THE EVOLUTION OF ANIMAL LIFE 



vast array of facts is provided by the theory of evolution. Mani-cind is not 

 exempt; the evidence clearly indicates that man originated as an evolutionary 

 offshoot from the mammalian stock that produced the higher apes, and that 

 man's more remote ancestry is in common with that of all other vertebrates. 



With the historic fact of organic evolution thus established, and its general 

 course indicated, the factors that have conditioned evolution remain to be 

 explained. A full explanation of these factors has not been accomplished, but 

 it is evident that variation and heredity are the beginning of evolutionary 

 modification; that isolation of incipient stages in small groups is important; 

 and that natural selection has directed evolution along pathways which fitted 

 the organism to its environment. These have certainly been among the 

 factors conditioning organic evolution. No case has been established for 

 the Lamarckian theory; the principal aspects of Darwin's theory of natural 

 selection, interpreted in the light of modern knowledge of genetics and 

 ecology, serve as a basis of a reasonable theory describing the probable 

 mechanisms of evolutionary change. 



As a fundamental generalization of biological science, only the Cell Theory 

 can rank with the Theory of Organic Evolution. Developing somewhat 

 earlier in the history of biology, the growth of the Cell Theory was correlated 

 with great advances in our understanding of biological science and in its 

 practical application. Attaining prominence and widespread acceptance 

 some years later, the Theory of Organic Evolution has had a similarly uni- 

 versal impact on biological thought and interpretation, and its importance 

 continues to grow with the accumulation of new evidence from many areas of 

 biological science. 



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