22 HOMING AND RELATED ACTIVITIES OF BIRDS. 



in order to direct themselves. It is the chart which serves this purpose; and if one con- 

 ceives of animals endowed with a sense which permits them to recognize only the direction 

 of magnetic north, these animals would have a constant notion of north, but would not be 

 capable of returning to their point of departure unless they had maintained an exact memory 

 of the length of the lines traversed in this or in that direction. The mariner, navigating by 

 dead reckoning, has to write down the length as well as the direction of all his rhumb lines. 

 Such a view demands at least a constant perception and a prodigious memory and does not 

 correspond at all to the simplicity of the notion of which I spoke above. But terrestrial 

 magnetism furnishes other indications. If we suppose that an animal is capable of perceiving 

 influences which actuate the needle of the compass of inclination, and that the position of this 

 animal remains always the same with respect to the vertical of the place, he will be able to 

 recognize the differences in intensity which cause the variations in the angle which we measure 

 with the aid of this compass, an angle which varies, as we know, from degrees at the 

 magnetic pole to 90 degrees at the magnetic equator. He would be able, then, to recognize 

 the direction where this angle varies most quickly (magnetic mei'idian or isogenic Hne), 

 and where it remains constant (magnetic parallel or isoclinic hne). Finally, there are other 

 data which we may gather from the use of our instruments, and which one can suppose 

 perceptible through an appropriate organ. We have in mind variations in magnetic 

 intensity. If one disturbs the position of equilibrium of the needle of the compass of declina- 

 tion, the needle comes back, by a series of oscillations, which show a variable force according 

 to the place. By joining all the points where this force is equal we can trace on our maps a 

 series of lines (isodynamic) whicli can be confused neither with isogonic nor with isoclinic 

 lines (although they show a development somewhat like that of the latter). One can then 

 admit that a line can be determined for an animal, endowed with the magnetic sense as 

 complete as we are assuming it to be, by the value of the magnetic actions in inclination and 

 declination, just as fairly as it would be determined for us by the intersection of given 

 magnetic lines. These magnetic conditions once known, the animal will be capable always 

 of going directly to this point. Carry it to considerable distances, cause it to describe com- 

 plicated circles, anaesthetize it even during transport, it will be able always to come back to 

 its point of departure. This sense is supposed to give the animal general direction. It will 

 suffice to bring it into a neighborhood where the five senses can be employed. 



He supposes that the magnetic currents act in such a way as to release 

 reflexes. The tissue affected is specified as follows: 



Assume, then, that terrestrial magnetism induces true currents in the endolymph of the 

 canals, the intensity of which varies with the position of these canals with respect to the 

 directions of the needles of inclination and declination, and wit h the intensity of the magnetic 

 phenomena. 



Thauzics,* likewise, as is well known, believes that the magnetic theory is 

 the only one which will account for the variou.s facts of homing. In his descrip- 

 tion of this theory he makes the following statement: 



The pigeon possesses a scnsibilile mngnclique which permits it to receive impressions 

 quite special and even somewhat mysterious — terrestrial magnetic currents, lines of force, 

 etc. — the influence of which combined with the work of the other senses puts the animal 

 on the true route and enables it to maintain it. 



Thauzies favors this theory because it is "logical" in appearance and 

 because it brings into play the "mental activity" of the bird. Thauzies has 

 had an immense amount of practical experience, f For 23 years he has made 



*ThauziE'.s, A.: L'Orientation lointaine, vi. Cong. Int. de Psy., 1909, p. 263. 



tHe states that the distance from which a pigeon can return is rarely greater than 500 km. 

 As a matter of fact, most of his flights lie well within this distance. He tells us further that, so 

 far as he knows, this faculty of orientation is not lost through age. At the age of 20 years a pigeon 

 is no longer tit for service. The quickness of flight attains to 110 to 120 km. to the hour, but it 

 varies according to the direction which the pigeon has to follow. These statements were made in 

 reply to questions asked at the time the paper from which we quote the above was read. 



