292 NATATORES. 



other birds are moving towards the south ; at all events they 

 quit the southern parts of the British shores in the latter 

 part of the season, and a few are found about the northern 

 islands all the winter over. No doubt their winter dress is 

 much better able to bear the cold, at least the part on the 

 upper surface of their bodies, than the summer dress j and 

 there is little difference in the temperature of the sea at the 

 two ends of the island. 



The fertility of the sea in food suited to diving birds, may 

 be the cause of their partiality for the north ; and the want 

 of suitable places for their multitudes to nestle and breed, 

 may be the cause why they disperse themselves southward in 

 the summer. 



It appears that the northern part of the Atlantic is better 

 supplied with food for diving birds than those portions of it 

 that are farther to the south. Much as they differ in form 

 and size, the food of the common black whale and the guille- 

 mot more nearly resemble each other than one who has not 

 reflected upon the subject would be apt to suppose. They 

 both subsist upon small animals that are found at no great 

 distance from the surface, and, as we have already said, the 

 accumulation of those animals is near the polar ice. But the 

 guillemots breed on the cliffs of rocks, and there is small 

 scope for their nesting in those arctic latitudes, unless they 

 were to range so far as to be at too great a distance from 

 their food. There are also small fishes upon our shores in 

 the summer, which take to the deep water in the winter ; so 

 that these birds find both nesting grounds and food with us 

 at that season. Further, the current in which the great float 

 of animal matter is contained, must, in consequence of the 

 freezing of the northern part of the sea, shift as far to the 

 southward in winter as the sea freezes, and thus, while the 

 diving birds, who do not at that time want to come to the 

 shores, move northward to more abundant food, that food is 



