110 PARA. Chap. III. 



that similar results are derivable from the comparison 

 of the birds of these countries. 



The interesting problem, how has the Amazons Delta 

 been formed ? receives light through this comparison of 

 Faunas. Although the portion of Guiana in question is 

 considerably nearer Para than are the middle and 

 southern parts of Brazil, yet it is separated from it by two 

 wide expanses of 'water, which must serve as a barrier to 

 migration in many cases. On the contrary, the land of 

 Brazil proper is quite continuous from Rio Janeiro and 

 Bahia up to Para ; and there are no signs of a barrier 

 ever having existed between these places within recent 

 geological epochs. Some of the species common to Para 

 and Guiana are not found higher up the river where it 

 is narrower, so they could not have passed round in that 

 direction. The question here arises, has the mouth of the 

 Amazons always existed as a barrier to migration since 

 the present species of the contiguous regions came into 

 existence ? It is difficult to decide the question ; but 

 the existing evidence goes far to show that it has not. 

 If the mouth of the great river, which, for a long 

 distance, is 170 miles broad, had been originally a wide 

 gulf, and had become gradually filled up by islands 

 formed of sediment brought down by the stream, we 

 vshould have to decide that an effectual barrier had in- 

 deed existed. But the delta of the Amazons is not an 

 alluvial formation like those of the Mississippi and the 

 Nile. The islands in its midst and the margins of both 

 shores have a foundation of rocks, which lie either bare 

 or very near the surface of the soil. This is especially 

 the case towards the sea-coast. In ascending the 



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