Vestigial structures are homologous of organs that have lost all or most of their original function in a species through evolution. Atavism is the recurrence of a trait after an absence of one or more generations due to a chance recombination of genes. The genome contains genes that were active in the past, but are no longer active. Those genes exist in all flora and fauna. Some of these disappear during embryonic development, but others are retained in adult Life. Those structures are sometimes the result of an intrauterine 'evolutionary' process.