The European horse, with his so-called ancestry interred, partly in the Tertiary deposits of Europe, but mostly in those of North America, is a typical instance of these anomalies in geographical distribution. It would, of course, be preposterous to suppose that two independent lines of descent could have fortuitously terminated in the production of one and the same type, namely, the genus _Equus_. Moreover, to admit for a moment that the extinct American _Equus_ and the extant European _Equus_ had converged by similar stages from distinct origins would be equivalent, as we have seen, to a surrender of the basic postulate that structural similarity rests on the principle of inheritance. Nothing remains, therefore, but to hypothecate a Tertiary land bridge between Europe and North America. Modern geologists, however, are beginning to resent these arbitrary interferences with their science in the interest of biological theories. Land bridges, they rightly insist, should be demonstrated by means of positive geological evidence and not by the mere exigencies of a hypothetical genealogy. Whosoever postulates a land bridge between continents should be able to adduce solid reasons, and to assign a mechanism capable of accomplishing the five-mile uplift necessary to bring a deep-sea bottom to the surface of the hydrosphere. Such an idea is extravagant and not to be easily entertained in our day, when geologists are beginning to understand the principle of _isostasy_. To-day, the crust of the earth, that is, the entire surface of the lithosphere, is conceived as being constituted of earth columns, all of which rest with equal weight upon the level of complete compensation, which exists at a depth of some 76 miles below land surfaces. At this depth viscous flows and undertows of the earth take place, compensating all differences of gravitational stress. Hence the materials constituting a mountain column are thought to be less dense than those constituting the surrounding lowland columns, and for this reason the mountains are buoyed up above the surrounding landscape. The columns under ocean bottoms, on the contrary, are thought to consist of heavy materials like basalt, which tend to depress the column. To raise a sea floor, therefore, some means of producing a dilatation of these materials would have to be available. Arthur B. Coleman called attention to this difficulty in his Presidential Address to the Geological Society of America (December 29, 1915), and we cannot do better than quote his own statement of the matter here: