222 THE STRUGGLE FOR LIFE. 
not yet understood, the living cells have the power, pari 
passu with the production of the toxine, to form a substance 
directly antagonistic to it, and to this has been given the 
name of antitoxine. The antitoxine is to be found in the 
blood serum, and has in some cases been separated trom 
it; but there is no evidence that it does not also occur in 
other portions of the body. There is every probability 
that it is the formation ot this substance which puts an end 
to the further growth of the Bacteria themselves, and this 
may perhaps explain why some epidemic diseases have a 
fixed and limited duration. 
I think that these facts explain why Vaccination protects 
the body against the poison of Small-pox. The vaccine 
disease is really Smallpox which has been modified by 
passing through the cow for countless generations, and it 
is so modified that it has never been known to produce 
‘true Small-pox when inoculated into the human subject, or 
to give rise to an epidemic. At the same time, it appar- 
ently makes sufficient antitoxine to protect the body, at all 
events for many years, from its parent disease. 
It has been proved, that if into an animal there be 
injected a small dose of the Bacteria causing one of these 
diseases, it will afterwards bear a stronger dose without 
injury, and this process can be repeated with larger and 
larger doses, until it cannot be affected by any possible 
amount that can be given. And now comes the most 
wonderful fact of all; if the serum of the blood of an 
animal so rendered insusceptible, or immune to that disease, 
be taken and injected into the tissues of another animal, 
even in a comparatively small quantity, that animal -will 
also share its immunity, and further still, this injection, if 
made into an animal already infected, will, to some extent, 
modify its progress. This has been proved, over and over 
again, by the injection of similar doses of the poison into 
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