Among the fishes generally, the habitat of particular species is fairly rigidly fixed in either fresh or salt water, and it is well known that the abrupt transfer of individuals from one to the other medium is quickly fatal. It has long been recognized that the deleterious influence of fresh water on individuals adapted to salt water, or vice versa, arises from physiological limitations in the maintenance of osmotic equilibrium. Though certain euryhaline forms, such as Anguilla and Fundulus, can be abruptly transferred without injury from one medium to the other, such forms are outstanding exceptions to the general rule. So unique is the adaptability of Fundulus that Breder ('34) has said that it "may almost be considered a generic characteristic. ' ' I n addition to these better known euryhaline species, certain typically marine forms can, under special ecological circumstances, establish themselves in fresh water (Breder, '34), but in this case adaptation appears to depend upon the presence of a high content of calcium in the water, which in some unknown manner protects against the deleterious influence, or aids in making the necessary compensations.
The precise nature of the physiological limitations which restrict the greater number of fishes to fresh or salt water is not known, but the recent studies of Smith ('32) on the 469