Serology : The chemistry ofblood provides most convincing evidence of relationship. Serology deals with the study of different aspects of blood serum. Thus, a comparative study of the blood serum of different vertebrates confirms their phylogenetic relationships. The blood proteins of various mammals are similar to a large extent, but are sharply distinguishable from those of other vertebrates. The degree of similarity between blood proteins of various animals is tested by the antigen-antibody technique. It has been observed that when a foreign protein is inoculated into the blood of an animal, the latter produces a complex protein compound against the foreign inoculated protein. These compounds are called antibodies and the foreign inoculated protein is referred to as antigen. When a reaction between antibody and antigen occurs, a soft white precipitate will be formed. The strength of ppt. depends upon the concentration of antigen. One .of the remarkable features of this test is that the antibodies formed, against one antigen, can also react with antigens of other sources, if the latter is chemically similar to the first antigen. The serum containing the antibody is called antiserum. This antiserum containing . antigen of one animal can be tested with antigens of other animals in order to show their relationships. The amount of precipitation produced during this test indicares the relationships of the organisms. The serum of closely related forms is unable to cause precipitation; whereas those of distantly related ‘forms produced more of precipitation. As a result of precipitation testS, it has been established that blood proteins of the mammals are basically similar but their degree of relationships varies. The blood protein tests have shown that the man is nearest to the great apes. These tests indicate a close relationship among dogs, cats and bears and also among sheep. goats, dears and cows. All the above mentioned similarities show that animals have evolved from the same ancestors.