DARWIN'S NATURAL SELECTION 55 



that these ten are better able to withstand the 

 frost than their fellows. 



Breeders of white sheep who supply the 

 white wool market have a very tangible guide 

 — they kill every lamb that shows the least 

 tinge of black. But even here, nature is not 

 to be out-done. In Virginia there is — or at 

 least was in Darwin's day — a wild hog of pure 

 black. One of its staple foods was known as 

 the "paint-root." Any hog with the least 

 speck of white on its body was poisoned by 

 this root while its all-black brothers found it 

 a health-sustaining and succulent food. 



In an environment which remained constant 

 and where a species of animals had reached a 

 population which strained the limits of sub- 

 sistence — food supply — those offspring which 

 most closely resemble their parents, who had 

 won out in that environment, would again 

 succeed and be selected. While if the envir- 

 onment changed — became warmer or colder 

 for example^ — those descendants which hap- 

 pened to vary in a direction making them 

 better able to cope with the new conditions 

 would be selected for survival as against those 

 who resembled their parents, which parents 

 had survived in their day because they were 

 adapted to the prior environment. 



For exfimple, a country is well supplied with 



