CONTRIVANCE A NECESSITY. 155 



enough to sustain the weight of the bird in flight. 

 And here again we have a wonderful example of 

 the skill with which inexorable mechanical laws 

 are subordinated to special purpose. It is a neces- 

 sary consequence of the area of the wing being so 

 reduced, in proportion to the size of the bird, that 

 great muscular power must be used in working it, 

 otherwise the Force of Gravity could not be over- 

 come at all. It is a farther consequence of this 

 proportion of weight to working power, that there 

 must be great momentum and therefore great velo- 

 city of flight. Accordingly this is the fact with all 

 the oceanic diving Birds. They have vast distances 

 to go, following shoals of fish, and moving from 

 their summer to their winter haunts. They all fly 

 with immense velocity, and the wing-strokes are 

 extremely rapid. But there is one quality which 

 their flight does not possess — because it is incom- 

 patible with their structure, and because it is not 

 required by their habits — they have no facility 

 in evolutions, no delicate power of steering ; 

 they cannot stop with ease, nor can they resume 

 their onward motion in a moment. They do not 

 want it : the trackless fields of ocean over which 



