THE BONY FISHES 115 



which is one reason why it is difiBcult to quote reliable 

 figures on renal function in these forms. 



Whatever the basic renal operations that permit some 

 fishes to migrate from fresh into salt water, or from salt 

 into fresh, it is appropriate to look on this migratory 

 habit as an extreme specialization in itself, involving in- 

 creased eflFectiveness of regulatory mechanisms common 

 to both fresh- and salt-water forms. For the great ma- 

 jority of fishes, the physiological adjustments involved 

 in even the slow transfer from one mediimi to the other 

 are inadequate to permit siurvival, and they remain 

 bound by physiological habitude to narrow ranges of 

 salinity. 



An exception to this statement is, however, observed 

 in certain semitropical fresh-water pools that communi- 

 cate sluggishly with the sea and that are located in areas 

 rich in limestone, so that the fresh water has a high cal- 

 cium and bicarbonate content. Hard-water pools of this 

 nature are characteristic of the Andros Islands and north- 

 em Florida. In such pools many typically marine fish, 

 such as snook, tarpon, and snappers, are to be found 

 very much at home with fresh- water forms. It is gen- 

 erally not difiicult for the paleontologist to distinguish 

 marine fossil beds from those that are continental in ori- 

 gin; but where, in the geologic past, fresh and salt water 

 have met under these conditions the faunal mixture may 

 belie all other evidences and reduce his confidence to 

 doubt. It would appear that either the high calcium con- 

 tent or the alkalinity of these hard waters makes the 

 transition from salt to fresh water easier— but why, no- 

 body knows. The question may seem trivial, but not to 

 anyone who is concerned with the intricate problem of 

 salt and water balance, with the proper functioning of 

 the heart and blood vessels, with the blood supply to 

 the brain, with the many problems that remain unsolved 

 in respect to the regulation and distribution of the in- 

 ternal environment in man. 



