CHAPTER VIII 



THE FACTORS OF EVOLUTION 



Variation along definite lines and Natural Selection are un- 

 doubtedly important factors of evolution Whether or not 

 sexual selection is a factor we are not yet in a position to decide 

 Modus operandi of Natural Selection Correlation an im- 

 portant factor Examples of correlation Correlation is a 

 subject that requires close study Isolation a factor in evolu- 

 tion Discriminate isolation Indiscriminate isolation Is 

 the latter a factor? Romanes' views Criticism of these 

 Indiscriminate isolation shown to be a factor Summary of 

 the methods in which new species arise Natural Selection 

 does not make species It merely decides which of certain 

 ready-made forms shall survive Natural Selection compared 

 to a competitive examination and to a medical board We 

 are yet in darkness as to the fundamental causes of the 

 Origin of Species In experiment and observation rather 

 than speculation lies the hope of discovering the nature of 

 these causes. 



WE have so far considered three factors 

 of evolution. The first of these is 

 the tendency of organisms to vary 

 along definite lines. This is a most 

 important factor, because, unless variation occurs 

 in any given direction, there can be no evolution 

 in that direction. Variations are the materials 

 upon which the other factors, or causes, of evolu- 

 tion work. The second great factor is natural 

 selection. Natural selection may be compared 



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