went from the small four-toed forest-dwelling Orohippus from the Eocene through three-toed species such as Miohippus to species more like the modern horse. By looking at their teeth he could see that, as the size grew larger and the toes reduced, the teeth changed from those of a browser to those of a grazer. All such changes could be explained by a general alteration in habitat from forest to grassland. And, it is now known, that is what did happen over large areas of North America from the Eocene to the Pleistocene: The ultimate causative agent was global temperature reduction (see Paleocene–Eocene Thermal Maximum). The modern account of the evolution of the horse has many other members, and the overall appearance of the tree of descent is more like a bush than a straight line. The horse series also strongly suggested that the process was gradual, and that the origin of the modern horse lay in North America, not in Eurasia. If so, then something must have happened to horses in North America, since none were there when Europeans arrived. The experience with Marsh was enough for Huxley to give credence to Darwin's gradualism, and to introduce the story of the horse into his lecture series. Marsh's and Huxley's conclusions were initially quite different. However, Marsh carefully showed Huxley his complete sequence of fossils. As Marsh put it, Huxley "then informed me that all this was new to him and that my facts demonstrated the evolution of the horse beyond question, and for the first time indicated the direct line of descent of an existing animal. With the generosity of true greatness, he gave up his own opinions in the face of new truth, and took my conclusions as the basis of his famous New York lecture on the horse." Support of Darwin Huxley was originally not persuaded of "development theory", as evolution was once called. This can be seen in his savage review of Robert Chambers' Vestiges of the Natural History of Creation, a book which contained some quite pertinent arguments in favour of evolution. Huxley had also rejected Lamarck's theory of transmutation, on the basis that there was insufficient evidence to support it. All this scepticism was brought together in a lecture to the Royal Institution, which made Darwin anxious enough to set about an effort to change young Huxley's mind. It was the kind of thing Darwin