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believed to be present in the one and absent in the other, should
always show so very wide a divergence, unless there were some
real difference in the liability to a fatal attack of those included
in the one class as compared with those comprised in the other.
We can see nothing to differentiate them in this respect, save
that the one class possessed, while the other did not, the protection
of vaccination, unless it be the circumstance suggested that
the unvaccinated were drawn from a more neglected, and
therefore from a less robust portion of the population. We have
already given our reasons for thinking this explanation quite
insufficient to account for the phenomenon.
We notice further that the same classes of vaccinated and
unvaccinated persons, which display when attacked by Small-Pox
so marked a contrast in the fatality of the disease, manifest a
contrast no less marked in the type of the disease from which
they suffer, viewed in relation to its severity or mildness. Here
again, unless vaccination be regarded as the determining cause
of the difference, it would remain to us, after considering all the
explanations which have been vouchsafed, an unsolved mystery.
The next point forced on our attention is the greater liability
to attack, which the evidence shows to exist in the case of the
unvaccinated than of the vaccinated. We are, of course, again
confronted by the possibility of error in the classification, but the
same test was applied in dividing into the two classes those who
inhabited the invaded houses as in making a similar division in
the case of the individuals attacked. It is possible, too, that the
inhabitants of the invaded houses included in the two classes
were not all equally within the reach of contagion, but any error
in this respect is just as likely to have affected the vaccinated as
the unvaccinated class. When the numbers dealt with are
considered, and it is remembered that the classification was made
in different towns, and always with the same result, we do not
think this source of possible error can be regarded as serious.