CHAPTER SEVEN 



The Two Main Problems 

 of Evolutio7i 



Evolution was defined above as "descent with modification." Now hered- 

 itary modifications must therefore be the basic materials for evokition 

 and the manner of origin of hereditary variations must be the first major 

 problem of evolution. Lamarck attempted its solution with the theses that 

 the action of the environment on an organism tends to produce adaptive 

 modifications, and that these acquired characters are inherited. He failed 

 because both of these theses are easily disprovable. 



Darwin side-stepped this problem. He simply accepted without ex- 

 planation the observed fact that organisms do vary one from another. 

 He did not distinguish between inheritable and non-inheritable variations. 

 Darwin's problem was the action of natural selection in the formation of 

 new species, given a variable progenitor. This is the second major problem 

 of evolution: how the varying arrays of organisms become sorted out into 

 species and higher categories. The role of selection and of other factors 

 in this process must be analyzed. The differences between the several 

 subspecies of a species may be quite as great as those between species of 

 a genus, but the former interbreed freely and blend from one to another 

 in nature while the latter generally do not. The origin and nature of the 

 barriers which account for this difference are therefore of great impor- 

 tance for evolution. 



The two main problems of evolution, therefore, are: 



( 1 ) Tlie origin of variation, and 



(2) The origin of species (and of higher categories). 



With these problems the subsequent chapters will be largely concerned. 

 But first, it may be profitable to survey the actual course of evolution as 

 shown by the paleontological record and by other data. 



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