256 LIFE : OUTLINES OF GENERAL BIOLOGY 



longations of sensory cells. Suspended in the fluid there are fine 

 particles of lime secreted by the wall of the sac, or particles of sand 

 and the like introduced from the outside world. These particles 

 jostle against the sensory hairs when the fluid is made to oscillate 

 by movements of the body. Among backboned animals the hearing 

 of the ear becomes more and more pronounced as we ascend the 

 scale; but even in man the balancing function of the ear is still very 

 important. 



A Sense of Direction? — Some very clever men are singularly 

 deticient in what is popularly called a sense of locality. Time after 

 time they visit the same town or the same big building without 

 learning in the least how to find their way about. In cases we have 

 known, the defect is associated with exceptional intellectual abiUty, 

 and possibly indicates that the mind is preoccupied with great 

 problems, and that the habit of attending to topography has never 

 been formed. On the other hand, we have known very gifted men 

 who could find their way without a compass on a moorland shrouded 

 in mist. They had indeed served a long apprenticeship to way- 

 finding, and were what might be called objectively-minded, but at 

 a juncture their only stipulation was that there should be no talking. 



Many a man can walk from the railway station for an hour into 

 the heart of an unknown city, and then retrace his steps without 

 mistake and without giving the matter much attention. In such a 

 case there is probably an unconscious registration of landmarks 

 and also some general registration of the muscular movements 

 performed. The useful habit that many people have of orientating 

 themselves in every new place must prevent big errors, but it is 

 necessary to distinguish the deliberate attending to details from the 

 almost automatic retracing of steps. 



Many very careful experiments have been made with ants, and 

 Rabaud has summed up the evidence in his book, How Animals 

 rind Their Way About (1928). His examination of the well-estab- 

 lished facts, excluding all hearsay, is entirely against the postulate 

 of any mysterious sense of direction. An ant has an instinct to 

 return to the nest with its treasure-trove, but it finds the way home 

 by serving an apprenticeship. 



In contrast to its instinctive activities, which depend on inborn 

 ready-made capacities, the ant learns its way about. There is, of 

 course, no difficulty when it gets on to a well-trodden ant-road, 

 such as often exists, for then the olfactory traces are probably in 

 most cases sufficient ; the problem is in regard to pathless territory. 

 The probability is that the ant gradually builds up a general 

 impression of the area around the nest — a general impression con- 

 sisting mainly of visual cues, but not excluding such features as the 

 slope and texture of the ground and the broad facts of light and 

 shade. 



