50 THE ZOOLOGIST. 



field of clearest vision in the flying bird lies in an antero-lateral 

 relation to the bird rather than directly ahead. The bird has a 

 simultaneous visual perception of objects lying on both sides of 

 itself. Owing to the rapidity of its flight and a preoccupation 

 with the object of flight, the bird does not form any definite 

 notions as to the nature and position of neutral objects, but is 

 concerned rather with wide surfaces and extensive lines occupy- 

 ing the visual fields. These surfaces and lines are used not to 

 direct the course followed, but to preserve the straightness of the 

 course originally projected from the starting point. Distances, 

 then, rather than objects furnish the data for keeping a true 

 course. And as birds fly with equal facility from all quarters 

 towards their object, there can^be no memory of the lateral 

 distances at the successive points on the course projected. 

 Hence the data are not geographical but mechanical. Continu- 

 ous observation, it need not be consciously, of the surfaces and 

 lines lying towards the limit of both lateral horizons furnishes a 

 more delicate test of the straightness of the course that is being 

 kept than binocular concentration on the objective lying directly 

 ahead. The idea, it may be noted, is not purely speculative, for 

 in some degree it can be given the trial of personal experience. 

 The explanation just given accounts for the Canary's failure to 

 reach the well-lighted cage when the room was dim, the more 

 so as the lateral deviation, when it arose, occurred towards the 

 really darker, or the farther away and therefore apparently 

 darker, side of the room. 



In more general relations it should be borne in mind that, 

 though the flight of small birds is often ill-directed in semi- 

 darkness, these birds, when they are migrating at night — as 

 Gatke has pointed out — have no apparent difficulty in keeping 

 their course.* Audition here is a possible factor of the second 

 component ; the more so as Prof. Watson found by observation 

 at night that the erratic flight of individual Terns was corrected 

 when their calls were answered from the nest.t At the same 

 time, it cannot be said that visual data based on fixed lines and 

 surfaces are essential to the mechanism of the second com- 

 ponent, for none is present on the open sea. Gatke observed 



* ' Heligoland as an Ornithological Observatory,' p. 63, 1895. 

 f Log. cit. p. 217. 



