CHAPTER XI 



EVOLUTION AGAINST DISEASE 1 



The decisive evidence afforded by disease Malaria Tuberculosis 

 Measles Whooping-cough Small-pox Dysentery Diarrhoea 

 Enteric fever. 



213. WE may now resume the main thread of our argu- 

 ment. The discussion in the last chapter will have had the 

 effect of clearing away some confusions as regards terms. 

 Probably, for instance, we are now agreed that " congenital " 

 syphilis and " congenital immunity to syphilis " are not, as is 

 so often implied in medical literature, true inborn characters, 

 but merely acquirements made in utero of exactly the same 

 kind as the mother's acquirements. We saw that diseases 

 afford a means of testing the three principal doctrines of 

 heredity. On any doctrine of heredity some changes they 

 must produce in races that have been long and severely 

 afflicted by them. What doctrines of heredity, then, does that 

 change confirm ? 



214. Is the doctrine of the spontaneous origin of variations 

 (and, therefore, the doctrine of evolution by Natural Selection) 

 the true doctrine ? Do all lethal and prevalent diseases 

 cause, through the survival of the fittest, an evolution of 

 immunity in the race the diseases against which immunity 

 cannot be acquired, an evolution of inborn immunity ; the 

 diseases against which immunity can be acquired an evolution 

 of the power of acquiring immunity 1 Do non-lethal diseases 

 produce no effect whatever on the race ? 



215. Or is the doctrine of the causation of variations by 

 the transmission of acquirements the true doctrine ? Do all 

 prevalent diseases, lethal or non-lethal, against which im- 

 munity cannot be acquired produce racial degeneration ? Do 

 all diseases, lethal or non-lethal, against which immunity can be 

 acquired, produce an evolution of inborn immunity ? 



1 The bulk of this chapter has been taken with some modification 

 from the Author's work, The Present Evolution of Man. 



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