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It is of course impracticable to make any correction for changes of this kind,
depending as they do not upon any change in the method of classification, or in the
incidence of disease, but upon changing fashion of certification, and such changes
render it difficult to trace, with any certainty, the course of any particular disease
over long periods of time, during which alterations of nomenclature, or certification
in greater detail of the causes leading to death, have resulted in changes, the extent
of which must be a matter of surmise. The course of mortality, certified as due to
old-age, is shown in a contour on the diagram and, from what is observed above in
regard to the increase, of the population at risk, it would be expected that the old age
mortality would show a continued increase, pari passu, with the increase of the
population, this cause of death being almost entirely limited to ages above 65.
Actually, however, it will be seen that the curve does not show any remarkable
increase and, in fact, during recent years, that is, the years when arterio-sclerosis
has come into fashion in certification, there has been a decrease. The other diseases
or groups of diseases shown in the diagram, which in addition to old age and heart
•diseases mainly affect persons of high age, are cancer and diseases of the urinary
system to which further reference is made below.
Zymotic
diseases.

It will be seen that at ages below fifteen years the decrease in mortality from
zymotic diseases, notwithstanding the practical disappearance of smallpox, cholera
and typhus, has not kept pace in recent years with the decline in the mortality
from all causes at these ages.
The proportions of deaths from the principal diseases