The theory of evolution says that all living things on Earth, including plants, animals and microbes, come from a common ancestor by slowly changing throughout the generations. Darwin suggested that the way living things changed over time is through "natural selection". This is the better survival and reproduction of those that best fit their environment. Fitting into the place where you live is called adapting. Those who fit best into the place where they live, the best adapted, have the best chance to survive and breed. Those who are less well-adapted tend not to survive. If they do not survive well enough to raise young, this means they do not pass on their genes. In this way, the species gradually changes.