GENERAL PHENOMENA OF IMMUNITY 19 



many centuries a certain degree of racial immunity has been estab- 

 lished by virtue of the elimination of more susceptible individuals 

 and the survival of the more resistant. It is apparently true that 

 when an infectious disease first attacks a race, it is more virulent 

 than in those races where it is commonly found. The native African 

 when brought into contact with tuberculosis appears to be attacked 

 violently. The decimation of the population of Iceland after the 

 introduction of measles was one of the horrors of improved com- 

 munications ; subsequent epidemics of the disease in the same people 

 have been considerably less fatal. The introduction of syphilis into 

 the American Indian showed a virulence unknown among the Cau- 

 casians. Smallpox materially aided the Spaniard in his conquest of 

 Mexico. The negro is supposed to be less susceptible to yellow 

 fever than is the Caucasian, but careful investigation would make 

 it appear that in infancy and childhood acquired immunity is estab- 

 lished by mild attacks of the disease. The recent work of Love and 

 Davenport shows that among 500,000 troops illness was 19 per cent, 

 more frequent among negro than among white troops. The negro 

 was apparently less resistant to pneumonia, tuberculosis, and small- 

 pox than the white. The negro was more resistant to skin diseases, 

 but contracted venereal disease readily and suffered more than the 

 whites from extension and complications of venereal disease. Borell 

 has reported that the Senegalese are very susceptible to pneumonia 

 even in their own country. On transportation to France during the 

 World War more than 5 per cent, succumbed to pneumonia before 

 they had become acclimated, but in those who had been in France 

 two or three years, the death-rate from pneumonia was much re- 

 duced ; only 2 in 7000 troops died of pneumonia. Whether this 

 reduction is due to acclimatization or the early elimination of the 

 more susceptible is an open question. An apparent racial immunity 

 to malaria may be explained by the persistence of this disease for 

 many years following a childhood infection. In Australia, New 

 Zealand, and Tasmania during the years 1906-1908 there were only 

 about half the deaths per thousand inhabitants as the result of tuber- 

 culosis than occurred in Ireland, Norway, and Japan, during the same 

 period ; whilst the rate decreased regularly in the former countries 

 it increased in the latter. This appears to favor the idea of racial 

 differences of susceptibility, but a careful analysis of all the condi- 

 tions may show that climate, mode of life, and hygienic conditions 

 have a considerable influence. In the lower animals racial differ- 

 ence may be more satisfactorily illustrated. Common sheep are sus- 

 ceptible to anthrax, whereas the Algerian sheep seem to be immune. 

 The culex mosquito rarely harbors the malarial parasite, whereas 

 the anopheles are commonly infected. The field mouse is highly 

 susceptible to glanders, whilst the white mouse is immune. The 

 gray mouse is more resistant to streptococcus infections than is the 

 white mouse. The common rat is more resistant to anthrax than 

 is the white rat. 



