Charles Darwin's theory of evolution by natural selection is the foundation upon which modern evolutionary biology is built. The theory was outlined in Darwin's landmark book On the Origin of Species, published in 1859. It states that all species of life have descended over time from common ancestors, and that the process of natural selection has resulted in the development of the various species we see today.Darwin's theory was based on his observations of the natural world, and on his study of the fossil record. He observed that species change over time, and that some species are better suited to their environment than others. He also noted that living things produce more offspring than can survive, and that these offspring vary in their traits. He reasoned that if some of these traits offer a survival advantage, then the organisms possessing them will be more likely to survive and reproduce. Over time, the advantageous traits will become more common in the population, while the disadvantageous traits will become less common. This process is known as natural selection.Darwin's theory is supported by a great deal of evidence from the natural world. It is the most widely accepted explanation for the diversity of life on earth.