Certain, specific, anatomical parts, have developed over time, with the process of evolution, from a usable, functional structure, to regress to an idle, ineffective attribute of the body. However, it is worth noting, that vestigial structures do not necessarily need to have lost all necessary functions, but may retain some. Though, a vestigial structure will never switch functions, which will be an example of exaptation. It is crucial to comprehend, the difference between the two, as they are commonly confused, due to the subjective nature of determining the purpose of a structural function. Such structures, arise from selective evolutionary pressure, when structures lose their value with changing environments, or especially when harmful towards the organism. A notable example of this process, often occurs with animals losing the ability to fly, when inhabiting islands. There are a plethora of examples throughout the animal kingdom, such as, whale’s hind legs, ostrich’s wings, caveman fish eyes, a humans appendix, and so on. Depicted above, is the vestigial structure, plica semilunaris, a remnant of the third eyelid, the nicitating membrane.