57 o THE POPULAR SCIENCE MONTHLY. 



aspects, it will be well, in the first place, to recall a fact in relation 

 to epidemics. 



Medical history proves, on the subject of epidemic and contagious 

 maladies, a marked fatality at the time of their first appearance, fol- 

 lowed by slowly-decreasing violence from generation to generation. 

 In our own memory the epidemic visits of cholera have diminished in 

 frequency and intensity within a short period of time. Previously to 

 our day, syphilis and varioloid, two infective diseases, differing in 

 their nature, and in their modes of transmission, had presented the 

 same phenomenon — Extreme intensity at the beginning, diminution 

 from period to period. 



If this diminution belonged to the nature of the maladies, popula- 

 tions infected for the first time in the nineteenth century should have 

 suffered less than those infected in previous centuries. But this is not 

 what has occurred. When a savage population has recently been 

 visited, for the first time, by the infection of small-pox, it has suffered 

 as much as the Europeans at the beginning of the malady in Europe. 

 It is the fact of invading a new field which renders epidemics de- 

 structive. Upon a little reflection, the reason of this is easy to com- 

 prehend. 



When an epidemic falls upon a population for the first time, the 

 greater part of the individuals disposed to receive the disease are at- 

 tacked. They die in great numbers. Subsequent births are the off- 

 spring of persons who did not contract the disease, or, at the least, 

 who contracted, yet survived it ; that is to say, of persons better con- 

 stituted than others to resist the disease. By virtue of the ordinary 

 resemblance of children to their parents, the new generation will be 

 less disposed to suffer from the epidemic. There will be then a dimi- 

 nution of the violence of the disease, or a temporary disappearance. 

 For the most part I presume a diminution, because that the resem- 

 blance of children to their grandparents (which is called atavism) is 

 not very rare, and tends to reproduce certain forms or physiological 

 conditions in families. At the end of two or three generations, that 

 special cause for the return of the epidemic is less felt, the resem- 

 blance to a great-grandfather, or ancestor still more removed, being 

 more rare than the resemblance to a grandfather. But then the bulk 

 of the population will no more have been exposed, by itself, or by its 

 fathers and mothers, to the malady in question, or will have been but 

 slightly exposed. Thus is constituted anew, by the very purity of the 

 disease, a proportion of individuals who have not been submitted to 

 the proof of the infection, or of whom the parents have not been sub- 

 mitted to the test ; a proportion on whom the malady will be severe, 

 and among whom the law of selection will recommence to operate. 



The law of events [force des choses) introduces then a variation in 

 the intensity of every disease, except that it does not act upon dis- 

 eases of which people rarely die, or which fall principally upon the 



