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

accumulated adapted admit affinities allied species America analogous ancient appear beak become bees believe belonging birds breeds cause cells characters cirripedes climate closely allied colour common progenitor continuous crossed crustaceans degree developed difficulty distinct species divergence doubt effects eggs embryo existing exterminated extinct extremely fact favourable fertilised fertility flowers formation forms fossil Fritz Müller genera genus geological geological period gradations greater number habits Hence hybrids hybrids produced important increase individuals inhabitants inherited insects instance instincts intercrossing intermediate kind lamellæ larvæ less living male mammals manner Marsupials migration modified descendants natural selection naturalists nearly nest occasionally offspring organisation organs parent peculiar perfect pigeons pistil plants pollen present preserved principle probably produced quadrupeds ranked reciprocal crosses remarked resemblance rudimentary seeds sexual selection slight South America stamens sterility structure successive supposed swimbladder theory variability variations varieties vary whilst whole widely different wings young