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acquired adult allied apes appear Argus pheasant beak beautiful become believe birds breeds cause chapter closely colour common confined crossed crustaceans degree descended developed distinct species domestic doubt eggs existing extinct extremely fact favourable feathers female fertility fishes formation forms genera genus gradations greater number groups habits horns hybrids important individuals inhabitants inherited insects instance instincts intermediate islands kind larvae latter Lepidoptera less likewise living Malay Archipelago males mammals manner Marsupials modified monkeys moult natural selection naturalists nearly nest observed occur ocelli offspring organs Origin of Species ornaments perfect period pigeons plants plumage pollen possess present probably produced progenitor quadrupeds races reason relation remarks resemble rudimentary seeds sexes sexual selection shew shewn slight South America sterility stridulating structure supposed tail tion transmitted variability variations varieties various vary vertebrate whilst whole widely wings young Zoolog