146
ing to the death registers of this Presidency) not attained in the first year of
invasion. The same law holds good in regard to other countries. The „mor-
tality in Great Britain was heavier in 1833, 1849, 1854, and 1866, than in 1832,
1848, 1853, and 1865, the actual years of invasion. So far as I can judge, the
mortality in many of our districts goes on increasing from the year of invasion
up to the third year, while other districts resemble more closely certain European
countries, in which the life of the epidemic is not prolonged beyond two
years.
201. When the rise and fall of small-pox epidemics come to be studied as we
now study cholera, I have some reason for thinking that we shall find the law of
movement on the outer edge of a circle, to be common to small-pox as well as to
cholera. In tracing the movement of small-pox during the last five years in
Southern India it has seemed to me that the wave of epidemic intensity passed
from north to south, though moving more slowly than a cholera wave. This is
not the place to detail the particular evidence on which I would establish the law
of movement of small-pox. In another place I shall enter fully into the question,
but would here protest against the assumption, so frequently repeated, in ignor-
ance of the facts, that there is no analogy between the laws of progress and
decline of a notoriously contagious disease like small-pox, and epidemic cholera.
To my mind there are some very striking points of resemblance in the behaviour
of the two contagia. Small-pox, in India, is wonderfully influenced in its pro-
gress by climate or season. It developes rapidly in the cool and dry months of
November, December, January, February, and March, and sometimes so late as
April or May, when it is as suddenly checked in its progress, pending the return
of the season of the year favourable to its growth. In the same way there
can be no doubt, I think, that the period of the south-west monsoon rains (when
small-pox is held in check) is the season of the year most favourable for the develop-
ment of the specific contagium of cholera, and that it is in these months it claims
the largest number of victims. In both diseases there is the same tendency to
increase through two or three years, up to a maximum of prevalence, which being
reached, is followed by rapid decline and extinction. Small-pox, moreover, has
its own cycle of increase and natural decline, as has cholera. These cycles,
curiously enough, in this part of India, alternate, so that the years of 1867 and
1868, which marked the decline of cholera all over Southern India, were notice-
able for a great and general augmentation of small-pox.
In connexion with the movement of epidemic waves of small-pox, I may
here casually allude to its westward activity in the south of Europe, France, and
England, during 1869 and 1870, following close upon the great Indian outburst
in 1866, 1867, and 1868. It was undoubtedly the fact that the epidemic attained
its maximum in France before it reached England, proving thereby a westward
progress. In well vaccinated populations the susceptibility to small-pox is
diminished, but its great development of recent years, in some of the European
States, shows that the epidemic waves flowing from the east, reach the west
now, as they did hundreds of years ago, though the people may be better
prepared to resist the subtle influence of the small-pox contagium than they
were prior to the days of Jenner.
202. In the study of the movement of cholera, I believe it is essential
that we should examine most carefully the history of localities that have escaped
attack during a season of epidemic invasion. If it be a fact that the occupation