Late last month, fishermen in Japan netted a surprise -- a bottlenose dolphin with two well-developed rear flippers. The flippers are remnants of the legs which grew in the dolphin's prehistoric ancestors.
Every animal's body is built by the genes contained in its DNA. But not all genes are active -- they have to be turned on by controller genes.
Over time, the DNA in a species changes -- new genes emerge, old ones become inactive. But in many cases, the old ones don't completely go away. The controller changes and stops activating the gene. But if there's a change in the controller, it may activate that forgotten gene again.
(The same thing happens with humans. As a baby develops inside its mother, it grows gills and a tail -- remnants of our animal heritage. These disappear before the baby is born.)