74 LIFE IN THE SEA [CH. 



water containing a relatively high concentration of 

 salt to water containing a low concentration, can it 

 perceive the change by means of its receptor taste 

 organs ? It does not seem likely that we could 

 perceive by taste alone the difference between water 

 containing 3 '45 % of salt and water containing 3*40 %, 

 nevertheless this is a change towards which a fish 

 might react. Now the atmosphere is to us very 

 much what the sea-water is to a fish, and we can 

 detect by means of our sense-organs changes less 

 than that instanced. Let us suppose that the tem- 

 perature falls rather suddenly from 20 C. to 15*6 C. : 

 we should certainly be able to detect such a change. 

 It would be a change of 1*5 % of the total possible 

 range of temperature, regarding 20 C. as the upper 

 limit; and the salinity change is also 1*5% of the 

 total range, regarding 3'45 % as the upper limit. 

 It seems quite probable then that the fish would be 

 able to detect, by means of its taste-buds, the change 

 in the saltness of the water, especially as we have 

 reason to believe that the olfactory receptor organs 

 in a fish have a lower threshold than in our own case, 

 and smell is only ' taste at a distance.' 



But we are not restricted to the supposition that 

 the fish can only be affected by salinity changes if 

 it can appreciate those by its organs of taste. Its 

 blood and lymph are watery fluids containing certain 

 salts of sodium, potassium, lime and magnesium in 



