CHAPTER 



20 



THE EVOLUTION 



OF 

 ANIMAL LIFE 



The term evolution connotes a gradual and continuous process of change, 

 over long periods of time. This process of change is apparently a universal 

 phenomenon; we know of nothing that is unchanged or unchanging. Com- 

 pelling evidence from studies in astronomy indicates that the universe, to- 

 gether with that infinitesimal part of it which comprises our solar system, has 

 undergone evolution on a cosmic scale through countless aeons. The study 

 of geology presents equally strong evidence that the planet Earth has been 

 subject to continuous evolutionary changes in its physicochemical features. 

 In the course of this inorganic evolution, conditions consonant with life arose 

 on earth; and at some subsequent time living systems appeared. Since their 

 origin, these systems have undergone the gradual alterations in form and 

 function which constitute organic evolution. Comparative studies of modern 

 plants and animals, and of the fossil remains of their extinct ancestors, fur- 

 nish abundant evidence of the reality of organic evolution. Although the 

 fact of organic evolution is established to the satisfaction of scientists, the 

 factors operating to cause or condition evolutionary processes are less easily 

 understood. One of the most challenging problems of the present day — one 

 which has intrigued several generations of biologists — is that of determining 

 the mechanisms or processes through which organic evolution has come about. 



In previous chapters the accounts of the various phyla of animals have 

 been presented from the evolutionary point of view. The nature of existing 

 animals, and the anatomical evidence of their phylogenetic interrelationships, 

 can be reasonably accounted for only by assuming that organic evolution has 

 occurred. In the present chapter we shall discuss the origin of life, consider 

 the nature of the several lines of evidence supporting the concept of organic 

 evolution, and present some of the theories that have been advanced to 

 explain the available facts. 



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