Abstract
A comparison of the amino acid sequences of the cytochromes c from horse, pig, rabbit, chicken, and man leads to the conclusion that all have derived phylogenetically from a common primordial cytochrome c. The significance of the constant and of the variable features of these sequences with regard, on the one hand, to the structural aspects of the functional activities of the protein, and on the other, to the evolutionary relations between the various cytochromes c is discussed.