THE NA TURAL HISTOR Y OF MAN 33 



acquired traits. One attack of many diseases 

 protects against subsequent attacks of the same 

 disease. Thus, he who has recovered from an 

 attack of measles or whooping-cough, chicken-pox, 

 small-pox, scarlatina, plague, cholera, etc., is very- 

 much less liable than he was before his illness to 

 take the complaint. Medical men have thought 

 some of this immunity, this increased resisting 

 power, has been transmitted by parents to offspring, 

 and that thus, during the lapse of generations, the 

 power of resistance has grown in the races afflicted. 

 At first sight, therefore, disease does not seem to 

 afford conclusive evidence against the Lamarckian 

 doctrine. 



Examined more closely, disease affords evidence 

 which is absolutely conclusive. If ever an acquired 

 character is transmitted, one would expect acquired 

 immunity to be that character. It affects not merely 

 this or that organ, or this or that structure, but the 

 whole body. The entire organism undergoes a pro- 

 found change. Before infection and recovery a man 

 is capable of affording shelter and nutrition to millions 

 of microbes. Experience of, and recovery from, a 

 disease so alters him that his body becomes 

 poisonous to that particular species of microbe. 

 They perish in him ; and as a rule this profound 

 change of constitution endures for the rest of his 

 life. If, then, it can be proved that acquired 



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