Species are merely those strongly marked races or local forms which, when in contact, do not intermix, and when inhabiting distinct areas are generally regarded to have had a separate origin, and to be incapable of producing a fertile hybrid offspring.

On the Phenomena of Variation and Geographical Distribution as Illustrated by the Papilionidae of the Malayan Region, Transactions of the Linnean Society of London, Volume 25, 1865, (p. 12)