Charles Darwin famously concluded On the Origin of Species with a vision of "endless forms most beautiful" continually evolving. More than 150 years later many evolutionary biologists see not endless forms but the same, or very similar, forms evolving repeatedly in many independent species lineages.[...]

Theoretical morphology--which seeks to sketch the range of forms that biological entities could take, with the ultimate goal of discovering why certain forms exist but others do not--is among paleontology's most significant contributions to the understanding of evolution. Today, with the aid of comp[...]