10 



present in some form in every instance in which this parti- 

 cular malady occurred. Further, when the disease spread (as 

 most of those in question do), it, so to speak, always bred true 

 — Small Pox always produced Small Pox, Scarlet Fever, 

 Scarlet Fever, and so on. Sydenham observed that in this 

 respect they were propagated just as truly as plants and 

 animals. Varieties might appear from time to time, but there 

 was nothing like the production of new species. These facts 

 being established the diseases in question came to be looked 

 upon as distinct entities, and so to be called Specific Diseases. 

 When the same set of phenomena were observed to present 

 themselves over and over again, with such regularity that a 

 skilled observer, coming on the scene at any moment, could 

 state with something like certainty what had already taken 

 place, and could also predict what was about to follow, it was 

 only natural to think that the results being so constant, the 

 causes should also be constant. What were the causes ? 

 Argument from analogy helped very much to elucidate this 

 problem. It was observed that if any given poison was 

 administered to an individual, not only were symptoms of 

 general illness developed, but also certain special indications 

 appeared, just as these specific diseases, by which the 

 special poison could be recognised. Mercury, Lead, and Phos- 

 phorus, Ergot of Rye afford examples. Again, if a healthy 

 man dwelt for a time in a marshy district, he certainly took 

 into his system something which would sooner or later set 

 up an illness of a very definite character, although that 

 illness might not show itself until after he had left the 

 malarious district. Observations of this kind naturally led 

 to the idea that these diseases were due to Specific Poisons, 

 and so Physicians came to speak of the poison of Small Pox, 

 the poison of Scarlet Fever, and so on. 



An important difference, however, will be at once noticed 

 between these two classes of poisons. In the first the 

 noxious material enters the system, does its work, and there 

 the matter ends. With the second class it is quite different. 

 A fraction of a drop, say of the virus of Small Pox, is in- 



