78 IOWA ACADEMY OF SCIENCES. 



older animals. The same thing holds true in the human race, and very properly 

 the term "children's diseases" is used for a number which are common to chil- 

 dren and not older people. In older people some diseases are rapidly fatal, while 

 other persons are exempt. The negro race is much more subject to tuberculous 

 troubles than the white race. Small-pox is much more severe in dark races than 

 fair skinned. The negro and latin races of tropical climates are more exempt 

 from yellow fever than northern people. It is said on good authority that 

 where cholera is indigenous, that the percentage of death is smaller than where it 

 is not." 



Dr. Sternberg says:^* "The tendency of continuous or repeated exposures to the 

 same pathogenic agent will evidently be to establish a race tolerance; and there is 

 reason to believe that such has been the effect in the case of some of the more com- 

 mon infectious diseases of man, which have been noticed to prevail with special 

 severity when first introduced among a virgin population, as in the islands of the 

 Pacific, etc." 



In bacterial diseases of plants the same thing has been noticed; every horticul- 

 turist is familiar with the fact that some varieties of apples are more subject to the 

 attacks of blight {Bacillus amylovorus) than others. It is certain that this sus- 

 ceptibility muft depend on certain conditions in the animal body or plant, either 

 favorable or unfavorable for the development of the pathogenic organism. It may 

 be that the temperature fluids, of the body, or the blood serum as Buchner^", Han- 

 kin'^'' and others claim have valuable germicidal properties. The products of cer- 

 tain glands like the thymal are said to afford immunity. Fokker'"'* has recently 

 published results which show that fresh milk has germicidal properties. It may 

 be that the tissues of plants or structure of parts of cells, or the fluids of the plant 

 are different from those attacked. Immunity from subsequent attacks varies in 

 different diseases, and the time also varies. The theories advanced for immunity 

 are the exhaustive theory, which holds that the organism growing in the animal 

 exhausts the supply of some substance essential for its growth. But this has been 

 set aside by Sternberg*'^ and others. • 



The retention theory, proposed by Chauveau: This investigator holds that cer- 

 tain products formed during the development of the germ in the body accumulate 

 and are retained. The vital resistance theory of Sternberg*^'- explains immunity upon 

 an acquired tolerance to the toxic products of pathogenic bacteria. There is much 

 evidence to support this theory. The theory of phagocytosis, first prominently 

 advocated by Metchnikoff, and sometimes called the Metchnikoff theory, is based 

 on the fact that bacteria in the blood are picked up by the leucocytes. That 

 immunity depends upon the power possessed by these leucocytes in destroying bac- 

 teria. There is no longer any doubt that the leucocytes pick up and destroy micro- 

 organisms in animals, for since the germs found m these leucocytes are often corro- 

 ded, and finally disappear entirely when health has been restored. Hankin*^ believes 

 there is found in the body, as a result of disease, autitoxine, and these substances 

 which are found in immune animals, he calls "defensive proteids;" these are das- 



57 Sims Woodhead, Bacteria and their Products, Chapters VIII and IX. 



58 Manual of Bactierology, p. 927. 



59 0entralblatt fur Bakt, and Parasitenkunde Vol. V, p. 817; Vol. VI, p. 1. 



60 Proc. Royal Soc. London, 1890, May 22. 

 60a Fortschritt der Medicin Vol. VIII, p. 7. 



6i.Iournal of Medical Sciences, April, 1881; Manual of Bacteriology, p. 238. 



62 American Journal of Medical Sciences, April, 1881. Manual of Bacteriology, 

 p. 2J0. 



63 See Sternberg's Manual of Bacteriology, p. 260. 



