CONTRIVANCE A NECESSITY. 177 



downwards, which would infallibly carry the body 

 onwards, is delivered at such an angle forwards 

 as to bring to an exact balance the upward, the 

 downward, and the forward forces which bear 

 upon the body of the bird. Mr Darwin says, 

 " when hovering by a flower, the tail is constantly 

 shut and expanded like a fan, the body being kept 

 in a nearly vertical position!' Mr Wallace, another 

 accurate observer, describes the Humming Birds 

 as "balancing themselves vertically in the air." 



These are a few, and a few only, of the adjust- 

 ments required in order to the giving of the power 

 of flight ; — adjustments of organic growth to in- 

 tensity of vital force — of external structure to ex- 

 ternal work — of shape in each separate feather to 

 definite shape in the series as a whole — of material 

 to resistance — of mass and form to required velo- 

 cities ; adjustments, in short, of law to law, of force 

 to force, and of all to Purpose. So many are these 

 contrivances, so various, so fine, so intricate, that 

 a volume might be written without exhausting the 

 beauty of the method in which this one mechanical 

 problem has been solved. It is by knowledge of 



unchanging laws that these victories over them 



M 



