l^(, ELEMENTS OF BIOLOGY 



producing devices are also responsive to vibrations of corresponding 

 frequencies. Then, too, reflexes set up by the sense of sight or hght 

 reception, by odors, and other stimuU produce a wide variety of 

 responses. For example, certain fish swim upstream when placed in 

 a water current. If the fish is placed in a stoppered bottle of water, 

 in which there are no currents, and the bottle towed behind a boat, 

 the fish similarly heads in the direction of motion of the boat, 

 showing that this orientation is due to some reflex set up by the 

 sense of sight. It is also true that the sight of food by acting through 

 nerve reflexes will cause activity in the digestive glands of mammals. 

 The total behavior of an animal is the sum of such reflexes, tropis- 

 tic responses and conditioned reflexes, plus other factors that are as 

 yet unknown or difficult to evaluate. It has not yet been possible to 

 account in a wholly satisfactory manner for the homing instinct 

 exhibited by some mammals, birds, some insects and a few mol- 

 luscs. The homing pigeon has been exploited as a means of com- 

 munication and methods of training have been worked out in 

 much detail, yet the nature of this instinct in the bird remains 

 unknown. From time immemorial the homing ability of the cat has 

 been the subject of anecdote and facetious adage; its powers are 

 indeed remarkable, but perhaps not so extraordinary as many anec- 

 dotes would indicate. Rather carefully controlled experiments and 

 observations by a skilled and reliable observer show that the homing 

 ability of different cats varies. It was found that when a female 

 with young was carried several miles from the kittens, hoodwinked 

 and covered in a manner to prevent any visual recognition of loca- 

 tion, the cat returned in a direct path within a few hours, crossing 

 a deep ravine and many railroad tracks, climbing a four-hundred- 

 foot rise, and traversing a maze of city streets. The same cat, com- 

 pletely anaesthetized with chloroform and carried to another point 

 before release, returned in seventy hours. A mollusc, Chiton (p. in) 

 has been observed to retire daily to the same crevice during storms 

 over a period of nine months, although other appropriate crevices 



