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

adapted admit affinities allied species America ancient animals appear Asa Gray beak become bees believe belonging birds breeds cause cells characters climate closely allied color continued crossed crustaceans degree developed difficulty distinct species disuse domestic doubt effects eggs embryo existing extinct extremely facts favorable female fertility flowers formation forms fossil Fritz Müller genera genus geological geological period Glacial period gradations greater number groups of species habits Hence hybrids hybrids produced important individuals inhabitants inherited insects instance instincts intercrossing intermediate intervals kind lamellæ larvæ less living male mammals manner Marsupials migration modified descendants natural selection naturalists nearly nest oceanic islands offspring organs Paleozoic parent peculiar perfect pigeons pistil plants pollen present principle probably produced quadrupeds ranked remarked resemblance rudimentary seeds Silurian slight South America stage stamens sterility structure successive suppose tend theory tion variability variations varieties vary whole wings young