ON ATLANTIC LAND BRIDGES 227 



complete and very accurate geological time scale for the entire 

 Tertiary Era, and to speak with precision regarding the time 

 of successive migrations. Much still remains to be done to 

 establish the approximate ages of the various Tertiary de- 

 posits in Europe and America before we can definitely say 

 whether a certain group of mammals made its first appearance 

 in Europe or in America. Possibly we may not reach such 

 a state of knowledge for a long time to come. 



In New Mexico what are known as the " Puerco " and the 

 " Torrejon " formations have yielded remains of archaic 

 mammals, some of which are related to mammals occurring 

 in late Mesozoic or early Tertiary deposits of Patagonia, others 

 indicating affinity with those of a similar age in France. 

 Now, as Professor Deperet tells us, it is inadmissible to argue 

 that mammals so nearly akin to each other could have arisen 

 independently in three distinct centres, in Patagonia, in 

 south-western North America and in France. We can only 

 explain these palaeontological affinities by migrations from 

 one area to the other. Professor Deperet * believes that the 

 cradle of these early placental mammals was in North 

 America, and that they crossed over to Europe by utilising 

 the " territories of the North Atlantic which had risen from 

 the sea." This is a somewhat vague statement, yet it indi- 

 cates clearly that the great French palaeontologist had in his 

 mind the existence, in these remote geological times, of some 

 kind of north Atlantic land bridge. He does not explain how 

 the North American mammals reached Patagonia, but Pro- 

 fessor Osborn f speaks distinctly of a, contemporary, that is 

 to say, early Eocene or previous (Cretaceous) land connection 

 between North America and South America, and he actually 

 places it in the same position as it occupies at present (see 

 Fig. 21). I propose to discuss it later on. At present we may 

 confine ourselves strictly to the European affinities of North 

 America. 



A momentous change occurs, according to Professor 

 Osborn, in the succeeding Wasatch formation of New Mexico 

 and Wyoming. The parallelism of similar stages in the 

 archaic mammals of western Europe and south-western 



* Deperet, C., "Transformations of the Animal World," pp. 308309. 

 t Osborn, H. F., " Cenozoio Mammal Horizons," p. 33. 



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