60 THE NEXT GENERATION 



thirst, that some did not die. When food failed for birds in 

 snow-covered Ohio during the winter of 1913, farmers found 

 many of them dead in the frozen fields ; yet there is such 

 difference in the power of birds to endure hunger for longer 

 or shorter periods, and such variety in their ability to hunt for 

 food, that many of them did not die. 



In 1885, when typhoid microbes found their way into the 

 water supply of Plymouth, Pennsylvania, noo men, women, 

 and children had the fever and were threatened with death ; 

 but there is such a difference in the power of individuals to 

 resist disease that, although 1 1 4 persons . died, 986 of those 

 who were ill did not die, and hundreds of others who had used 

 the same contaminated water were not even affected by it. 1 



When seven men offered themselves for the yellow-fever 

 tests in Cuba in 1900, all were bitten by mosquitoes loaded 

 with blood from yellow-fever victims. But even against this 

 disease there is a difference in the power of the body to pro- 

 tect itself. One man escaped with no touch of the fever 

 whatever, six were ill with it, and one man died. 2 



Many other cases might be cited of persons who, overtaken 

 by calamity, have escaped death because they had some power 

 of endurance which those who died lacked. 



IV. SURVIVAL OF THE FITTEST 3 



We are now within sight of the fourth link of Darwin's 

 chain. He says that, because of the universal law of variation, 



1 Described in "Town and City," of the Gulick Hygiene Series, p. 107. 



2 Described in " Town and City," pp. 234235. 



3 Darwin first called this the law of natural selection, meaning that the 

 selecting was done without man's help ; but some people misunderstood 

 his meaning, so he adopted the phrase survival of the fittest, which Herbert 

 Spencer used. It means precisely the same thing as natural selection, is 

 easy to understand, and is the term in common use to-day. 



