ANCIENT LAND BRIDGES 357 



between the western mam'malian fauna of North America and 

 those of Europe on the one hand and South America on the 

 other. Since south-western North America was then prac- 

 tically isolated and separated from the remainder of North 

 America by great ocean belts, how can we imagine these Euro- 

 pean and South American affinities to have been brought 

 about ? Surely only by some land connection that lay to the 

 south. I suggest that it was from western Mexico that these 

 earliest mammals invaded south-western North America. 

 Then followed a time when the Gulf of Mexico and the Pacific 

 Ocean probably communicated with one another, thus separat- 

 ing the supposed mid-Atlantic land bridge from North 

 America. Professor Smith speaks only of a temporary con- 

 nection between the oceans, accounted for by the occurrence 

 in Oregon, as well as in California, of the Atlantic marine 

 mollusk Venericardia planicosta. Before the Miocene Period 

 this Atlantic connection had ceased, and the faunas of the 

 later Tertiary were wholly of the Pacific type, continues Pro- 

 fessor Smith.* He does not allude to Oligocene deposits, but 

 it is not long since that these were recognised at all outside 

 Europe. At any rate, after the Eocene follows a time during 

 which the Pacific recedes from the west coast, thus giving 

 full opportunities for an invasion of animals from the theo- 

 retical western land. We may suppose that this corresponded 

 with the Oligocene Period and with the time when, as Pro- 

 fessor Osborn tells us, there was a re-establishment of the 

 faunal resemblance of south-western North America with 

 Europe. Possibly Chile, which was connected at an earlier 

 period with this same western belt of land, became separated 

 from it. This again was succeeded by a period of marine 

 transgression in the west. Even northern Mexico was largely 

 covered by the sea, as well as both sides of Lower California 

 and a large portion of western California. All this time 

 western South America must have risen gradually above the 

 sea, and I presume that certain fragments of land, like 

 Peru, became joined to the long peninsula which stretched 

 far southward running parallel with the newly formed west 

 coast of South America. Thus while North America no longer 



* Smith, J. Pen-in, " Geological History of California," p. 348. 



