seems due to this faculty. Ability to travel with precision over un- 

 marked trails is not limited either to birds or to man. It is likewise 

 possessed by many other mammals as well as by some insects and fishes, 

 the well-known migrations of the salmon and the eel being notable 

 examples. 



Ability to follow a more or less definite course to a definite goal is 

 evidently part of an inherited faculty. Both the path and the goal 

 must have been determined either when the habit originated or in the 

 course of its subsequent evolution. The theory is sometimes advanced 

 that the older and more experienced birds lead the way, showing the 

 route to their younger companions. This explanation may be accept- 

 able for some species, but not for those in which adults and the young 

 migrate at different times. The young cowbird that is reared by 

 foster parents flocks with others of its kind when grown and in many 

 cases can hardly be said to have adult guidance in migration. An in- 

 herited migratory instinct with a definite sense of the goal to be reached 

 and the route to be followed must be attributed to these birds. 



It is well known that birds possess wonderful vision. If they also 

 have retentive memories subsequent trips over the route may well be 

 steered in part by recognizable landmarks. The arguments against 

 the theory of vision and memory are chiefly that much migration takes 

 place by night and that great stretches of the open sea are crossed with- 

 out hesitation. Nevertheless, the nights are rarely so dark that all 

 terrestrial objects are totally obscured, and such features as coastlines 

 and rivers are just those that are most likely to be seen in the faintest 

 light, particularly by the acute vision of the bird and from its aerial 

 points of observation. But some birds fly unerringly through the 

 densest fog. Members of the Biological Survey, proceeding by steamer 

 from the island of Unalaska to Bogoslof Island in Bering Sea, through 

 a fog that was so heavy as to make invisible every object beyond a 

 hundred yards, recorded the fact that flocks of murres, returning to 

 Bogoslof, after quests for food, broke through the wall of fog astern, 

 flew by the vessel, and disappeared into the mists ahead. The ship was 

 heading direct for the island by the use of compass and chart, but its 

 course was no more sure than that of the birds. 



Some investigators have asserted that the sense of direction has its 

 seat in the ears or nasal passages and thus that the bird is enabled to 

 identify air currents and other phenomena. It has been found that 

 disturbance of the columella or the semicircular canals of the inner ear 



889511—50- 



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