GEOGRAPHIC DISTRIBUTION OF ANIMALS: CONTINENTS 265 



but the examples cited will suffice to demonstrate the far-reaching similari- 

 ties characterizing the faunas of these northern continents. 



In the preceding paragraphs we have presented evidence that the three 

 continents in the Southern Hemisphere differ markedly in their animal 

 inhabitants, whereas the two great land masses in the Northern Hemi- 

 sphere are closely similar in this respect. Can we now find some reasonable 

 explanation for the contrasted situations in the two hemispheres? 



Accessibility 



As we examine a map, preferably a globe, we are struck by the fact that 

 the three southern continents are widely separated from one another by 

 expanses of ocean, whereas the northern land masses are nearly in contact, 

 being separated from each other by only the 56 miles of Bering Strait. Per- 

 haps, then, accessibility forms the clue we are seeking. This seems all the 

 more likely when we realize that Bering Strait is shallow as well as nar- 

 row, and that there is every reason to believe that in various past geologic 

 periods the region lay above sea level to create a continuous bridge of land 

 between North America and Asia. A similar bridge may have existed be- 

 tween Europe and North America, perhaps by way of Iceland and Green- 

 land, but the evidence for it is less conclusive than is the evidence for the 

 Alaska-Siberia connection. The latter must have made possible extensive 

 migrations between Eurasia and North America, which would explain in 

 large measure the great similarities of animal life observed to occur in 

 these two great land masses. 



Turning to the Southern Hemisphere, we note that inaccessibility charac- 

 terizes the three southern continents. Widely separated from one another, 

 such connections as they have are with northern continents. 



Of the three, Australia is the most completely isolated. Perhaps it was 

 at one time connected to the continent of Asia by a land bridge of which 

 the East Indian islands to the northwest represent the unsubmerged rem- 

 nants. But the connection, if it ever existed, must have been extremely 

 ancient. While only shallow ocean separates many of these islands (e.g., 

 Borneo, Sumatra, Java) from Asia, there are deeper stretches between 

 Australia and the islands mentioned. Evidence seems to indicate that Aus- 

 tralia has been isolated from Asia since at least Cretaceous times (p. 137). 



Did the ancestors of the monotremes and marsupials reach Australia by 

 land before the present isolation of the continent occurred? Such an ex- 

 planation is possible, though, as Simpson (1943, 1953) has pointed out, it 

 may be more probable that these ancestors reached Australia by being 



