554 Critical Periods in the History of the Earth. [September, 
regions of the shallow sea between the Aleutian Isles and Behr- 
ing Strait. Through this gate-way came an invasion from Asia, 
including probably the mammoth. With this invasion probably 
came also man. It seems probable, therefore, that the earliest 
remains of man in America will be found on the Pacific coast. 
Also the great Pliocene lake, which stretched from near the 
shores of the Gulf far into British America, and possibly into 
arctic regions, and formed a more or Jess complete barrier to the 
mammalian fauna east and west, was abolished by upheaval, and 
free communication was established. It is impossible that all 
these changes of climate and all these migrations, partly enforced 
by changes of climate and partly permitted by removal of bar- 
riers, and in this latter case especially attended with the fiercest 
struggle for life, should not produce rapid and profound changes 
in the mammalian fauna. 
In Europe the process has-been more accurately studied and is 
better known. In Quaternary times at least four different mam- 
malian faune struggled for mastery on European soil. (1.) The 
Pliocene autochthones. (2.) Invasions from Africa by opening 
of gate-ways through the Mediterranean: one by way of Italy, 
Sicily, and Malta, and one by Gibraltar, both of which have been 
again closed. (3.) Invasions from Asia, by removal of a great 
sea barrier connecting the Black and Caspian seas with the Arctic 
Ocean. This gate-way has remained open ever since. (4.) In- 
vasions from arctic regions, enforced by changes of climate. 
Probably more than one such invasion took place; certainly, one 
occurred during the second Glacial epoch. The final result of all 
these climatic changes and these struggles-for mastery was that 
the Pliocene autocthones, adapted to a more genial climate, 
were mostly destroyed or else driven southward with some change 
into Africa: the African invaders were driven back also into Af 
rica, and with some Pliocene autocthones isolated there by sub- 
sidence in the Mediterranean region closing the southern gate- 
ways, and still exist there under slightly modified forms; the 
Arctic invaders were again driven northward by return of more 
genial climate, and there exist to this day; while the Asiaties re- 
mained masters of the field, though greatly modified by the con- 
flict. Or perhaps, more accurately, we might say that the existing 
European mammalian fauna is a resultant of all these factors 
but the controlling factor is the Asiatic. With the Asiatic ray 
sion came man, and was a prime agent in determining the fina 
result, 
