those from the older rocks, and the three-toed forms from more recent layers. The value of this kind of evidence might have been open to greater doubt had the series been made up of forms found scattered over the whole world, for it is well known how difficult it is to compare in point of time the rocks of different continents. But in certain parts of the world, especially in North America, series of fossil horses have been found in sedimentary deposits that appear to be perfectly continuous. This series, by itself, and without regard to the point as to whether in other parts of the world other series may exist, shows exactly those results which the theory of descent postulates, and we find here, in all probability, a direct line of descent. While it may be freely admitted that no such series can demonstrate the theory of descent with absolute certainty, yet it would be folly to disregard evidence as clear as this. In regard to the other point raised by Fleischmann concerning the large number of species of fossil horses that have existed in past times, it is obvious that while this greatly increases the difficulty of the paleontologist it is not an objection to the descent theory. In fact, our experience with living species would lead us to expect that many types have been represented at each geological period by a number of related species that may have inhabited the same country. On the descent theory, one species only in each geological period could have been in the line of descent of the present species of horse. The difficulty of determining which species (if there were several living in a given epoch) is the ancestor of the horse is increased, but this is not in itself an objection to the theory.