76 



STRUCTURE AND COMPARISON. 



lizards. The bat's wing was the simplest possible device ; the 

 bird's wing is a great invention. 



The bird's wing was not made hit-or-miss, but by the 

 nicest adjustments and by the correct solution of many prob- 

 lems. First of all, the use to be made of it, which decides 

 its shape. Then, its size, which depends largely, but not 

 entirely, upon the weight of the bird's body. Then there are 

 the problems of making the wing strong enough to resist the 

 pressure of the air; of making it as light as possible; of 

 giving to the individual feather lightness, stiffness, and a firm 

 attachment to the -bone ; of making the feather impervious to 

 the air on the downward stroke ; of making it shed the air on 

 the upward beat; of providing muscles strong enough to spread 

 these great fans and to keep them moving ; of placing these 



FIG. 15. WING OF BIRD. 



(By courtesy of McClure's Magazine. Copyrighted, 1895, by the S. S. McClure Co.) 



muscles where they will not make the bird top-heavy ; of pro- 

 viding lungs large enough to keep the blood fresh and warm, 

 and of devising some way of breathing that will not interfere 

 with the motions of flying. The invention and construction 

 of a great locomotive are simplicity itself to the skill required 

 to make it possible for a bird to fly with a bird's wings. 



