13 



America and Europe are characterized by a large predominance of forms of 

 the corresponding southern continent. It is, indeed, evident that migration 

 from the one continent to the other has taken place, and is amply sufficient 

 to account for the abrupt changes in the life of each, without necessitating 

 the intervention of creative acts. If glacial periods be dependent on cosmic 

 movements, the increased obliquity of the earth's axis to the sun, at periods 

 25,000 years apart, due to the same causes as precession, would cause a cor- 

 responding alternation of cold periods in the opposite hemispheres. This is 

 well known as a most potent cause of migration and extinction, and the 

 known relations of the faunae would thus result from a greater or less alter- 

 nate invasion of the one hemisphere by the life of the other. 



But within the great time-boundaries are distinct land-faunse whose 

 striking distinction may not thus be accounted for. Thus, the Miocene and 

 Loup Fork faunae of Western America are. entirely distinct, but with corre- 

 sponding members. The alternate presence and absence of water-areas 

 adapted for the preservation of the remains of the animals will abundantly 

 account for such minor interruptions. Such changing topography is well 

 known as due to the slow vertical oscillations of the earth's crust. 



The original question, the exactitude of the chronological significance of 

 structural types, has been momentarily held in abeyance. Is paleontology a 

 science so far exact as to furnish a chronological scale of terrestrial strata? 

 The admission that the known Tertiary faunas, for instance, are but fragments 

 of a continuous succession, would appear to invalidate any such claim. It 

 would indicate that the restriction of a given type to a given horizon is only 

 a matter of discovery, and that another accident may at any time give it a new 

 range. This objection has but little weight. Fragments though they be, 

 nearly-related formations, as the Tertiaries, are obviously the visible portions 

 of a serial succession of life. Like the bright lines in a spectrum, the order 

 is not disturbed by the temporary obliteration of a part of the colors, but the 

 visible portions indicate the relations of the component parts with infallible 

 certainty. The more universal the physical interruption, the more far-reach- 

 ing the break in the succession of life in any one locality, the greater 

 is the significance of remains of animals as indication of relation in time. The 

 change of faunae in Arctogaea at the close of the Cretaceous is a case in point. 

 A dinosaur, sauropterygian, ammonite, or rudist are as definite indicators of 



