328 DR. WILLIAM ROBERTS. 



of diphtheria at the time, neither at the school nor in 

 Bowden.^ 



Take another illustration : cholera suddenly breaks out in 

 some remote district in India, and spreads from that centre 

 over half the globe. In three or four seasons, the epidemic 

 dies away and ceases altogether from among them. A few 

 years later, it reappears and spreads again, and disappears 

 as before. Does not this look as if the cholera virus were 

 an occasional sport from some Indian saprophyte, which by 

 variation has acquired a parasitic habit, and, having run 

 through countless generations, either dies out or reverts 

 -again to its original type ? Similarly, typhoid fever might 

 be explained as due to a variation from some common sapro- 

 phyte of our stagnant pools or sewers, which, under certain 

 conditions of its own surroundings, or certain conditions 

 within the human body, acquires a parasitic habit. Having 

 acquired this habit, it becomes a contagious virus, which is 

 transmitted with its new habit through a certain number of 

 generations ; but finally, these conditions ceasing, it reverts 

 again to its original non-parasitic type. 



In regard to some contagia, such as smallpox and scarlet 

 fever, it might be said that the variation was a very rare one, 

 but also a very permanent one, with little or no tendency to 

 reversion ; while others, like erysipelas and typhoid fever, 

 were frequent sports, with a more decided tendency to re- 

 version to the original type. In regard to some pathogenic 

 organisms, it might be assumed that the parent type had 

 disappeared, and the parasitic variety only remained, just 

 as the wild parents of many of our cultivated flowers and 

 vegetables have disappeared, leaving behind them only their 

 altered descendants. 



How aptly, too, this view explains what used to be called 

 the " Epidemic Constitution," and the hybrid forms and 

 subvarieties of eruptive and other fevers. 



I must not pursue this vein further. I have said enough 

 to indicate that this conception enables us, if it does nothing 

 else, to have coherent ideas about the origin and the spread 

 of zymotic diseases. 



In applying the doctrine of pathogenic organisms, or 

 pathophytes, as they might be termed, to the explanation 

 of the phenomena of infective diseases, we must be on our 

 guard against hard-and-fast lines of interpretation. So far as 



' Complex cases of mingled scarlet fever and diphtheria are sometimes 

 seen. Similarly the peach-tree will occasionally, among a multitude of 

 ordinary fruit, produce one fruit of which one half has the peach character 

 and the other half the nectarine character. — Darwin. 



