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

action adapted advantage allied America amount animals appear become believe birds breeds cause characters climate closely common compared considered continuous crossed descended developed difficulty distinct domestic doubt effected existing explained extinct extremely facts families favourable fertility flowers follow formations forms genera genus geological give given greater groups habits hand Hence hybrids important improved increase individuals inhabitants inherited insects instance instincts intermediate islands kind known land laws less living look males manner means modification namely natural selection naturalists nearly observed occasionally occur offspring organic organisation origin parent perfect period plants points pollen present principle probably produced range ranked reason remains remarked resemble seeds seems seen separated similar single slight sometimes species sterility structure struggle successive supposed tend theory variability variations varieties vary whole widely