So far as North America is concerned, the best available evidence points to the conclusion that we should regard the Fort Union, Puerco and Torrejon as the most ancient of the Cenozoic formations (see Table, p. 17), though retaining so many features of Mesozoic life that a separate division of the Tertiary, the Paleocene epoch, is made for them. Such a separation is not the common practice in this country, where it is more usual to employ the terms “Lowest” or “Basal” Eocene. In my judgment, however, the balance of advantage is in favour of giving to this so-called Basal Eocene a rank equivalent to that of the four other universally recognized and admitted epochs of the Tertiary period. No marine rocks of Paleocene date have yet been found in North America, which indicates that the continent was at least as extensive as it is now. The very scanty development of deposits representing this epoch in Europe renders the comparison with the fossils of the Old World unsatisfactory and hence leads to uncertainty, when it is attempted to determine the land-connections of the time. During the Mesozoic era the shallow Bering Sea had repeatedly been elevated into a land joining North America with Asia and had as often been depressed, so as to separate the continents and allow the waters of the Arctic Ocean to mingle with those of the Pacific. A like alternation of junction and separation went on during the Tertiary and Quaternary periods and, by a comparison of the fossil mammals of Europe and America for any particular division of geological time, it is almost always feasible to say whether the two continents were connected, or altogether separated. This statement does not imply that the proportion of common elements in the two faunas during epochs of continental connection was a constant one at all times, for that was by no means true. Mere land-connections or separations are not the only factors which limit the spread of terrestrial animals; if they were, the community of forms between North and South America would be much greater than it actually is. Climatic barriers are of almost equal importance in determining animal distribution, and changes of climate may greatly alter the conditions of migration between connected continents. As the connections between North America and the Old World were probably in high latitudes, where the seas are narrow, changes of climate produced a greater effect upon migration than they could have done had the land-bridges been in the tropical or warm temperate zones. That these vicissitudes of climate really did occur and are not mere guesses to bolster up a tottering hypothesis, there is abundant evidence to prove.