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 disease, and the children born to them will profit by their immunity. If the disease is less fatal, the purification will be less. We thus discover, 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 maladies which attack youth, rule from year to year in a mode more continuous.
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, notwithstanding; 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 vaccination, should also vary. When Jenner discovered the utility of vaccination, 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, passably 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 repetition, and the dangerous practice of inoculation would not have continued, 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 composed 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
