THE EVOLUTION OF ANIMAL LIFE 



from an ancestral protoplasmic system which possessed these basic capacities. 

 Taken alone, this general uniformity of fundamental functions is less im- 

 pressive than the many special cases of structural and distributional evidence; 

 it does, however, serve to corroborate the evidence from other fields. 



There is one very impressive line of specific evidence for evolution, from a 

 field related to physiology. It has been pointed out that one aspect of the 

 individuality of organisms is the specificity of the proteins which each 

 organism produces. This protein specificity is the basis of the development 

 of immunity to certain diseases in vertebrates, by the production of specific 

 antibodies in the blood which react very precisely against foreign proteins. 

 The immune reaction is one of the important mechanisms by which the 

 body is protected against invasion by microorganisms, but when suitably 

 stimulated a vertebrate produces antibodies against foreign proteins from any 

 source. Developments in the field of serology have made available a 

 technique, involving this capacity for immunization, that is very useful in 

 the determination of degrees of interrelationship between different organisms. 

 The technique and its application may be briefly described as follows. A 

 rabbit is immunized against the blood proteins of a sheep, for example, by 

 repeated small injections of proteins from the serum of sheep blood. Subse- 

 quently, the antibodies that have formed in the blood of the rabbit will 

 react very strongly when in contact with sheep serum under controlled 

 conditions. They will also react, though less positively, against serum 

 proteins from the goat, the cow, the horse, or even man. The basis of the 

 varying degree of reactivity is the varying degree of chemical similarity 

 between the proteins of the diflferent sera. It should follow that the blood 

 proteins of closely related animals are more alike than those of distantly 

 related forms; and the serological tests should make it possible to express 

 in some quantitative terms lie relative closeness or distance of the relationship. 



Extensive tests of the serological technique have borne out its usefulness 

 in this connection. The proteins of birds have proved to resemble those of 

 other birds very closely, those of reptiles less closely, and those of amphibians 

 not at all closely. Mammalian proteins are more similar to those of reptiles 

 than to those of birds; human proteins react more strongly against proteins 

 from other primates than against those from other mammalian types; and 

 so on. In all animals the degree of similarity between the proteins of 

 different groups, as shown by serological methods, confirms the degree of 

 relationship between the groups inferred on the basis of other evidence. 

 The technique is also applicable to purified protein extracts of the body 

 fluids of invertebrates, and it has been demonstrated that the chemical 

 affinities of chordates lie with hemichords and echinoderms rather than with 

 any other invertebrate groups. This technique should prove very useful in 

 resolving problems of phylogenetic affinities in many instances where other 

 evidence is equivocal. The existence of degrees of chemical similaritv 

 between the proteins of different kinds of animals supports very stronglv the 

 inference of different lines of evolutionary descent from common ancestors. 



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