66 THE FOUNDATIONS OF SCIENCE 



part in spreading the infection. Moreover, the 

 various races of mankind manifest different degrees 

 of susceptibility to different forms of infection ; and 

 in a few cases persons have been found who, like 

 some animals, are apparently immune to the conse- 

 quences of infection, but are nevertheless acting as 

 hosts to the parasite, and are spreading the disease 

 amongst those with whom they come in contact. 



The application of Pasteur's results to the pre- 

 vention of microbic infection in the operations of 

 surgery by Lord Lister (1827-1912), in conjunction 

 with the discovery of anaesthetics, made possible the 

 surgical advances which have done so much to 

 relieve human suffering, and have changed our hos- 

 pitals from institutions in which recovery was at 

 best doubtful into the most effective agencies for the 

 preservation of human life. 



The result of the improvements in hygiene, 

 medicine, and surgery is best measured by the drop 

 in the annual death-rate in cities from some eighty 

 per thousand of the population two hundred years 

 ago to the present value of about fifteen per thousand. 

 Not only does this change mean lives prolonged 

 which otherwise would have been sacrificed, but it 

 means also increased health and strength for some 

 who, in old conditions, might have survived with 

 impaired vitality. Nevertheless, we must not forget 

 that this alteration in the incidence of death and 

 disease involves dangers of its own. Mental and 



