The origin of species by means of natural selection

action adapted advantage allied America amount ancient animals appear become believe birds breeds cause characters climate closely common considered continuous crossed descendants difl'erent distinct domestic doubt evidence existing extinct extremely facts families favourable fertility find first flowers follow formations forms genera genus give given greater groups habits hand Hence hybrids important increase individuals inhabitants inherited insects instance instincts intermediate islands kind known land laws less lines living look males manner means migration modification namely natural selection naturalists nearly never observed occur organic origin parent perfect period plants points present principle probably produced range ranked reason regions remains remarked resemble seeds seems seen separated shown single slight sometimes species sterility structure struggle successive suppose tend theory tion trees variability variations varieties vary whole widely