Charles Darwin's Essay of 1844. Wikipedia