HISTORICAL AND EXPERIMENTAL STUDY OF HOMING. 17 



recognizable to man it must present inequalities of elevation which amounts 

 to at least 30" and these must be relatively abrupt. 



The fourth column of table 1 gives the least observable inequality in the 

 coast-line for the distances in column 1 for man. 



The only data upon acuity in birds has been presented by Johnson* on the 

 chick and places its acuity at about 4'. The fifth column in the table gives 

 the smallest inequality in the coast line which can stimulate such an eye at 

 the distances indicated in the first column. The most irregular mountains 

 in the region of the Gulf of Mexico, where our observations were made, the 

 Sierra Madre, would have, at a distance of 300 miles, the same stimulating 

 value for the eye of this bird as a fine straight line seen a few feet away, the 

 irregularities being lost in the imperfections of its dioptric mechanism. If 

 the visual acuity of the homing bird is no greater than that of the chick, or 

 even of man, the possibility of its reaction even to an "imprecise" visual 

 impression seems precluded for distances of more than 100 miles. 



A THEORY BASED UPON CUTANEOUS SENSIBILITY. 



Cyon,t although he has contributed largely to our knowledge of the structure 

 and function of the labyrinth, does not wish to be confused with those writers 

 who attribute to them any special function in distant orientation. Naturally 

 they are brought into play in the flight of the bird, but only as a reflex mechan- 

 ism involved in the maintenance of equilibration. 



In 1897 Cyon first announced his own theory. The theory advanced 

 posited a Spursinn, which has its location in the nose. This sense may be 

 wholly independent and separate from the ordinary olfactory sense. In sup- 

 port of his theory Cyon refers to the well-kno\vn fact that the hunting-dog can 

 at great distances find a person to whom he is attached. Why should not the 

 homing sense be bound up in other animals with this sense? The fact that the 

 sense of smell is so little developed in birds can mean nothing but that they are 

 not sensitive to odors which affect the human being. This fact does not mili- 

 tate against the view that the organ of homing may have its seat in the same 

 organ, without being identical with the olfactory organs. Many people from 

 their youth are lacking in the sense of smell ; they show a remarkable inabil- 

 ity to orient themselves, not only in cities with which they are most familiar, 

 but also in large buildings, especially if the latter are laid out symmetrically. 

 They require time to orient themselves in a new building. Cyon suggests also 

 that the work of Bethe on ants and bees seems to speak for a division in the 

 olfactory sense. 



Cyon's preliminary experiments showed that the birds were insensitive to 

 ordinary smell substances. The birds showed no reactions when oil of cajuput 

 and asafcetida were offered. On the other hand, they were found to be so 

 very sensitive to ammonia that it may be used to test the ansesthetizing effect 

 of cocaine. Fanciers sometimes hold that pigeons react to the nest by 

 reason of smell. To see if this was well founded, Cyon blinded a brooding 

 pigeon by closing the eyes with cotton soaked in collodion. The experiments 

 were carried out in a house the third story of which contained a cote. The 



*Journ. Animal Beha\-ior, 1914, p. 340. 



tCyon, E. von: Ohrlabyrinth, Raumsinn und Orientirung, Pfliiger's Archiv, lxxex. 



