In 1848, a study "On the fossil horses of America" by Joseph Leidy systematically examined Pleistocene horse fossils from various collections, including that of the Academy of Natural Sciences, and concluded at least two ancient horse species had existed in North America: "Equus curvidens" and another, which he named "Equus americanus". A decade later, however, he found the latter name had already been taken and renamed it "Equus complicatus". In the same year, he visited Europe and was introduced by Owen to Darwin.