The original sequence of species believed to have evolved into the horse was based on fossils discovered in North America in 1879 by paleontologist Othniel Charles Marsh. The sequence, from "Eohippus" to the modern horse ("Equus"), was popularized by Thomas Huxley and became one of the most widely known examples of a clear evolutionary progression. The horse's evolutionary lineage became a common feature of biology textbooks, and the sequence of transitional fossils was assembled by the American Museum of Natural History into an exhibit that emphasized the gradual, "straight-line" evolution of the horse.