34 THE RIDDLE OF MIGRATION 



mental appreciation of sound distinctions possible 

 in a mammal. Experiments readily demonstrate 

 this general conclusion to be correct for despite 

 obviously acute hearing, birds are remarkably indif- 

 ferent to the character of the sounds produced. 



There is one interesting feature about the vesti- 

 bule and canals of birds which will be further dis- 

 cussed below. This section of the ear shows struc- 

 tural differences in various groups, being most 

 highly developed in birds that may be termed 

 "good" flyers, e.g. swifts and swallows. 



It is hardly necessary to dwell long on the struc- 

 ture of the eye, an organ that has reached remark- 

 able perfection in birds. In general principle it 

 somewhat resembles a camera possessing a dia- 

 phragm {iris) which controls the amount of light 

 admitted, a lens for focussing (in which the cornea 

 is also partially concerned) and at the back of the 

 chamber a sensitive nervous layer {retina) for the 

 reception of the image. It is obvious that the lens 

 cannot be wracked back and forth as in a camera 

 and an alternative principle is therefore adopted. 

 The convexity of the lens is altered as required by 

 appropriate muscles for close or distant focus. In 

 the higher mammals the retinal layer is made up of 

 two kinds of cells, the rods and the cones. They 

 differ in structure and in their connections with the 



