Recapitulation theory
For example, Haeckel realized that before they are born, there is a time when humans have slits in their necks. These slits look like the gills on adult fishes. Haeckel took this to mean that some time before our species became human, we were fish-like. This view is supported by the fossil record and other evidence. As the unborn human gets closer to being born, it develops a tail. In fact, some human babies are even born with tails. Haeckel concluded, again correctly, that at some time in humanities more recent evolutionary past (after we had gone through our fish stage) the species that would go on to become humans had a tail.