22 LAND MAMMALS IN THE WESTERN HEMISPHERE 



should afford testimony in confirmation of the inferences 

 drawn from a study of the mammals. Of course, the separating 

 sea did not necessarily cross the site of the present isthmus ; 

 it might have cut through some part of Central America, but 

 a glance at the map immediately suggests the isthmus as the 

 place of separation and subsequent connection. As a matter of 

 fact, isthmian geology is in complete accord with the evidence 

 derived from the mammals. Even near the summit of the hills 

 which form the watershed between the Atlantic and the Pacific 

 and through which the great Culebra Cut passes, are beds of 

 marine Tertiary shells, showing that at that time the land was 

 completely submerged. This does not at all preclude the 

 possibility of other transverse seas at the same period ; indeed, 

 much of Central America was probably under the sea also, but 

 the geology of that region is still too imperfectly known to 

 permit positive statements. 



When several different kinds of testimony, each inde- 

 pendent of the other, can be secured and all are found 

 to be in harmony, the strength of the conclusion is thereby 

 greatly increased. Many distinct lines of evidence support 

 the inference that North and South America were com- 

 pletely severed for a great part of the Tertiary period. 

 This is indicated in the clearest manner, not only by the 

 geological structure of the Isthmus and by the mammals, 

 living and extinct, as already described, but also by the fresh- 

 water fishes, the land-shells, the reptiles and many other 

 groups of animals and plants. 



The distribution of marine fossils may render the same sort 

 of service in elucidating the history of the sea as land-mammals 

 do for the continents, demonstrating the opening and closing 

 of connections between land-areas and between oceans. The 

 sea, it is true, is one and undivided, the continental masses 

 being great islands in it, but, nevertheless, the sea is divisible 

 into zoological provinces, just as is the land. Temperature, 

 depth of water, character of the bottom, etc., are factors that 



