36 A STUDY IN HEREDITY 



there are some diseases against which immunity 

 may be acquired, but which, though prevalent, are 

 not deadly. Chicken-pox is an example. Against 

 it no evolution is observable, for chicken-pox is 

 neither more nor less severe in type when attacking 

 Englishmen than when attacking Polynesians. In 

 this case, in the absence of selection, the constant 

 acquirement of immunity has not tended to render 

 the race more resistant. It is clear, then, that 

 races grow resistant, not through the transmission 

 of acquirements, but solely through the survival 

 of the fit.^ 



1 It is to be noted that though one attack of certain diseases usually 

 confers immunity on the individual, yet, in such cases, the race never 

 attains to immunity. Each succeeding generation remains as sus- 

 ceptible as the preceding. Thus Englishmen are as susceptible to 

 infection by measles as Polynesians. But, since measles weeds out 

 those who cannot recover from it {i.e., those who -cannot acquire 

 immunity against it), the direction the evolution takes is towards an 

 increase oii}a.& power of acqtciring immunity. For that reason, though 

 Englishmen are as susceptible to infection by measles as Polynesians, 

 they recover from it much more easily. The only diseases against 

 which inborn immunity is, or tends to be, evolved, are those against 

 which the individual cannot acquire immunity — consumption for 

 example. When immunity against disease can be acquired by the 

 individual, then the /flw^r (j/' ac^razr/;;^ it is evolved in the race by 

 Natural Selection. When it cannot be acquired by the individual, 

 when one attack weakens rather than strengthens, then inborn 

 immunity is evolved in the race. In the one case the capacity to 

 recover from infection is evolved : in the other the capacity to resist 

 infection. In both cases the evolution proceeds wholly on lines of 

 Natural Selection, not on lines of the transmission of acquirements, 



