12 HOMING AND RELATED ACTIVITIES OF BIRDS. 



the animal reacts to each of these serial landmarks as it would react to 

 the goal itself. An obvious logical implication is that the animal granted 

 that it moves about at random until the first of these landmarks is reached 

 must be able to follow these clues in the direction towards the goal and not in 

 the reverse way (in the case of a dog, e. g., there must be no back-tracking). 

 Were this not the case, the animal, approaching the chain of landmarks at 

 right angles, by turning, e. g., to the right, would meet a series which, if fol- 

 lowed up, would take it home; if to the left, a series which would lead it still 

 farther afield. We can conceive, under this hypothesis, of an animal being 

 effectually lost in a wilderness of landmarks. In other words, if orientation is 

 obtained by reference to landmarks, the trail must be polarized. It is also 

 obvious that this hypothesis calls, first, for the ability on the part of the bird 

 to form habits of very great complexity; second, that it must form them with 

 extreme rapidity to account for the facts of homing (even in cases where land- 

 marks are possible). For convenience we shall group all such theories under 

 the heading of "habit theories of return." Such theories are advanced by 

 Hachet-Souplet, Hodge, and others. 



It is clear that if it can be proven that birds can return only over stretches 

 which afford landmarks, then there is no real problem of distant orientation at 

 all. The problem simplifies itself to the determination of what sense-organs 

 the animal employs in its normal habits of reacting to objects, the intensity of 

 the stimuli which call out such habits, and the rapidity with which such habits 

 are laid down, etc. In essence, the problem is as simple as determining 

 whether a young animal goes to its nest by reason of the presence of an audi- 

 tory stimulus (cry of the parent) or through vision (sight of the nest) ; or 

 gets out of the way of a hawk through the sight of the hawk or its shadow, 

 or through the auditory stimulus set up by the crying of its neighbors. 



On the other hand, the animal conceivably may be guided back reflexly 

 through the action of some unknown intra-organic or extra-organic stimulus, 

 acting upon some assumed but unknown sense-organ. We will call this 

 hypothesis (2). We shall develop this conception in connection with the 

 "contrepied theory" of Reynaud, the attitude theory of Bonnier, and the 

 magnetic sense of Viguier and Thauzies, etc. In such reflex returns no special 

 locality or position habits have to be formed. No landmarks are needed. All 

 such theories we shall group under the heading of " reflex theories of return." 



In the historical sketch which follows we have not tried to do justice to all 

 theories nor to all experimental data. We have tried to present the more 

 important theories and where possible some of the experimental details upon 

 which they are grounded. We have spent a disproportionate amount of time 

 upon the historical aspect of homing in vertebrates because no one hitherto has 

 attempted to treat the subject in monographic form. 



