Structures in organisms (when no longer adequate for theiroriginal purpose) often become secondarily adapted for a differentuse. In the course of their evolutionary history, vertebrates haveutilized three different \"urinary\" systems. Today, however, we onlyuse one. What has happened to the other two? Why were they nolonger adequate for mammalian needs? What do you think would happento such vestigial structures if a secondary \"use\" did not arise?Why?
