what would be expected to account for such a correlation. However, if the experiments are considered as throwing light on a physiological mechanism, they have much more meaning.
It is well known that carbon dioxide has a marked effect on the transport of oxygen by the blood of mammals. In the presence of relatively low concentrations of carbon dioxide their blood can become saturated with oxygen at relatively low tensions of that gas. On the other hand if the carbon dioxide tension is increased the blood cannot take up or hold oxygen to nearly the same extent. Accordingly the moderate tension of carbon dioxide in the alveoli and the higher tension in the tissues facilitate the shift of oxygen from the lungs to the blood and from the blood to the tissues. This effect of carbon dioxide on the ability of blood to take up oxygen is known as the "Bohr effect." Carbon dioxide has a similar effect on the blood of fishes, as has been shown by many workers. Particularly since the discovery of the biological anticoagulant heparin, it has been shown that the magnitude of the Bohr effect in fish blood differs greatly from one fish to another. It is often much greater than that displayed by the blood of mammals or there may be hardly any effect at all as Black (personal communication) has found to be the case in the catfish.
Blood curves for two fresh-water fish, the sucker and the carp, from Black ( 1938), are given in the upper left panel of Fig. 3 which shows the oxygen tension necessary to half saturate the blood in the presence of a given tension of carbon dioxide. These two blood curves occupy about the center of the range of sensitivity which has been found in fishes, and by way of a landmark it might be pointed out that a similar curve for man lies very closely to that of the carp.
When the curves for the levels to which oxygen can be removed in the presence of different concentrations of carbon dioxide by these two species, shown in the upper right panel of Fig. 3, are compared with the curves for the drawn blood, it will be apparent that the sensitivity toward carbon dioxide displayed by the fish may be dependent upon the magnitude of the Bohr effect shown by their bloods. This is what would be expected on a priori grounds, for fish with blood very sensitive to carbon dioxide would be less able to take up oxygen in the presence of increasing carbon dioxide than would fish with blood that was more indifferent to the presence of that gas. Thus, and there is further evidence to support this view, the measure of sensitivity towards carbon dioxide as shown in Fig. 3 is in all likelihood a measure of difference in the Bohr effect displayed by the bloods of different species. Consequently it seems reasonable to infer that the correlation observed between sensitivity and vertical distribution of these species in summer is due to differences in the oxygen transport capacity of the blood.
With this brief mention of the part experiment plays in limnology our consideration of the position of fish and other higher animals in the economy of lakes is finished. No attempt has been made to review the subject exhaustively or to give more than a few references to works which are easily accessible in America. It may be felt that vertebrates other than fish have been shamelessly neglected throughout this discussion. It must be confessed that they have, but in justification it can be pointed out that the principles which have been mentioned here apply to other forms of life as well as fish. Again it may be felt that applied limnology has been overstressed. In defense it may be maintained that the only valid classification of limnology is into the practical and the not practical rather than into pure and applied. The differences between pure and applied science lie not in the subject matter but in the attitude taken towards it.
BATTLE HELEN I., HUNTSMAN A. G., JEFFERS ANNE M. , JEPFERS G. W., JOHNSON W. H. and McNAIRN N. A. 1936. "Fatness, Digestion and Food of Passamaquoddy Young Herring". Jour. Biol. Bd. Can., 2: 401.
BLACK E. C. 1938. Oxygen Transport by theBlood of the Carp and the Common Sucker