352 



PRINCIPLES OF ANIMAL BIOLOGY 



garoo) are known to have existed since Mesozoic time. There are reasons 

 for supposing that, at that time, AustraHa was connected with the Ameri- 



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can continent by a land bridge across the antarctic region, or indirectly 

 through the other continents (Fig. 243), or by both of these means. As 

 a result, marsupials are found in both AustraHa and the Americas. But 



