from an Agricultural and Veterinary Point of Vieic. 89 
of some substance which is detrimental to a second invasion of 
them. This theory would seem to have ^ood foundation in 
what is known to occur in fermentation. The poisonous sub- 
stance may remain persistently in the blood and tissues, in 
which case immunity will be permanently established ; or it 
may be removed in the course of time, and the body become 
again susceptible, and then the protection is only temporary. 
Both theories are perhaps equally acceptable, in explanation of 
what occurs. 
Vaccination, as a defence from small-pox, seems to have led 
Pasteur to ask of himself why, if contagious maladies do not 
recur, there should not be found for each of them a different 
disease, but resembling them, which acting upon them as cow- 
pox does upon small-pox, would prove preventives of them ? 
And what has been called a "chance occurrence" appears to 
have thrown open the way to enable him to give a reply to the 
question. Having shown that the microbe of fowl-cholera 
could be cultivated in an artificial medium for hundreds of 
times, without its virulence being diminished in the slightest 
degree, he found that this virulence could only be assured when 
no great lapse of time had occurred between the successive 
cultivations — the second culture being sown twenty-four hours 
after the first, the third twenty-four after the second, and so 
on to the hundredth or more. When several days, weeks, or 
even months, were allowed to elapse between the cultures, a 
progressive weakening in power of the microbe was apparent. 
So that if fowls are inoculated with the successive cultivations 
made at short intervals, and die in the course of one or two 
days, those which are inoculated with that which has been 
made at an interval of some weeks or months, will suffer much 
less and recover. But the most startling phenomenon in this 
respect is, that after recovering, should tliey be re-inoculated with 
the most active virus which would kill its hundred per cent, of 
those inoculated with it in a few hours, they will not die, per- 
haps even show no signs of illness : they are protected by 
inoculation with the delayed or attenuated cultivation. 
The discovery of this power in the weakened microbe of a 
contagious disease, to protect an animal from the lethal action of 
the same kind of microbe when in full virulency, must be held 
to constitute the most important of Pasteur's services to medical 
science and to mankind ; inasmuch as its effects may be of 
the greatest magnitude, and far-reaching, when made applicable 
to the many contagious disorders — so deadly and so little under 
the control of man — which affect our own species and the lower 
animals. The destructive microbe is transformed in the labora- 
tory, by the skill of man, and at will, into a benign, life- 
