Vestiges

In Chapter 3, Remnants: Vestiges, Embryos, and Bad Design, Mr. Coyne talks about the vestigial organs of organisms, and how these vestiges never really disappear. He also takes note that we have not yet been able to discover the reason for vestigial organs, and why they aren't simply bred out of a population. On page 62, in the second paragraph, he gives several possible reasons for vestiges, but what do you think? What are other reasons certain organisms never lose particular traits?