On the Origin of Species by Means of Natural Selection

accumulated action adapted advantage allied America amount ancient animals appear become believe birds breeds cause chapter characters climate closely common compared considered continuous crossed descendants developed difficulty distinct domestic doubt effects 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 land laws less living look male manner means migration modification namely natural selection naturalists nearly never observed occasionally occur offspring organic origin parent perfect period plants points present principle probably produced range ranked reason remarked resemblance seeds seems seen separated single slight sometimes species sterility structure struggle successive suppose tend theory understand variability variation varieties vary whole widely