I was reading a book about biology recently and read this passage:

“The principle “form follows function” is found in many contexts. For example, birds and fish have streamlined bodies that allow them to move quickly through the medium in which they live, be it air or water. It means that, in general, one can deduce the function of a structure by looking at its form, because the two are matched.”

It made me think of something I read a while back about evolution and how originally when evolution was first being talked about, there was this idea that the embyronic form would later match its later full-formed stage. It was rejected. Just trying to find out what this theory was called.