FOSSIL PEDIGREES 113 



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. Who- 

 soever 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, com- 

 pensating 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 dilata- 

 tion of these materials would have to be available. Arthur 

 B. Coleman called attention to this difficulty in his Presidential 



