Progressions has evolved from a Journal exclusively engaged with Peer-led Team Learning (PLTL) to one that is concerned not only with all things PLTL but also increasingly with the exploration of new visions of content in undergraduate science education. Instructional content has lagged far behind current research. Textbooks often copy one another - a fact that Steven J. Gould wrote about in the The Case of the Creeping Fox Terrier Clone where he traced to 1904 the incorrect claim that the prehistoric horses were the size of "fox terriers".