CHAPTER II 



VARIATION 



1. THE MOST INVARIABLE THING IN NATURE 



IN the introductory chapter it was shown that "or- 

 ganic resemblance based on descent," by which is 

 meant heredity, is due principally to the fact that off- 

 spring are material continuations of their parents and 

 consequently may be expected to be like them. The 

 fact that this is the case in the great majority of in- 

 stances has given rise to the popular formula, "like 

 produces like," as a rule of heredity. 



But this formula by no means always fits the facts. 

 Like often produces something apparently unlike. 

 For instance, two brown-eyed parents may produce a 

 blue-eyed child, although brown-eyed children are more 

 usual from such a parentage. It is a common expe- 

 rience, indeed, for breeders of plants and animals to 

 meet with continual difficulties in getting organisms to 

 "breed true." 



On the other hand, it is exactly these variations which 

 so constantly interfere with breeding true that fur- 

 nish the sole foothold for improvement. If all organ- 

 isms did breed strictly true, one generation could not 

 stand on the shoulders of the preceding generation, 

 and there would be no evolutionary advance. 



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