Competitive exclusion is also known as Gause's Principle. It describes what occurs when two species are competing for the same resources. Usually, one will out-compete the other due to variations in genetic makeup. This will lead to one species being successful, while the other may become extinct. This occurred in a famous experiment performed by Gause with two species of Paramecia-- P. caudatum and P. aurelia. They were in the same test tube with a food supply. In many trials, P. aurelia was able to drive P. caudatum to extinction. Gause's law is valid when all other ecologicaly factors are constant. In nature, rather than compete directly, organisms tend to occupy a specific niche, thus reducing competition. For example, many different species of finches can live in one tree in the Galapagos islands, feeding in different locations, on different foods, or at different times. They co-exist in the same habitat because they are not directly competing.