Several investigators have tested for the presence or absence of the blood characters of humans (A, B and O; M and N) in the bloods of the anthropoid apes and lower monkeys. From their results, coupled with evidence from other sources, it appears that there are 3 categories of genes which make for resemblances and differences between related spp.: genes in each species which are homologous, i.e., produce indistinguishable effects; genes which are related, but not identical, their effects being similar but not identical; and genes which are not alike and whose effects make for the differences between one species and another.[long dash]In backcross individuals following a species cross in both doves and pigeons, it was possible to isolate cellular characters which distinguish one of the species of the cross from the other. Although certain of the characters, which set one species apart from another, are in turn shared either in part or in toto with still other related spp., yet the present evidence indicates that a single species probably possesses certain antigenic characters, hence the causative genes, not found in any other species.[long dash]A division of the species of Columba has been noted, so that those of the Old World form a group in contrast to those of the New World, and vice versa.