THE OEIGIN AND PROPAGATION OF DISEASE. 2 '2 7 



give expression, in definite terms, to a doctrine which has heretofore had 

 only a vague and uncertain existence ? 



If there be any one direction in which progress is now so marked as 

 to constitute a dominant feature of the present state of medicine, and to 

 embrace a practically new medical idea, I should say it was that of the 

 origin and propagation of disease hy independent organic germs. Perhaps 

 it would be wrong to say that this doctrine is even yet distinctly formu- 

 lated. It is certainly far from being definitely established as a general 

 truth. Some very wild and reckless statements have been made in regard 

 to it by observers possessed of more zeal than knowledge ; and some 

 elaborate but baseless theories relating to the^ specific development and 

 transformation of organic germs have been tried at the bar of scientific 

 investigation, and, being convicted of incomj)etency, have sulfered ac- 

 cordingly the just penalty of extermination. Perhaps the doctrine 

 itself will also be finally abandoned. It may be that the evidence in its 

 favor, which is yet only partial, will hereafter lose its special signifi- 

 cance; and the appearances which now seem to sustain it may come to 

 be naturally explained in some other way. Still there can be no doubt 

 that the idea is at present entertained, and that it is by no means con- 

 fined to the minds of careless or irresponsible theorizers. So far, it ex- 

 ists in the form rather of a scientific instinct than of a positive belief; 

 and its gray light hangs about the edge of the medical horizon like the 

 coming dawn of a new period. 



Now, can this instinct of the medical mind be justified in any way ? 

 Are there any facts and discoveries, already established beyond the 

 possibility of doubt, which have naturally led it in this direction, and 

 which point, like the telegraphic reports of successive meteorological 

 stations, to a continuous and definite movement of scientific pathology? 



I think it really began many years ago, in the early investigation of 

 parasitic diseases. Perhaps we can hardly include under this designa- 

 tion the effects produced by ordinary intestinal worms, like taenia or 

 ascaris, because the animal and parasitic nature of these worms was 

 perfectly palpable, and could not be mistaken by any one. But scabies 

 was on a different footing. It was a contagious, eruptive affection, 

 capable of spreading over a large portion of the body, and of giving the 

 patient great discomfort ; and, when it was found to be due simply to the 

 presence and propagation of a parasitic insect, the discovery was a 

 great achievement, and for the first time made it possible to have a dis- 

 tinct and rational comprehension of the origin of the disease, as well as 

 of its propagation and means of cure. A remarkable circumstance in 

 the history of our knowledge in regard to Sarcoptes scabiei is, that its 

 discovery in the present century was in fact a rediscovery of something 

 which had been known centuries before and long forgotten ; or, at least, 

 the method of finding the insect having been lost, the most eminent 

 dermatologists of forty years ago had never seen it, and were really in 

 doubt as to its existence. However, this uncertainty was terminated in 



