Each organism contains an individual combination of inherited traits. Organisms that have useful traits reproduce in greater numbers. Their offspring inherit the traits. Organisms with unfavorable traits eventually die off. The fittest survive.
Nature selects different traits at different times.
Varieties within a species gradually create a new species. The publication of this theory started a sensational controversy. Many writers applied Darwin's theory to sociology. They developed a controversial theory called Social Darwinism. Many people, from Karl Marx to Captain Mahan to Adolf Hitler, employed Social Darwinism in their arguments.