THE SENSE OF DIRECTION. 43 



primarily determines the direction of flight, and it may be 

 regarded as static in operation. The second component is the 

 means by which a true course is kept, and it is dynamic in mode 

 of operation. While it fulfils the conditions of the first com- 

 ponent, the theory based on a sense of displacements does not 

 adequately comprise those of the second. For, if a persistent 

 wind blows across the track of birds travelling a long way over 

 open water, then the birds are faced with the difficulty of 

 neutralizing drift, as Darwin pointed out.* Should the travellers 

 have no perception of a strong leeward drift from their true 

 course, then at any given part of the journey the birds will not 

 point towards their destination, but along a line which is parallel 

 to the original course projected from the point of departure. 

 Such a misfortune might readily be attributed to birds far 

 beyond sight of land. Nevertheless, it is advisable to believe 

 that birds possess the power to correct deviation in the absence 

 of landfalls. 



The observations which I am about to record tend to show 

 that the theory of a sense of displacements accounts for the first 

 component of the sense of direction more satisfactorily than the 

 theory of a memory of localities ; that a memory of localities is 

 not employed under conditions in which one would expect a 

 useful knowledge of landmarks to be demonstrated ; and that the 

 former theory does not sustain the keeping of a true course. 



Obs. 1.— Fog. Visibility range 1500 yards. Wind N.W., 1. 

 Twelve Geese from inland on course N. by E. magn., made the 

 sea at Eedhouse Point, Bast Lothian, and turned W.N.W. 

 After going so far on this course, the three leaders turned about 

 S.W. ; the rest continued W.N.W. for some time, and then 

 followed the leaders. They were thereafter watched going S.W. 

 along the coast of Gullane Bay as long as they were visible. 

 The Geese invariably rest in Aberlady Bay after having been 

 inland. The direct line from Eedhouse Point to Gullane Point 

 was afterwards found to be W.S.W. magn. 



Obs. 2. — Fog. Visibility range 300 yards. Calm. Obser- 

 vation made from " sill " opposite Eyebroughty Brig, Firth of 

 Forth. The usual flight of waders, during high water of spring 



* Darwin, " Posthumous Essay on Instinct," 1859. Appendix to 

 ' Mental Evolution in Animals (Romanes),' p. 357. 



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