CHAPTER XXIV 

 VARIATION AND HEREDITY 



INTRODUCTORY STATEMENT 



In the preliminary discussion of "The Causal Factors of Organic 

 Evolution" (chapter xv) it was pointed out that variation and heredity 

 are to be considered as perhaps the primary factors of evolution, and 

 that natural selection is surely an important secondary directing factor. 

 The Lamarckian factor, isolation, orthogenesis, etc., may be con- 

 sidered as tertiary factors, if they are real factors at all. The main 

 task of genetics is to study and to explain the primary factors of evolu- 

 tion: variation and heredity. Both of these factors are essential to 

 evolution, for evolution is defined as descent with modification. Im- 

 agine descent without modification, and we should have heredity alone. 

 Offspring would be exactly like parents, and there would be a world of 

 life at a standstill except that each generation would have to start back 

 at the beginning of the life-cycle. Each life-cycle of a member of a 

 species would be identical with that of every other. Imagine, again, 

 modification without descent, and we should have the variation factor 

 alone. Individuals of a species would vary in all sorts of ways and 

 would be entirely different in successive generations, for there would be 

 no genetic continuity between the characteristics of one generation and 

 the next. That would be a random world without sequence or possi- 

 bility of progress, for nothing gained by one generation could be trans- 

 mitted to the next, and there could be no accumulation from generation 

 to generation of advantageous changes gained by individuals. Neither 

 variation alone nor heredity alone could produce racial progress, but 

 the two together, with the aid of selection, and possibly some other 

 factors, seem to constitute an ideal mechanism of evolution. 



In attempting to present the facts of variation and heredity, the 

 teacher of genetics inevitably finds himself in a peculiar dilemma as 

 to which of the two subjects to consider first. Shall he deal with varia- 

 tion first and consider heredity as a sort of persistence beyond the 

 immediate generation of certain types of variations, or shall he con- 

 sider heredity first and deal with variation as a sort of aberration of 



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