THE IMPENETRABLE SEA 



or change their abodes from the sea to inland lakes or 

 vice versa. 



The term "diver" is often applied, vaguely, to birds 

 which have no right to the name, such as several of the 

 sea ducks, some of the mergansers, and certain auks and 

 grebes; but British ornithologists agree that the word 

 should be restricted to the Colymbidae, a clearly defined 

 group possessing considerable powers of submergence. 



In common with the grebes — but thereby differing 

 from other birds — the divers possess curious anatomical 

 structures which help them to swim. The wings are small, 

 concave and composed of stiff feathers, so that they can 

 use them as oars when underwater and giving chase to 

 submerged fishes, or escaping from their underwater 

 enemies. 



In the diver, the crest of the tibia is prolonged up- 

 wards to unite with the kneecap (patella) , so that a spike- 

 like projection is formed at the extremity of the bone, 

 which gives the bird a considerable advantage in the act 

 of swimming by reason of its efficient leverage. The limbs 

 are placed as far back as possible, and the tarsus is 

 flattened laterally to cleave the water. The toes, which 

 are either lobated or webbed, are so formed that they 

 close into a small compass when drawn towards the body 

 in preparation for a stroke. The plumage is close, silky 

 and very glossy. The tail is either short or wanting alto- 

 gether. The body is flat, oval and ''stream lined", and 

 from its rather depressed contour appears to float more 

 deeply in the water than it actually does. 



The great northern diver {Colymbus glacialis) is the 

 largest species of the genus, and may attain a length of as 

 much as three feet. It is met chiefly in the Arctic regions, 

 but comes farther south with the approach of winter. It is 

 a beautiful bird, characterized by its glossy black head 

 and neck and the presence of two gorgets (or semi- 

 collars) of velvet-black and pure white vertical stripes on 

 its throat, and belts of white spots contrasting sharply 



158 



