CONTRIVANCE A NECESSITY". 153 



own size of wing : that size of wing must be large 

 enough to act upon a volume of air sufficient to 

 lift the animal's whole weight : lastly, and conse- 

 quently, the weight must not be too great, or dis- 

 persed over too large a bulk. But within these 

 limits there is room for great varieties of adjust- 

 ment, having reference to corresponding varieties 

 of purpose. To some birds the air is almost their 

 perpetual home — the only region in which they 

 find their food — a region which they never leave, 

 whether in storm or sunshine, except during the 

 hours of darkness and the yearly days which are 

 devoted to their nests. Other birds are mainly 

 terrestrial, and never betake themselves to flight 

 except to escape an enemy, or to follow the sea- 

 sons and the sun. Between these extremes there 

 is every possible variety of habit. And all these 

 have corresponding varieties of structure. The 

 birds which seek their food in the air have long 

 and powerful wings, and so nice an adjustment of 

 their weight to that power and to that length, that 

 the faculty of self-command in them is perfect, 

 and their power of direction so accurate that they 

 can pick up a flying gnat whilst passing through 



