ii3G GENERAL ANALOGIES. 



the upper level of sea-food, unless in that portion of 

 the food of those birds which is cast up by the waves. 

 But still there is a correspondence : they keep more 

 to their own element, and have their motion more 

 fitted for that, and less for the other two. Those 

 sea-birds which seem never to tire on the wing, are 

 all imperfect walkers ; and though they float very 

 buoyantly, much more so indeed than those which 

 are habitually on the water, they are by no means 

 so expert at swimming. 



The most perfect flight and the most expert action 

 in the water are indeed incompatible with each other. 

 Birds which float about the livelong day require to 

 be light for their bulk and extent of feathers, though 

 a heavier bird succeeds better on a momentary rush ; 

 aud the air birds which fly over the sea in 'search of 

 their food do not require the same rapid motion as 

 birds which fly in the air in pursuit of their food. But 

 a bird which is to have the most perfect command of 

 itself in the water, and be able to dive, and come up, 

 and drive along, sometimes wholly immersed, and 

 sometimes not, must, in order to perform its various 

 evolutions with as little muscular exertion as possible, 

 be of nearly the same specific gravity as sea-water. 



It is absurd to say, as has sometimes been said, 

 that if birds have not a certain specific gravity they 

 cannot dive. The lightest substance, the lightest 

 gas, can be forced under water by mechanical means, 

 and it is rather too much for us to suppose that we 

 can beat nature with our small second-hand mechanics. 

 But still it accords with goneral principles that that 

 which is the most nearly of the same specific gravity 

 as water, should admit of motion in all directions in 

 water with the least effort. This principle is trace- 

 able in aquatic birds ; for we find that the diving 



