ANCIENT LAND BRIDGES 
857 
between the western mammalian 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. Perrin, “ Geological History of California,” p. 348. 
