A CCORDING to current doctrines the mammals are the product of an evolutionary development which is traced back through the lower classes of vertebrates and is usually considered to have had its origin among the invertebrate phyla. In recent years the respiratory function of mammalian blood has been studied in great detail. Considered as a physico-chemical system concerned with the transport of oxygen and carbon dioxide, its characteristics are now well known and in many ways satisfactorily understood (35). Recent publications make it possible to compare these properties of the blood in at least one or more representatives of the various classes of lower vertebrates and of certain invertebrates. One can consequently inquire what changes in the characteristics of the blood parallel the supposed evolutionary development. In considering the evolution of the vertebrates certain general tendencies are apparent which have an obvious relation to the respiratory function of the blood. In the first place, the change from an aquatic to a terrestrial habit of life involves a profound alteration in the method of aerating the blood and in the conditions to which the blood is exposed in the respiratory organs. In the second place, the general tendency throughout the development has led to organisms increasingly capable of intense activity. This has required the development of more effective arrangements for the circulation of the blood and the transport of gases by it in order to care for greater metabolic requirements. One of the factors which has favored the greater activity of which the birds and mammals are capable is the development of a constant, high body temperature which has at the same time fixed rather definitely one of the conditions under which the respiratory function of the blood takes place. The transport of oxygen by vertebrate blood is due practically entirely to the hemoglobin present in it. Consequently the conditions under which oxygen is carried depend upon the specific properties of this substance. The transport of carbon dioxide, on the other hand, while depending in large part on reactions in which hemoglobin is involved, is a less specific phenomenon, since other proteins such as serum albumen and globulin and certain inorganic substances play a considerable role. The transport of carbon dioxide is consequently dependent in its details upon the general processes whereby the kidney and other organs regulate the composition of the blood. The hemoglobins are substances which may be decomposed into hematin and a protein, globin. The former compound contains iron in complex combination with protoporphyrin. It is to the hematin fraction that the transport of oxygen by hemoglobin is due, oxygen being combined with hemoglobin in strict

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