18 GENETICS 



The most invariable thing in nature is variation. 

 This fact is at once the hope and the despair of the 

 breeder who seeks to hold fast to whatever he has 

 found that is good and at the same time tries to find 

 something better. Variation is a veritable Pandora's 

 box and the chaos that would ensue if it were not con- 

 fined within certain predictable limits can hardly be 

 imagined. Obviously the entire subject of variation is 

 intimately and inevitably bound up with any considera- 

 tion of genetics, for when the similarities and dissimi- 

 larities between succeeding generations are clear, then 

 heredity can be explained. 



2. THE UNIVERSALITY OF VARIATION 



Much of the variation in nature is patent to the most 

 casual observer, but it requires a trained eye to see the 

 universal extent of many minor differences. A flock of 

 sheep may all look alike to a passing stranger, but not 

 to the man who tends them. A dozen blue violet plants 

 from different localities might easily be identified by the 

 amateur botanist as belonging to the same species when, 

 to a specialist on the genus Viola, unmistakable differ- 

 ences would doubtless be clearly apparent. 



"Identical twins," for example, constitute so marked 

 an exception to the universal rule of variational differ- 

 ence that they challenge the attention at once, yet even 

 here upon critical examination there appears some de- 

 gree of variation. 



The fact that every attempt at an intimate acquaint- 

 ance with any group of organisms whatsoever invariably 



