88 
Pasteur and his Work, 
it has been an unique instance of one form of disease proving 
an antidote to another form. Cow-pox is not the same disease 
as small-pox, and by no known means can it be transformed 
into the latter ; neither is cow-pox human small-pox changed 
in almost every characteristic leature by transmission to the 
cow. Physicians and others have made the strange blunder of 
considering the two diseases as dependent on the same virus, 
notwithstanding the most obvious reasons to the contrary ; and 
this blunder is repeated in medical books. But it is a patent 
fact, nevertheless, that cow-pox virus, when inoculated in man, 
protects him from small-pox, which the pox of no other animal 
will do, that I am aware of, except it be that of the camel 
and horse. The microbes of cow-pox find a congenial soil in 
mankind, multiply there, and make that soil unsuitable for the 
microbes of small-pox. 
Why one attack of a contagious disease should render the 
body of a creature proof against another, for a more or less 
extended period, or even for the entire duration of life, is 
not at present known. Various hypotheses have been pre- 
sented at different times to account for it, but that which has 
perhaps received most favour is the one that explains the 
immunity, by suggesting that something has been removed from 
the blood in the first attack which is necessary for the growth 
and multiplication of the disease-producing germs, and which 
is only slowly, or never, rfeproduced. In the words of Pro- 
fessor Tyndall, " When a tree or a bundle of wheat or barley- 
straw is burnt, a certain amount of mineral matter remains in 
the ashes, extremely small in comparison with the bulk of the 
tree or of the straw, but absolutely essential to its growth. In 
a soil lacking, or exhausted of, the necessary mineral consti- 
tuents, the tree lives, the crop cannot grow. Now contagia 
are living things, which demand certain elements of life just as 
inexorably as trees, or wheat, or barley ; and it is not difficult 
to see that a crop of a given parasite may so far use up a con- 
stituent existing in small quantities in the body, but essential 
to the growth of the parasite, as to render the body unfit for the 
production of a second crop. The soil is exhausted, and, until 
the lost constituent is restored, the body is protected from any 
further attack of the same disorder. . . . To exhaust a soil, 
however, a parasite less vigorous and destructive than the really 
virulent one may suffice, and if, after having by means of a 
feebler organism exhausted the soil without fatal results, the 
most highly virulent parasite be introduced into the system, it 
will prove powerless." Another theory, highly favoured by 
some eminent authorities, is to the effect that the microbes, 
during their multiplication in the body, cause the production 
