A FEATHER. 689 



When the wing is moved downward through the air, the feathers 

 are pressed together ; the air is confined in the concave surface of the 

 wing, and the bird is raised up ; but, when the wing is moved up to 

 repeat the stroke, the air rushes down between the feathers. This is 

 the paddling motion popularly supposed to be flight, but really only 

 a small part of it. The air, being gaseous-, does not remain passive 

 under the descending wing, but tends to slide out, and as the front is 

 unyielding, while the back is flexible, the air finds this exit, bending up 

 the tips of the feathers, and sliding out backward and upward, while the 

 feathers, and with them the bird, slide forward and downward. The 

 bird can rotate its wings as well as flap them, and, by fixing them at 

 such an angle that the fall occasioned by sliding shall just balance 

 the lift given by the downward flap, it is able to move forward with- 

 out rising or falling, although the motion of its wings is up and down ; 

 and, by changing the inclination of the wings a little, it can go up 

 or down at the same time that it moves forward. 



This is only an outline of what is known of the mechanism of 

 flight — and many parts of the process are not yet understood — but we 

 know enough to be able to appreciate the wonderful way in which 

 every part, and every curve and outline, is adapted to its use ; and the 

 attention of thoughtful men has long been attracted by these and the 

 countless similar adaptations in Nature, and many of the greatest 

 thinkers have occupied themselves in attempts to understand the way 

 in which they have been produced. Some have decided that adapta- 

 tion implies design ; and hence these adaptations must be the direct 

 work of a personal designer and creator ; but adaptation alone does 

 not always imply design. I may go into the woods and find a young- 

 tree adapted for a cane, but no one will say that it was designed for 

 a cane ; and I once knew a very unskillful amateur carpenter, who 

 was asked, at the close of a very industrious day's work, what he had 

 made. He answered, " Well, I designed it to be a rustic chair, but I 

 think it will answer nicely as a saw-buck." In this case the adapta- 

 tion was certainly not the fruit of design. 



But, even if design can be shown, it does not follow that the adap- 

 tation is the fruit of direct creative interposition ; and the fact that it 

 is not always perfect — that, perfect as the wing is in most birds, more 

 than one species has become extinct in recent times, on account of the 

 rudimentary and useless state of the wings — has been held by many to 

 be sufficient proof that the adaptation was not produced in this way. 



We shall be able to take a more fair view of this question after 

 we have examined the ultimate nature — the homology, as it is called 

 — of the organs of flight. 



Feathers evidently take the place occupied by hair in mammals ; 



and, in some birds which do not fly — such as the ostrich — they are 



very like hair ; and examination of the microscopic structure and 



mode of growth of a feather shows that it is formed in the same way 



vol. iv. — 44 



