CHAPTER XIX 



VARIATION AND HEREDITY 



IN organic variation, our interest centers on the 

 mechanism and forces concerned in the adjustment of 

 an organism to its environment. Variation signifies 

 change, and may be evidence of past and present influences ; 

 heredity gives a record of the past and certain promises 

 for the future. Both of these are important aspects of 

 evolution. 



Many theories have been advanced in explanation of 

 the facts of variation and heredity. No single theory re- 

 ceives at present universal sanction. Every reasonable 

 hypothesis merits careful consideration, and the present 

 widespread interest in experimental evolution makes it 

 particularly desirable to view new facts in an unpreju- 

 diced light. The limited scope of this book makes it 

 possible to include only a brief presentation of some of the 

 important facts and views, as an introduction to the sub- 

 ject; the fuller theoretical treatment and application 

 must be sought in the literature. 



VARIATION 



The capacity for variation is a fundamental possession. 

 It is universal with living organisms. Though heredity 



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