RESISTANCE OF FISHES. 339 



is therefore a matter of much importance that the adults cho<>M 

 a location for depositing the eggs that will enable them to develop 

 normally, and which will prove efficacious for the first stages of 

 growth of the fry. 



That the adults make the selection successfully in a majority 

 of cases is undoubtedly true and the fact that the conditions 

 at the breeding grounds may be very different from those of the 

 normal habitat does not seem to interfere with the choice. The 

 conditions necessary for early stages of different species vary 

 widely. Thus the adults of two or more species may live in the 

 same general habitat up to the 1: reeding season when the process 

 of selection of suitable breeding grounds often results in their 

 becoming widely separated. This results in the eggs of the 

 different species being deposited, and the young hatched, under 

 conditions which may differ greatly, but which in most instances 

 prove to be the best for the development of the first stages of 

 each species. 



By what process the adults of each species are enabled to 

 select from the great variety of combinations of conditions that 

 combination that is best suited to the development of their own 

 eggs and fry is a matter for investigation. It is probably true 

 that, to a large degree, the choice is a result of reactions, chemical 

 and otherwise, to the factors of the environment. Everman ('98) 

 states that in Louisiana the blue catfish (Ictalurus furcatus] 

 and goujon (Leptos olivaris] are influenced in their movements 

 by the temperature of the water. During the winter they 

 come farther down the river, where the water is warmest, and 

 in the summer run further up stream or retire to the deeper 

 waters. In some recent experiments with fishes in temperature 

 gradients I have found that many species are very sensitive to 

 slight differences in the temperature of the water, detecting and 

 reacting to differences as small as i to 2 degrees C. Gurley ('02) 

 thinks that temperature together with salinity are the factors 

 by which the salmon and other fishes which come from salt into 

 fresh water to breed are enabled to find the freshwater streams. 



Green ('09) states that salmon, when coming into fresh water 

 during the breeding season, often spend from 2 to 4 days swim- 

 ming back and forth in the brackish water before passing on into 



