Evolution 325 



The blood of all vertebrates contains hemoglobin as the respiratory 

 pigment. The hemoglobin varies slightly, and this is best shown by 

 the hematin crystals which can be formed. The form of these crystals 

 shows relationships which parallel those shown by various structures. 

 Relationships among animals further can be demonstrated by serologi- 

 cal tests, that is, the antigen-antibody technique. In this test, serum of 

 an animal is injected into another animal, often a rabbit. After a time, 

 the rabbit develops antibodies to the injected serum. The serum of the 

 rabbit's blood is then withdrawn and mixed with sera from other 

 animals. The more closely related the animals being tested, the 

 greater the precipitate formed; distantly related forms show little pre- 

 cipitate. Thus the chemistry of the body also shows evolutionary de- 

 velopment. 



Paleontology. — Before the principle of evolution was generally 

 accepted, the presence of fossils in rocks was a disquieting fact. Some 

 believed that these represented early attempts at creating the "perfect 

 forms." The fossils were "rejects" of an earlier experimental period. 

 Other individuals saw them as remains of the great flood. 



When viewed in the light of the theory of organic evolution, these 

 remains of animals seem logical. Many represent ancient forms that 

 have died out as new progressive ones evolved. Some vertebrates 

 such as the horse and the elephant have a long fossil history. It is 

 possible to trace their gradual development. 



Classification. — The fact that the millions of present-day animals 

 and plants can be logically classified gives ample evidence that they are 

 related. Species are grouped into genera, genera into families, families 

 into orders, and orders into phyla. From the seeming chaos of animal 

 and plant forms, order may be established by a recognition of common 

 characters and descent. 



HOW EVOLUTION OCCURS 



As each generation of biologists tried to explain the multiplicity 

 of species, a number of theories developed to account for the variation 

 and relationships which so obviously existed. Even after Darwin had 

 successfully demonstrated that evolution was the key to species forma- 

 tion, the necessity of explaining how variations arose and how they 

 were ultimately translated into new species remained. Even today bi- 

 ologists still work on this problem and the final explanation has not 

 been achieved. 



