703 
Paris, 1920) has gone so far as to propound a theory of 
migration which dispenses with instinctive behaviour 
in favour of “ galvanotropism,” and reduces birds to 
the status of mere automata acting under the com- 
pulsion of “des grands courants a¢riens électro-ma- 
enétiques Gquinoxiaux.” Unfortunately for his argu- 
ment, it is based on a conception of migration which is 
not consistent with many of the established facts, and 
it presupposes the existence of physical phenomena as 
to which the physicists are silent. Nor is its credibility 
increased by the absence of any suggestion as to a 
possible physiological mechanism linking the supposed 
physical causes to the alleged biological effects. At 
the best it is one of those “ explanations ”’ which call 
for more explaining than the original phenomena. 
In these circumstances one welcomes a serious 
attempt, by a biologist and a physicist in collaboration, 
to set forth the possibilities of the case. This has been 
done by Dr. Rochon-Duvigneaud and Prof. Ch. Maurain 
(La Nature, 1923, 232) in respect of homing pigeons. 
In this paper Dr. Rochon-Duvigneaud begins by stating 
the biological data;and Prof. Maurain, who is director 
of the Institute of Terrestrial Physics in Paris, follows 
with a discussion of the physical facts, particularly 
those of terrestrial magnetism, which might be relevant. 
Prof. Maurain confines himself to a statement of ap- 
parent possibilities; he holds none of them as proved, 
and he urges the need for experimental inquiry. 
Whether his tentative hypothesis is sufficiently plausible 
from a biological point of view to constitute a prima 
facie case for further research on these lines, however, 
is perhaps open to doubt. 
Prof. Maurain’s suggestion may be stated as follows : 
The magnetic declination (angle of magnetic needle’s 
lateral deviation from the geographical meridian) and 
the magnetic dip or inclination (angle of needle’s vertical 
deviation from the horizontal plane) both vary from 
place to place over the surface of the globe. The 
periodical variations at any given place, and the 
irregular disturbances which also occur, are small in 
proportion to the otherwise constant geographical 
differences. Roughly speaking, therefore, every 
locality has its characteristic declination and dip. If 
lines be drawn through the places having the same 
declination, and other lines through the places having 
the same dip, these lines are (in Europe) roughly at 
right angles to each other. The lines thus serve as 
co-ordinates, which fix the position of any given locality 
like lines of longitude and latitude. Moreover, the 
declination and dip increase or decrease progressively 
as distance from a given locality is increased, except 
along those lines where one or other factor remains 
constant. So much is a matter of common knowledge. 
It has then to be supposed that the pigeons are 
sensitive to changes in declination and dip, and indeed 
simultaneously sensitive to each factor independently 
of the other, and that when removed to a new locality 
the birds have a natural tendency, so to speak, to seek 
their own magnetic level. If a pigeon be removed to 
another place having the same declination but a greater 
(or less) dip, it would fly along the line of equal declina- 
tion in the direction of decreasing (or increasing) dip. 
Similarly, if removed to a place having the same dip, it 
would follow the direction in which the declination 
changes towards the amount found at the home locality. 
NO. 2795, VOL. 111] 

NATURE 

[May 26, 1923 
__ = eee eee 
Again, in the more general case of a bird removed to a 
place where both declination and dip are different, it 
would be affected by both factors and its homeward 
path would be the resultant of the two tendencies. (It 
is noted that there is no question of remembering the 
magnetic changes experienced on the outward journey, 
as a bird removed by a circuitous route will find a 
direct path home.) 
Ornithologists will be grateful to the physicist for 
the statement of a possible case, but they will regret 
that Prof. Maurain has confined his argument to the 
relatively short journeys performed by homing pigeons 
and to the magnetic phenomena as they exist in Europe 
(for it is not in every part of the world that the lines of 
equal declination and of equal dip run at right angles 
to each other, and that there is only one point at which 
a given pair of values for these factors is to be found). 
They would have liked to see a case similarly stated in 
respect of the migrations of, say, swallows from South 
Africa to England (cf. NaTurE, March 16, 1922, p. 346), 
over an area in which more complicated changes in terres- 
trial magnetism have to be reckoned with. Dr. Cathelin 
notwithstanding (“ Le retour au nid reste done pour 
nous une des grandes hérésies ornithologiques”), an 
ever-increasing volume of records of marked birds 
shows that swallows and others commonly perform very 
accurate feats of “ homing ” from great distances. 
Returning to homing pigeons, however, we may 
examine the argument more closely. The physical 
phenomena exist, and a remarkable power of orienta- 
tion is undoubtedly involved in homing; can a con- 
nexion be traced between them? The most serious 
objection seems to be the entire absence of any evidence 
of sensibility to magnetism on the part of birds or other 
animals ; and without this physiological link specula- 
tion must needs be barren. Kelvin got negative results 
from his experiment of subjecting the human head to 
the influence of a powerful magnetic field; Du Bois 
observed no effect on protozoa ; and the writers of the 
paper under discussion havesimilarly failed with pigeons. 
Within a limited field, it must be remembered, the 
strong electro-magnets used in such experiments are very 
many times more powerful than terrestrial magnetism : 
yet for Prof. Maurain’s hypothesis we must suppose 
that birds are sensitive not only to minute changes 
in terrestrial magnetism, but also to changes in two 
of its factors separately. It does not seem, therefore, 
that the theory can be regarded as a promising one. 
Prof. Maurain apparently holds, nevertheless, that 
there is a good case for further investigation, and he 
discusses in some detail the conditions necessary for 
an experiment on pigeons during their actual homing 
flight. It is not, of course, possible to interfere with 
terrestrial magnetism by means of artificial magnets 
over an area of any size, although it is admitted that 
pigeons find their way back as easily to lofts in great 
cities, where electric cables and the like cause an appre- 
ciable disturbance, as to lofts in the open country. 
The pigeon might be made to carry a small magnet and 
thus be kept within its field, but negative results would 
not be considered altogether convincing, because the 
amount of interference with terrestrial magnetism 
would be constant throughout ; whether a portable 
apparatus giving varying magnetic effects could be 
| designed is not discussed. Our author considers that 
