On the Origin of Species by Means of Natural Selection: Or, the Preservation of Favored Races in the Struggle for Life

according action adapted admit allied America amount animals appear become believe belonging birds breeds called cause certain characters climate closely colour common considered continued crossed descendants developed difficulty distinct domestic doubt effects existing extinct extremely facts families favourable fertility flowers follow formations forms genera genus give given groups habits hand Hence hybrids important increase individuals inhabitants inherited insects instance instincts intermediate islands kind known land laws less living look male manner means modified namely natural selection naturalists nearly never observed occur offspring organs origin parent perfect period plants points present preserved principle probably produced range ranked reason remarked resemblance respect seeds seems seen separated shown side similar single slight sometimes species stage sterility structure successive supposed tend theory variability variations varieties various vary whole widely young