Atavism (Latin atavus — the ancestor) — emergence in an organism of the signs inherent in the remote ancestors, but absent at individuals of this generation. An example of atavism at the modern person is the hvostoobrazny appendage.
Atavisms figured at Charles Darwin's theory. They served as the proof of filetichesky origin of animals. Filetichesky evolution (from Greek phyle - the tribe, a sort) is called the evolution expressed in the gradual unidirectional change of structure of organisms. In representations of modern genetics and experimental embryology the concept of atavism is narrower. Understood all unexpectedly shown signs as atavism earlier. Now atavisms call "single variations the" of signs approximately similar to features of far ancestors with which genetic relationship is obvious or probable. Tell about spontaneous atavism when unexpectedly there are signs not characteristic of individuals of this look in their modern look, but hypothetically inherent in ancestors from other systematic category. The tail-shaped appendage of the person belongs to spontaneous atavism. As a rule, atavisms are formed at an animal in a stage of an embryonal development. The polimastiya (mnogososkovost) and a hyper trichosis (excessive hairiness) belong to embryonic deviations as atavism. The most frequent are atavism cases as result of crossing. About "Crossing as the direct reason of atavism" still Darwin wrote. Hybridization of organisms is postulated by the main reason of atavism not accidentally: rudiments of the inherited signs can remain latent long time. Thanks to crossing they pass into an active state and come to light in posterity. Modern geneticists are inclined to consider that manifestation of signs depends on a set of factors. When crossing there can be a recombination of genes; at the exit new signs turn out. Receiving hybrids is based on knowledge of this fact. The phenomenon of atavism should be distinguished from rudiments. The rudiment (Latin of rudimentum - a rudiment) is the some sign which is present at all individuals of a look, but lost the functionality. Examples of rudiments are: appendix, ear muscles, tailbone.