SOME CONSEQUENCES OF EVOLUTIONARY RELATIONSHIP 369 



plasma instead of being contained in special cells. Furthermore, it occurs 

 in organisms built on a structural plan very different from that of the 

 vertebrates, so that there can be no question of close relationship. 



Biochemical determinations of kinship. Since we know that each 

 kind of zygote is predestined to develop into a particular kind of adult 

 organism, it follows that the differences between organisms are inherent 

 in the protoplasm of the zygote and of the cells formed from that zygote. 

 These protoplasmic differences are the result of differences in the gene 

 complex, just as truly as are the more obvious morphological and onto- 

 genetic differences. Protoplasmic differences should manifest themselves 

 in the nature of the chemical products of cell activity. Similar protoplasms 

 should make similar products; unlike protoplasms should make unlike 

 products. If we could test the degree of likeness in protoplasms and their 

 products, it should give us a measure of the degree of kinship between the 

 organisms tested. 



Some very delicate tests of this sort have actually been devised and 

 applied to a wide range of species. The most successful approach has been 

 through the study of immunology, which deals with the reactions of 

 blood to toxic foreign substances, chiefly proteins. Immunology is one 

 of the borderline sciences with applications in many fields — medicine, 

 physiology, biochemistry, genetics, and, as it proves, evolution. 



In our treatment of the human body we have already touched upon the 

 defenses against disease that are provided by the blood. One of these is 

 the ability of the blood to form antibodies capable of neutralizing toxic 

 foreign proteins and certain polysaccharides. These antibodies are formed 

 after the entry of the foreign substance as a response to its presence and 

 are specific — that is, made to order for each different protein or poly- 

 saccharide. Any substance that can evoke this reaction is called an 

 antigen. When we consider that for each of thousands of possible antigens 

 a counterpart antibody can be formed in the blood of a single animal, the 

 wonderful nature of this property of blood becomes apparent. It seems 

 probable that the antigen must by its own chemical configuration in some 

 way guide the formation of its counterpart antibody. Antibodies act in 

 various ways. Those which furnish the basis for the tests of relationship 

 here described are called precipitins, for they combine with the toxic 

 foreign molecules to form an insoluble precipitate, thus rendering them 

 innocuous. Analysis of precipitates formed with antigens of known chem- 

 ical composition shows that at least some precipitins are typical serum 

 proteins. 



The general procedure for making precipitin tests of biochemical relationship 

 is as follows. Some particular antigen-containing material is chosen as the basis 

 with which others may be compared. Suppose that we select human serum as 



