38 PHYSICAL BASIS OF CIVILIZATION 



vorable traits, the former is far more efficient and 

 rapid as a factor in the extermination of either 

 unfavorable traits or varieties of organisms affected 

 by them. Panmixia is the important, decisive 

 element by which extermination is accomplished. 



If unfavorable traits, or varieties of creatures 

 afflicted with them, can be exterminated by the 

 direct results of these traits alone, that is to say, 

 without the occurrence of panmixia, then such 

 traits must be of so excessively destructive a nature 

 that no two creatures of opposite sex afflicted with 

 them can survive long enough for a chance of repro- 

 duction. 



If in each generation, during many generations, 

 but one pair of the afflicted type remains alive, that 

 is entirely sufficient to preserve the variety, and even 

 to increase and accentuate the unfavorable traits, 

 provided only that there is no panmixia. For in 

 each generation the offspring of the one pair would 

 then inherit these unfavorable traits from both 

 parents, therefore in increased amount and accen- 

 tuated quality. So that these qualities unfavorable 

 to adaptation would continue to develop more and 

 more ad infinitum, provided only panmixia can be 

 prevented. 



But can this be done? Excepting only man's 

 power to control domesticated animals and plants, 

 is there any agency in nature that can prevent 

 closely related varieties from interbreeding? So 

 far but one instance, presently to be mentioned, has 



