28 SOCIAL EVOLUTION 



and all the chickens had a spotted coat. So by recording 

 the variation in any trait we can tell whether it has a 

 survival value and whether natural selection has caused 

 its uniformity. This is the reason why we find that all 

 the individuals of any species are more or less alike, 

 because variation outside of certain safe limits is hazard- 

 ous. 



A rain storm once washed a large number of sparrows 

 out of their nests. An observer gathered the injured 

 sparrows together and tried to revive them. A large 

 number of the birds recovered but some did not survive. 

 Measurements of all the dead and revived sparrows were 

 taken and the curve showing their distribution (the fre- 

 quency with which each measure occurred) was plotted. 

 Then the measurements of the revived sparrows were 

 taken separately and their distribution was plotted. It 

 was found that the measurements of the surviving spar- 

 rows varied less from the average degree of the charac- 

 ter measured than did the measurements of the dead 

 sparrows ; that is, the dead sparrows were more variable. 

 The curve representing the birds which survived was a 

 narrower and steeper curve, which showed that the birds 

 killed were more largely the unusual, the extreme, those 

 widely differing from the average. We find pretty gen- 

 erally that the extreme variates from the normal are less 

 likely to survive the dangers of their surroundings. The 

 more normal are more likely to survive. 



By examining the smooth curve in figure 2 which rep- 

 resents the approximate distribution of the frequencies 

 at which different heights occur among ten and one-half 

 year old American boys, one can better understand this 

 normal order. If there should be an epidemic of scarlet 

 fever and all of the boys contracted the disease, 30 



