pole as they appeared to von Middendorf to do in Russia. So far 
as I am aware there is among plants and animals no known case 
of response in any way to magnetic force. 
Semi-circular The attempt to locate some organ within the 
Canal Theory vvhich this function of path-finding 
might be referable resulted in the Mach-Breuer theory of the semi¬ 
circular canals which was elaborated particularly to explain how 
the carrier pigeon finds its way home. This theory rests upon the 
supposition that the semi-circular canals of the inner ear 
form an organ of equilibration by means of which an animal can 
orient itself in any of the three planes of space. Each of these 
three canals, which are situated at right angles to each other, en¬ 
larges at one of its ends into an ampulla, within which, surrounded 
by endolymphatic fluid, are located delicate nerve endings coming 
from the eighth cranial nerve. In whatever position the ear is held 
the endolymphatic fluid within the semi-circular canals presses 
more upon the nerve endings within one ampulla than upon those 
of the other two, and the particular stimulation thus received, upon 
being transferred to the brain, records for the animal its position 
in space. Along with this sensory registry of positions in space 
it is assumed that there has been developed the ability to record 
intervals of time upon the brain automatically. It is interesting 
in this connection to notice that the recognition of time intervals 
is a basic principle of music and birds are notably muscal. Be- 
sles a registry of position in space an of time intervals there may 
be developed a registry of -the distance traversed in case a uni¬ 
form speed is manitained. Thus when a pigeon going away from 
home travels at a given speed, for example, east twenty min¬ 
utes. north thirty minutes and east again ten minutes, all these 
changes in direction, together with the time occupied in making 
them and consequently the distance traveled in following each 
direction, are recorded upon the brain as sounds are recorded 
upon the cylinder of a phonograph. To accomplish the return 
journey it is only necessary to reverse the record made upon 
the brain in order to get back to the starting point. 
A theory of this kind has the advantage of being capable of ex- 
veriments to test its soundness, and such a test was made in 1893 
by Professor Sigmund Exner in Vienna. Exner attempted to 
find experimentally whether the brain of the carrier pigeon records 
automatically the direction and distance taken in the outward 
journey in such a way as to be equipped to make the journev 
home. He first took two covered cages of pigeons several miles 
away from home to a locality unknown to the birds and out of 
