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1OO
the United Kingdom, apart from the spread of the practice of
vaccination ; amongst other things, to improvement of sanitary
conditions.
It is beyond doubt than an infectious disease like small-pox
is, other things being equal, more likely to spread in towns than
in country districts, and more likely to spread in crowded town
districts than in others not so densely populated; so that we
should expect a lessened proportion of over-crowded dwellings, by
diminishing the opportunities for contagion, to check the prevalence
of the disease and consequently to render its mortality less.
On the other hand it is certain that, during the period of
the decline, there has been in England and Wales and in Scotland,
though not in Ireland, a large increase of the population; so that
the density of the population in two out of these parts of the
United Kingdom, taking each of them as a whole, has been
increasing.
And it is equally certain, and probably far more important,
that in all of them, during the period of the decline, there has been
a continually growing proportion of the population living in the
towns, and particularly in the larger towns.
This growth of the proportion of the population living in
towns has been a condition tending to an increased prevalence of,
and mortality from, small-pox.
There has also been, during the period of the decline,
another change in the conditions of life, affecting all three
countries, which would seem, at all events on a priori grounds, to
have largely tended to an increased prevalence of small-pox;
namely, the enormous and continued extension of movement
among the population, and of communication with other countries,
following the increasing facilities for such movement and communication.
We have already pointed out that on a priori grounds it is