THE SENSE OF DIRECTION. 53 



severed by more than a moderate distance, the knowledge pos- 

 sessed by the bird of the appearance of its home is not suffi- 

 ciently accurate — or, at any rate, dominant — to overcome the 

 confusion resulting from the disturbance of its position in space. 

 To summarize : the sense of direction is an expression of the 

 means by which an animal is able to return to any locality it 

 has recently occupied. The sense of direction is susceptible of 

 analysis into two components. The first component is a con- 

 stant knowledge of the position of a locality in space. It is 

 internal in origin ; that is, independent of the environment, 

 except in so far as the sensation of space is attached to the 

 external world, and is probably a capacity to sense the position 

 of the objective in terms of the bodily displacements in space 

 experienced during the outward journey. The second com- 

 ponent consists of the means of keeping a true course. In birds 

 it is external in origin ; that is, dependent on the environment. 

 The wide surfaces and extensive lines passing through the visual 

 fields are used not to direct the course followed, but to preserve 

 the straightness of the course originally projected from the 

 starting point. The object is attained by the maintenance of 

 an equality of the distances extending from the bird to related 

 surfaces and lines passing through the visual fields. Deviation 

 from the course is indicated by an angular distortion appearing 

 in the apparent paths of the surfaces and lines occupying the 

 visual fields. Deviation is corrected during flight by an in- 

 voluntary declination of the long axis of the bird's body contrary 

 in direction and sufficient in degree to neutralize the rotation 

 or distortion then apparent in the fields of vision. 



