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generally from cholera in 1870 that I hardly expect to hear of any seasonal
reproduction there in this present year, but in the district of Kistna which
was only lightly affected, the seasonal reproduction has already commenced
(February 12th), and will, I doubt not, go on, until the unoccupied tracts of last
year have been more or less completely invaded. In the same way I explain the
facts of the invasion of the Western Coast territory in 1870, which had also
escaped the downward wave in 1869. The attraction to cholera lay in the new
country, and, although the epidemic had to pass down to the extreme south of India
to turn the mountain barrier, it travelled up northwards just as easily as it has
hitherto done southwards, because the unoccupied ground, and the population
endowed with a proneness or susceptibility for the disease, lay in front of it. That
this was the fact may also be inferred by the splitting up, as it were, of the
southern wave, one limb passing on south to invade Ceylon (also ready for the
reception of cholera by reason of long exemption), and the other doubling back
upon itself to seize the unoccupied tract of the Western Coast.
Ihe reluctance of cholera to retrace its steps over a lately invaded area is
also well exemplified in the case of the Cuddapah, Bellary, and Kurnool Districts.
When the pilgrims dispersed from Tripatty in May 1870, many must have
travelled north-west through these districts ; but they soon left the cholera behind
them, for these districts had been recently invaded by a cholera wave passing on
to the south, and the population, apparently, was not in a condition to receive
and multiply the imported cholera. Moreover these districts were already under
the influence of a wave of malaria, and on that account, it may be, cholera could
establish no hold on them.
200. In its behaviour to the human race, cholera presents some striking analo-
gies to small-pox. As regards an individual community,there are in both diseases the
same peculiarities of invasion ; of growth, or increment (modified and influenced by
season), culmination, and rapid decline, followed by an interval of complete rest,
the latter broken only by a new invasion. As regards small-pox we have no
difficulty in recognising that it is a disease multiplying in, and spreading by means
of, the human body, but we are still very ignorant of the laws which govern its peri-
odical appearance in epidemic waves, or of the influence of season in hastening or
repressing the culmination of an epidemic, and of the causes of its complete
extinction for long periods together over vast tracts of country, which have never
been invaded by the vaccinator. Those who dispute the influence of human
intercourse in the distribution of cholera are, I think, bound to explain why small-
pox, an acknowledged communicable disease, should become epidemic only peri-
odically, spread up to its maximum power, subject all the while to seasonal influ-
ences, and then as suddenly die out. We know that the contagious particles of
small-pox in a locality, at the acme of an epidemic, must be infinitely more numer-
ous than at the beginning, and this being so, the question may well be asked why
should the disease ever decline if it was capable of unlimited propagation ? But
the truth seems to be, in regard to this matter, as in regard to cholera diffusion,
that there is a limit to epidemic distribution, and that this limit is reached, just as
the minute particles of which the contagium consists, exist in the greatest
abundance. By a law peculiar to themselves, the particles of contagium of, pro-
bably, all diseases of the infectious type cease to multiply beyond a certain number
of years.
The maximum power of cholera, if we may judge from its fatality, is (accord-