Charles Darwin's book, On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, was published in 1859 and is the foundation of evolutionary biology.
Darwin went on a scientific expedition to the Galapagos Islands in 1831 and there he saw life forms that were unique to this volcanic island chain. He spent 20 years of his life contemplating how these unique life forms arose in the short time that these islands existed. He reasoned they must have had ancestors from Ecuador that somehow found their way to the Galapagos, and over time, they somehow changed.
The Theory of Natural Selection included the idea that living things tend to produce more offspring than can possibly survive. The amount of resources is stable therefore a struggle for existence occurs. Since no two individuals are exactly alike, those with variations better suited to the environment may live, reproduce and pass on these variations. Survival of the fittest occurs and these "fit" individuals pass their heritable traits on to the next generation. Natural selection occurs--those best adapted to their environment win the struggle for existence. Over time, as variations accumulate, eventually, a new species may arise. This is what evolution is--change over time.
His ideas included the premise that living things had ancestors--and didn't remain the same over time. This is known as descent with modification...as time passes, future generations will differ from the ancestral population due to the variations they passed down in response to environmental pressures. Different species branch off a common ancestral species. These ideas can be used to describe how flora and fauna in an area could arise over time. For instance, the various finch species in the Galapagos Islands can be traced to an ancestral finch species from Ecuador. Because the early finches that arrived on these islands were separated geographically and because each island had its own selection pressures and habitats, natural selection resulted in the different species of finch today which differ by beak size, shape, behavior and diet.
The main ideas of the work can be summarized thusly: (1) heritable variation leads to (2) differential reproductive success.