Whereas innumerable adaptations of structure and habit are known, fitting specific animals to their natural environments, we have as yet very little knowledge of the physical and chemical causes or of the physiological effects of animal distribution.
In the case of aquatic animals, Krogh and Leitch (1919), and more recently Root (1931), have shown that the bloods of fishes which inhabit waters often deficient in oxygen contain haemoglobins with a greater oxygen affinity than those of fishes living in swift streams or in the open waters of the sea. In trout and mackerel the oxygen is readily given up from blood to tissues, permitting of an active life, but when oxygen is deficient in the water a sufficient quantity cannot be secured by the haemoglobin. Carp, on the other hand, lead a more sluggish life ; their haemoglobin parts reluctantly with its oxygen, but the blood can capture...
