48 THE ZOOLOGIST. 



changes in the locality of the nest until after they arrived, while 

 in those birds without binocular vision a clear perception of the 

 objective can only be obtained by turning the head to one side. 



Direct vision of the objective is not required by certain 

 diving birds, which are able to fly distances of from one-half 

 to two miles over the sea to feeding areas of small extent and 

 lying two to five fathoms under muddy water. The absence of 

 landway after the first dive forms a test of the accuracy of the 

 orientation, which, indeed, rarely errs by more than a few yards. 



It is thus possible to exclude the use of an immediate visual 

 perception of the objective and a knowledge of landmarks, even 

 though these are available, when it is clearly recognised that 

 habitual flights of longer or shorter duration can proceed 

 towards a successful conclusion without the aid of either of 

 these methods of orientation. And therewith we exclude the 

 whole environment from the mechanism of the first component 

 of the directive sense. The advantage so far gained lies in the 

 circumstance that, whether the flights extend over a few yards 

 or several hundreds of miles, the mechanism of the first compo- 

 nent is reduced to one kind only— is the same for all habitual 

 flights. 



The data on which the first component rests must then be 

 internal and independent of the environment. To account for 

 the nature of the internal data, Bonnier put forward the theory 

 which is based on the sense of displacements.* Bonnier asserted 

 that there is no known instance of an animal making a definite 

 movement to a certain place where it had never been, and, so far 

 as I am aware, the assertion is still true to-day. All movements 

 made by animals with the object of reaching definite localities 

 are return movements. Further, the capacity to return to the 

 original point of departure does not extend over unlimited 

 distances. The limitation suggests a mechanism dependent 

 on a knowledge of places, and it has, indeed, been so in- 

 terpreted. But the objections to this view are too weighty to be 

 overcome. From the two facts just cited it is apparent that the 

 mechanism of the first component is dependent on impressions 

 acquired on the outward journey, and that these impressions are 

 internal and not due to the environment is maintained by the 

 * Oompt. Rend. Soc. Biol. t. iv. p. 1051, 1897. 



