What is Vestigial organ?
'''Vestigial organs''' are [[Organ (anatomy)|organs]] of the body which are smaller and simpler than those in related [[species]]. They have lost, or almost lost, their original feature. Vestigiality is evidence for [[evolution]], since they only make sense if evolution has occurred.<ref>Futuyma D.J. 1995. ''Science on trial: the case for evolution''. Sunderland, MA: Sinauer p49 {{ISBN|0-878-93184-8}}.</ref> They were one of the puzzles of pre-Darwinian [[natural history]]. The puzzle vanished once biologists realised they once were working [[adaptation]]s, in the [[ancestor]]s of present-day animals. They occur in animals (and plants) which have changed their style of life from their ancestors. Thus [[snakes]] lost their legs as their system of movement changed. But one type of snake – the [[boa]]s – have vestigial rear legs and [[pelvis]]. The human [[vermiform appendix]] is another example. That was much larger, and stored [[microbe]]s which produced cellulase to break down plant [[cell wall]]s. [[Leaves]] are the main diet of [[ape]]s, but they are not a main part of man's diet. [[Cellulose]] cannot be digested by our species. An unused organ usually degenerates, and becomes smaller or vanishes altogether. Thus [[amphibia]] living in dark caves lose their sight and their body colour.<ref name=Costa>Charles Darwin. 2009. ''The annotated Origin: a facsimile of the first edition of the ''Origin of Species'', annotated by James T. Costa''. Harvard University Press. p137 {{ISBN|978-0-674-03281-1}}</ref> [[Mutation]]s pile up. All structures need energy for their development, maintenance, and weight. This, and the risk of disease in the part (e.g., infection, cancer), provide some selection for the removal of parts which no longer help an organism's fitness. The human [[Vermiform appendix|appendix]] had selection against it because of deaths from [[appendicitis]]. Appendicitis was untreatable until modern times. The selection pressure was probably weak, because most cases occur after the age of peak [[fertility