ALTERNATIONS IN DISEASES. 571 



aged. The more fatal a disease among youth, the quicker is the work 

 of the law of selection, and the more prompt the diminution of the 

 malady. If a first invasion, for instance, destroys a moiety of the 

 population below marriageable age, the survivors should be very 

 little liable, in their physical or physiological conditions, to the dis- 

 ease, and the children born to them will profit by their immunity. If 

 the disease is less fatal, the purification will be less. We thus dis- 

 cover, I do not say the cause, but a cause why pestilences and other 

 very serious maladies attack populations at intervals, and are, as it is 

 said, epidemic ; while certain diseases less serious, even among mala- 

 dies which attack youth, rule from year to year in a mode more con- 

 tinuous. 



Such are the clear laws — one might add the rigid laws — which rule 

 in diseases, to produce aggravation or diminution, independently of 

 all these natural circumstances. Without doubt there may be other 

 circumstances, physical or physiological, and physicians may discover 

 preventive or curative means which exert influence upon them. But 

 the incessant effect of heredity, and of the law of selection, exists, not- 

 withstanding ; and, when other influences cannot be demonstrated, we 

 may be assured that heredity and selection perform their part. 



We now see that the efficacy of preventive means, such as vaccina- 

 tion, should also vary. When Jenner discovered the utility of vac- 

 cination, the small-pox had in a slight degree lost, in Europe, its 

 primitive intensity. The people who then existed proceeded from 

 many generations which could, thanks to the process of selection, pass- 

 ably resist the epidemic. Individuals were not so readily affected as 

 at the origin of the disease, or, if they had the disease, they succumbed 

 to it in a smaller proportion ; or, yet again, those who survived 

 rarely contracted the disease a second time. It was supposed that 

 those who had the disease by inoculation were sheltered from a repe- 

 tition, and the dangerous practice of inoculation would not have con- 

 tinued, but for this opinion. Vaccination, then, came at an epoch 

 when the European population found itself in ameliorated conditions 

 with regard to epidemic small-pox. Practised with ardor, it had the 

 effect to render small-pox very rare. But, precisely because it had 

 become rare in the generation which immediately succeeded Jenner, 

 in the generation which issued from that was found a majority com- 

 posed of persons who had not been exposed to the epidemic. Among 

 them must have been some persons who naturally, or by atavism, 

 were disposed to take the infection. From that cause arose a certain 

 renewed sensitiveness (recrudescence), which vaccination could less 

 easily control. 



In other words, after two or three vaccinated generations, the 

 European population having been slightly exposed to the small-pox, 

 found itself approximating to the conditions of a population in which 

 the disease appears for the first time. The attack is not altogether so 



