306 FRANCIS B. SUMNER. 



or two days after transfer to fresh water. On the other hand, it 

 will be remembered that fishes of this species commonly did not 

 die for a considerable number of days, while many survived for 

 a week and some even for several weeks. Again, it will be 

 recalled that the fatal effects of fresh water upon this and some 

 other species were nullified by the admixture of a very small 

 percentage of salt water. Analyses showed that in this latter 

 case there was little or no decrease in the salt content of the 

 body. A rough approximation was pointed out between the per- 

 centage of salts in this faintly saline water and that in the fishes 

 themselves. All of these facts point to the conclusion that one factor 

 in t/ie deatli of salt-water fishes in fresh water is the extraction 

 from their tissues of an amount of salts sufficient to reduce the per- 

 centage below a certain necessary minimum. 



If the question be asked : Why are not fresh-water fishes thus 

 affected in their own medium ? it is replied that their mem- 

 branes have been adapted to resisting such an extraction of salts. 

 It is perhaps also true that the irreducible minimum of salts in 

 these species is lower than in the case of salt-water ones. In any 

 case, the percentage actually present is, on the average, less 

 (Atwater, 1 Katz, 2 Quinton 3 and several others). 



Whether or not salt water ever has a toxic effect, in the nar- 

 rower sense, upon fresh-water fishes cannot be stated definitely. 

 Bert denied that such was the case, but he is not entirely con- 

 sistent in this position. In view of the fatal effects upon salt- 

 water fishes of some of the individual components of sea salt, 

 when taken separately (Loeb, 4 Siedlecki 5 ), it seems quite possi- 

 ble that sea-water may act as a poison to fresh-water organisms, 

 independently of any osmotic effects. Indeed both of the last- 

 named writers have shown that it is the chemical nature of the 

 solutions used rather than their osmotic pressures which deter- 

 mines, in many cases, whether or not they shall prove fatal. 



COLLEGE OF THE CITY OF NEW YORK, 

 March 2J, 1906. 



1 Report U. S. Com. Fish and Fisheries for 1888 (1891). 



2 Archiv filr die gesammte Physiologie, 1896. 



3 " L'eau de mer, milieu organique," Paris, 1904. 



4 American Journal of Physiology, 1900. 



5 Comptes Rendus de r Academic des Sciences, 1903, 



^ o f* r~" i c*i r~ 

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