38 



This will appear more evident when it is con- 

 sidered that the same causes which operate to 

 drive them away from the northern and temperate 

 nations of Europe and Asia, namely, rigorous 

 weather, and scarcity of food in winter, do not 

 exist in Senegal and other tropical countries, 

 where the weather is constantly warm, and the 

 air always abounds with winged insects. 



It has often been said, in objection to the 

 migration of swallows, that considering the num- 

 ber of these birds which annually inhabit Europe, 

 if all were to cross the ocean twice a year, they 

 would oftener be seen by mariners on their pas- 

 sage, than they appear to be*. But it seems 

 to me very unlikely that they should very fre- 

 quently be seen; because, from the extraordinary 

 length of their wings, it is probable they perform 

 their aerial journies at too great a height to be 

 discerned; and most likely those which have 



* It does not appear, upon inquiry, that these birds 

 are so seldom seen at sea, as, from the scarcity of pub- 

 lished accounts, one might be inclined to imagine. I have 

 often heard seamen say, that they have seen swallows many 

 hundred miles from land, during their voyages. 



