158 PASTEUR AND HYDEOPHOBIA III 



able to form satisfactorily. Most diseases which are 

 propagated by a virus — as, for instance, small-pox, 

 scarlet fever, typhoid, syphilis — have a fixed and 

 definite ''incubation period." What is going on in 

 the victimised animal or man durino^ that incubation 

 period ? On the supposition that the virus is a living 

 thing, we may imagine that the virus is slowly multi- 

 plying during this period, until it is sufiiciently abun- 

 dant to cause poisonous effects in the animal attacked. 

 It is difficult to suggest an explanation of the incuba- 

 tion period if we do not assume that the virus is a 

 living thing which can grow. 



The poisonous effects are, at any rate, deferred 

 during this incubation period. If you could introduce 

 a modified and mild form of the same virus with a 

 shorter incubation period into the animal which has 

 been infected with a stronger virus with a long incuba- 

 tion period, you might get the protoplasm of the 

 infected animal accustomed first to mild and then 

 gradually to stronger doses of the poison before the 

 critical period of the long and strong virus arrived ; 

 and so, when the assumed hour of deadly maturity of 

 the latter was reached, the animal tissues would 

 exhibit complete indifference, having in the meantime 

 learnt to tolerate without the slightest tremor of dis- 

 organisation the poison (or it may be the vibration !) 

 which, previous to their education, would have been 

 rapidly fatal. Almost equally well we may figure to 

 ourselves the state of preparation brought about if we 

 choose to employ the terms of the first or of the second 

 supposition above given. The point of importance to 



