Sensory Discrimination: The Chemical Sense iii 



more dilute solutions than the other two, thus being ca- 

 pable of acting as a distance sense (550). A number of 

 species of fish have been shown to possess smell, by demon- 

 strating that they can discriminate between small bags 

 filled with food and similar bags stuffed with inedible 

 substances, and that this discrimination is lost when the 

 olfactory nerves are cut or the nostrils are closed (547, 

 142). SheKord and his associates (673) have thrown light 

 on a very interesting problem in animal behavior, the 

 migrations of fish. It is well known that salmon return 

 to fresh water to spawn, ascending rivers, and that other 

 fish perform migrations that are of great economic impor- 

 tance to the fishing industry. Shelford has demonstrated 

 that fish are very sensitive to slight variations in the chemi- 

 cal constitution, the saHnity, for instance, of the water in 

 which they live, and their responses to such changes may 

 well account for all their wanderings. 



Among amphibians, the spotted newt seems to show a 

 relation between smell and the "common chemical sense" 

 not unlike that existing in fishes. The olfactory nerves 

 seem to be required for the discrimination of food. When 

 chemicals are applied to the body, the head end is much the 

 most sensitive region, even when the olfactory nerves are 

 cut. Acids and alkalies cause very marked reactions ; salt 

 is less effective and sugar not effective at all (629). Cole 

 (135) studied the time required for the reflex withdrawal 

 of the hind legs of leopard frogs when four chlorides, those 

 of ammonium, potassium, sodium and Kthium, were applied 

 in solution. He found that the speed of reaction corre- 

 sponded to the order in which these salts affect the human 

 sense of taste. That a common chemical sense, and not 

 pain, was involved in these skin reactions was indicated 

 by the fact that they persisted when ordinary pain reactions, 



