Atavism
An atavism is a real or supposed evolutionary throwback, such as traits reappearing which had disappeared generations ago. Atavisms occur because genes for previously existing phenotypical features are often preserved in DNA, even though the genes are not expressed in some or most of the organisms possessing them.

# Examples
Examples observed include:
- hind legs on whales
- hind fins on dolphins
- extra toes on horses, as in archaic horses
- reemergence of sexual reproduction in Hieracium pilosella and Crotoniidae
Atavisms have been observed in humans as well. For example, babies have been born with a vestigial tail, called "coccygeal process", "coccygeal projection", and "caudal appendage". It can also be evidenced in humans who possess large, ape-like teeth.

# Atavism in history
During the interval between the acceptance of evolution and the rise of modern understanding of genetics, atavism was used to account for the reappearance in an individual of a trait after several generations of absence. Such an individual was sometimes called a "throwback". The term is often used in connection with the unexpected reappearance of primitive traits in organisms.
The notion of atavism was used frequently by social Darwinists, who claimed that inferior races displayed atavistic traits, and represented more primitive traits than their own race.  Both the notion of atavism, and Haeckel's recapitulation theory, are saturated with notions of evolution as progress, as a march towards greater complexity and superior ability.
In addition, the concept of atavism as part of an individualistic explanation of the causes of criminal deviance was popularised by the Italian criminologist Cesare Lombroso in the 1870s. He attempted to identify physical characteristics common to criminals and labeled those he found as atavistic, throwback traits that determined 'primitive' criminal behavior. His statistical evidence and the notion that physical traits determine inevitable criminality (an idea closely related to the concepts of eugenics) have long since been debunked, but the concept that physical traits may affect the likelihood of criminal behavior in the individual remains popular in some circles.
The notion that somehow, atavisms could be made to accumulate by selective breeding led to breeds such as the Heck cattle. This had been bred from ancient landraces with selected primitive traits, in an attempt of "reviving" the extinct aurochs.

# Cultural references to atavism
The term atavism is sometimes also applied in the discussion of culture.  Some social scientists describe the return of older, "more primitive" tendencies (e.g., warlike attitudes, "clan identity," etc. -- anything suggesting the social and political atmosphere of thousands of years ago) as "atavistic." "Resurgent Atavism" is a common name for the belief that people in the modern era are beginning to revert to ways of thinking and acting that are throwbacks to a former time.  This is especially used by sociologists in reference to violence.  Marxists refer to pre-capitalist classes (such as the peasantry, the aristocracy and the petit-bourgeoisie) as "atavistic" to indicate that they do not fit into the bipolar class division (bourgeoisie/proletariat) of modern capitalist society.  Marxists therefore view them as a reactionary force that will try to stop not only socialism, but also bourgeois progress itself.
The neo-pagan subculture also uses this same terminology ("atavism" or "resurgent atavism") to describe how modern, Western countries are experiencing both the decline of Christianity and the rise of religious movements inspired by the pagan religions of centuries past.  Some cite the rise of environmentalism, scientific inquiry, and liberalization of society as contributing to an increasingly secular society, one in which religious sentiments are more frequently tied with an appreciation of the physical world rather than set against it.  Occasionally, the use of these terms in reference to "alternative" spirituality or in an occult context implies the use of violence to assert these changing religious views--for example, in the book Lords of Chaos a rash of church burnings across Scandinavia has been described as a part of this trend because many of the perpetrators were self-described "pagans" seeking to overthrow what they deemed to be centuries of religious oppression by Christianity.
Atavism is a key term in Joseph Schumpeter's explanation of World War I in 20th Century liberal Europe.  He defends the liberal belief in international relations that an international society built on commerce will avoid war because of its destructiveness and comparative cost.  His reason for WWI is termed "Atavism," in which he claims the vestigial governments in Europe (the German Empire, Russian Empire, Ottoman Empire, and Austro-Hungarian Empire) pulled the liberal Europe into war, and that the liberal structure of the continent did not cause it.  He uses this idea to say that liberalism and commerce will continue to have a soothing effect in international relations, and that war will not arise in nations who are built on commercial ties.