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the absence or the prevalence of them, as the case may be, as a
test of the sanitary condition of a district. But, without underrating
the importance of this test, there are limitations to its
applicability which must be borne in mind if we would draw
sound conclusions. What I mean may be best explained by an
illustration or two founded on our own local experiences within
the last few years. Thus Measles was very fatal in 1878; the
deaths were far above the average: it was, so to speak, the
zymotic disease that gave a special character to the year. In
1879, on the other hand, the deaths from Measles were below
the average; but the reduced mortality—evidence of the diminished
prevalence of the disease, was as it were the corollary of the excessive
prevalence and fatality of the malady in the previous year: in
saying this, I do not, of course, ignore the fact that one epidemic
of a zymotic disease may be more severe than another; still less
am I forgetful of the fact that the fatality of an epidemic is largely
influenced by the means taken, or the neglect to take means, to
limit the spread of infection. Again, the rate of mortality in the
Metropolis from Whooping Cough in 1881, was the lowest on record;
it is not surprising to find therefore that in the current year the
disease has been fatal almost beyond precedent. Diarrhoea may be
cited as an illustration of quite another kind. The mortality from this
disease amongst infants was excessive in 1877; the mortality in 1878
was far below the average: but the diminished mortality in the second
year had no relation to the excessive mortality in the first; the
conditions were altogether different. The summer season of 1878
was cold and wet, and in a cold and wet summer the mortality from
infantile diarrhoea is always low, just as it is always high when
the summer is hot and dry, as it was in 1877. Per contra, it may
be observed that the conditions that were so favourable to infantile
life in respect of diarrhoea, in 1878, were very unfavourable
to life, both in the young and in the aged, in respect of another
class of diseases, viz., the pulmonary, the mortality from which
throughout the year was excessive, as it always is in cold and
wet seasons.
Again, the significance of a high rate of prevalence of enteric
fever may vary widely in different circumstances: the disease
may be constantly present in one district owing to drainage defects,
or a bad water supply; in another its introduction may be due to