DEFENSIVE FACTORS OF THE ANIMAL ORGANISM 191 



of the bacteria. The fact that the bacillus of avian tuberculosis — 

 whose natural host has a normal body temperature of 40° C. and above 

 — will grow on cu}.ture media at 40 to 50° C, whereas B. tuberculosis 

 of man can not be cultivated at a temperature above 40° C, would 

 seem to lend some support to this view. The difference between warm- 

 and cold-blooded animals has already been noted. The necessity for 

 cultural adaptation, too, would seem to be borne out by the great 

 enhancement observed in the virulence of certain microorganisms for 

 a given species after repeated passage through individuals of this species. 



Racial Immunity. — Just as differences in susceptibility and im- 

 munity exist among the various animal species, so the separate races or 

 varieties within the same species may display differences in their reac- 

 tions toward pathogenic germs. Algerian sheep, for instance, show 

 a much higher resistance to anthrax than do our own domestic sheep, 

 and the various races of mice differ in their susceptibility to anthrax 

 and to glanders. 



Similar racial differences are common among human beings. As a 

 general rule, it may be said that a race among whom a certain disease 

 has been endemic for many ages is less susceptible to this disease than 

 are other races among whom it has been more recently introduced. The 

 appalling ravages of tuberculosis among negroes, American Indians, and 

 Esquimaux, bear striking witness to this fact. Conversely, the compar- 

 ative ipimunity of the negro from yellow fever, a disease of the greatest 

 virulence for Caucasians, furnishes further evidence in favor of this 

 opinion. It must not be forgotten, however, in judging of these rela- 

 tions, that the great differences in the customs of personal and social 

 hygiene existing among the various races may considerably affect the 

 transmission of disease and lead to false conclusions. 



In so far as the statement made above is true, however, it seems to 

 indicate that the endemic diseases have carried in their train a certain 

 degree of inherited immunity. 



In other cases ' — as in the instance of the malaria-immunity of 

 negroes — the resistance seems to be acquired rather than inherited, for, 

 as Hirsch was first to note, death from this disease occurred frequently 

 among the children, while adult negroes were rarely attacked. 



Differences in Individual Resistance. — In bacteriological ex- 

 perimentation with smaller test animals, a direct ratio may often exist 

 between body weight and dosage in determining the outcome of an 



■ Hahn, in KoUe und Wassermann, loc. cit. 



