80 THE RIDDLE OF MIGRATION 



When one attempts to invoke the magnetic field 

 one is immediately faced with serious difficulties. 

 The idea has many times proved attractive to in- 

 vestigators and experiments have been designed to 

 examine the concept but they have led to nothing. 

 Known facts of the anatomy and physiology of a 

 bird suggest no means by which terrestrial magnet- 

 ism could be perceived. The inner ear, with its 

 semi-circular canals might logically be surmised to 

 be the seat of such perception (if it exists) but evi- 

 dence is entirely wanting. 



An attractive theory has been propounded at- 

 tempting to show, from the physicist's viewpoint, 

 that since the magnetic dip and declination vary 

 from place to place and offer unlimited variety of 

 combination, a bird could thus detect and recognise 

 different areas of the earth. This assumes that the 

 relationship between dip and declination is not 

 exactly similar at any two points of the globe but 

 even if this were not open to question, as it appears 

 to be, there still remains no known mode of percep- 

 tion by which a bird might become aware of the 

 situation. 



Magnetic sensibility might be an integral com- 

 ponent of the homing sense as well as of true mi- 

 gration but the two things are essentially distinct. 

 Pigeons have been used many times for the investi- 



