but there is one other almost as remarkable 

 that opens up a question that is even harder 

 to answer. One day in the early spring I 

 saw two eider-ducks swimming about the 

 Hummock Pond on the island of Nantucket. 

 The keen-eyed critic will interfere here and 

 say I was mistaken ; for eiders are salt-water 

 ducks that haunt only the open sea and are 

 supposed never to enter fresh water, not 

 even to breed. That is what I also sup- 

 posed until I saw these two; so I sat down 

 to watch a while and find out, if possible, 

 what had caused them to change their 

 habits. At this time of year the birds are 

 almost invariably found in pairs, and some- 

 times a flock a hundred yards long will pass 

 you, flying close to the water and sweeping 

 around the point where you are watching, 

 first a pretty brown female and then a gor- 

 geous black-and-white drake just behind her, 

 alternating with perfect regularity, female and 

 male, throughout the whole length of the 

 long line. The two birds before me, however, 



were both females ; and ^ =ri 



that was another reason - 



