378 THE AMERICAN NATURALIST [Vol. LVI 



As pointed out by Gurley (1902), the minnows migrate 

 to warming water for the purpose of spawning, while the 

 cod and the salmon migrate to cooling water for the same 

 purpose. Chamberlain believes that the salmon come into 

 water warmer than the sea water (1906). 



Field records for Fundulus heteroclitus secured by the 

 writer in connection with another investigation (1916) 

 indicate the importance of temperature. The fish began 

 coming inland in the spring when the water was about 

 15° C. and continued to run in and out until the inland 

 pools had reached a temperature in August of about 

 24° C. Then for a period of over two weeks, they ceased 

 running. About September 1, when the temperature had 

 again lowered, they appeared again and continued to run 

 until the temperature ran down to 10° C. 



Stream Pressure 



When sea water was introduced through the f -in. glass 

 tube with a force sending it horizontally to a distance of 

 6 inches, while fresh water was introduced through the 

 experimental tube into the adjoining trough with a force 

 sending it 12 inches from the end of the tube, there was 

 no difficulty in luring the fish away into the fresh water 

 and keeping them directed towards it. 



Many experiments were made, toxic substances such as 

 KCl and double sea water also being introduced, but the 

 increased pressure always proved the powerful factor. 

 Chamberlain (1906), Prince (1920) and others have pre- 

 viously shown that in the case of the salmon, migration 

 into fresh water is delayed until the floods come down 

 into the bays and small streams. The arrival of a vol- 

 ume of rushing water furnishes the needed stimulus and 

 the fish proceed forthwith to obey their instinct to swim 

 against the current. 



That fish can determine the presence of toxic sub- 

 stances in sea water or in fresh water is unquestionably 

 demonstrable. But we have much evidence that those 

 fish lying offshore and habitually migrating up a certain 



