LIFE ON THE EARTH. 37 



work into the substance of coral, and Teredines are 

 found to have rasped their way through the masses 

 of drift wood in the Portland oolite, the Woburn 

 sand, the Chalk and London clay. 



Movements in air are accomplished by animals 

 of the articulated and vertebrated types with a con- 

 siderable variety of organization, curiously engrafted 

 on the original structure by modification of some of 

 its parts. If we consider the mechanism for flight in 

 its completeness, the general idea appears to be the 

 employment of two anterior limbs for rowing through 

 the air by an expanded elastic wing, and employing 

 some retral instrument for steerage. Thus the ge- 

 neral arrangement is in a considerable degree the 

 inverse of that employed for swimming. The wing 

 or air-fin is in like manner distinct in principle of 

 construction from the paddle or water-fin. Its an- 

 terior edge is always strongly fortified by bones in 

 Birds, by strong tracheal tubes in Insectsthe sur- 

 face is made flexible by feathers in Birds, by clear or 

 scaly membranes in Insects. Thus a yielding blade 

 is presented to the air, and this yielding is so ma- 

 naged in Birds as to be nothing anteriorly but great 

 retrally, very little on the stroke, very great on the 

 return. This is managed (1) by the tile-like arrange- 

 ment of the feathers as a whole ; for thus they are 

 strongly compacted in one direction, and feeble in 



