INFECTION. 119 



tions do not contradict the validity of a rule, though they would a 

 law of nature. The same is true of pleuro-pneumonia. 



Another form of acquired immunity is that produced by inocula- 

 tion, as in variola, pleuro-2)neuin()nia, and, according to Pasteur, in 

 chicken-cholera and anthrax. By this procedure the artificial pro- 

 duction of a mild form of the disease is able to prevent natural 

 infection. According to Pasteur, we have a still mure striking 

 phenomenon in anthrax. 



His experiments led him to affirm that the inoculation of ani- 

 mals with a virus which has been attended by a certain series of arti- 

 ficial cultivation, made in a peculiar manner, was capable of prevent- 

 ing the eniption of the diseases when inoculated with blood of a 

 most virulent character. 



He says : " Fifty sheep were placed at my disposition for inocu- 

 lation. Twenty-five of them were inoculated with a known virulent 

 nuiterial, and twenty-five were vaccinated with a prepared attenuated 

 virus. The first twenty-five all died. Two weeks subsequent to 

 the vaccination the other twenty-five were reinoculated witli a vims 

 of known malignancy : they all resisted infection. (This subject is 

 of the greatest importance, but governmental support is necessary 

 before we can thus advance the science of preventive medicine in 

 this country.) 



Again, there is a species of acquired immunity of another kind. 

 It is known that syphilis is slowly becoming less virulent among the 

 people of the civilized world. AVe are less open to infection, and 

 the disease is not so malignant. People acquire a certain degree of 

 immunity against the poisonous action of certain drugs, by gradually 

 mcreasing the amount taken, as illustrated both by the sick-bed and 

 by acquired habits in the use of opium, arsenic, or even those luxu- 

 ries tobacco and alcohol. 



The first cigar makes us sick, but finally we can smoke all day. 

 So it is with alcohol : we can gradually adapt ourselves to larger and 

 larger quantities. 



To my mind there is but one explanation to all this, and tliat of a 

 very philosophical (i. e., theoretical) nature, although we have observa- 

 tions and experiences in the natural worhl whidi serve to confirm 

 our theory. 



The two great forces which Darwin claims have exerted an in- 

 fluence upon the changes that centuries have produced in the forms 

 and characters of any species of animals, are — 



1. The struggle for existence. 



2. The gradual adaptation to natural conditions and surround- 



