Since then, as the number of equid fossils has increased, the actual evolutionary progression from "Eohippus" to "Equus" has been discovered to be much more complex and multibranched than was initially supposed. The straight, direct progression from the former to the latter has been replaced by a more elaborate model with numerous branches in different directions, of which the modern horse is only one of many. George Gaylord Simpson in 1951 first recognized that the modern horse was not the "goal" of the entire lineage of equids, but is simply the only genus of the many horse lineages to survive.