1. THE MOST INVARIABLE THING IN NATURE 



IN the introductory chapter it was shown that 

 "organic resemblance based on descent," by which 

 is meant heredity, is due principally to the fact that 

 offspring 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 

 instances 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 

 experience, indeed, for breeders of plants and animals 

 to meet with continual difficulties in getting or- 

 ganisms to "breed true." 



On the other hand, it is exactly these variations 

 which so constantly interfere with breeding true 

 that furnish the sole foothold for improvement. If 

 all organisms 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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