EVOLUTION OF MAN 171 



connection with disease is seen in the relation of savage 

 peoples to certain mild diseases prevalent among civilized 

 races. Measles is not very serious in civilized communities. 

 It has long been a common disease. Those, in the past, 

 who were unable to resist this disease have died ; and, as 

 it is mostly a disease of children, they have died before 

 reaching adult life and becoming parents. They have, 

 therefore, not transmitted to the next generation their consti- 

 tution with its slight powers of resistance to this disease. 



Many of those children, on the other hand, who have 

 been strong enough to survive attacks of measles have 

 reached maturity and have handed down to their children 

 something of their natural ability to resist its attacks. There 

 has thus been developed among civilized peoples a consider- 

 able degree of power to throw off this disease. 



But among savage races, the North American Indians, 

 for example, measles has often been a fearful scourge. It 

 has not been prevalent among them for many generations, 

 as among the white peoples, and they have not acquired 

 through natural selection the ability to resist it. Therefore, 

 it was but natural that when introduced among them it 

 should wipe out whole communities, slaying adults as well 

 as children. 



Were vaccination now to be universally given up, it is 

 possible that small pox would be more dangerous than it 

 used to be before Jenner found a way to save us from its 

 ravages ; though perhaps vaccination has not been used 

 long enough to allow much deterioration in the power of 

 resistance to small pox which was to a degree acquired dur- 

 ing those centuries when the disease had free course. 



How far will the deterioration which results from par- 



