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. 308—309. 
t Osborn, II. F., “ Cenozoie Mammal Horizons,” p. 33. 
Q 2 
