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25
into account the corresponding figures for all other causes of death may lead to
erroneous conclusions. If the phthisis deaths are calculated as a percentage of the
deaths from all causes the resulting curves, also shown in the diagram on page 23,
exhibit little difference from one another. The young adult, middle-age and the oldage
phthisis types have disappeared. The greatest proportion of phthisis deaths
is shown to occur at about the same age in all three groups.
A consideration of the age-distribution of deaths among common lodging-house
inmates in conjunction with the high mortality from phthisis after the age of 45 in
London as compared with England and Wales suggests the possibility that some part
of the London mortality at the higher ages is due to causes other than any particularly
adverse effects of the London environment upon health. A comparison
of the mortality of London children with that of children in England and Wales as
a whole shows that at school ages the death-rate in London from phthisis is lower
than that of England and Wales. In 1921-25 the average death-rate from phthisis
at ages 5-15 in England and Wales was 164 per million as compared with 140 in
London. There is in this fact prima facie evidence against any assumption of specific
tubercular environment. Some part of the lower mortality among the London
children may no doubt be due to the fact that deaths among London
children attending special schools outside the County are not transferred back to the
London population as are the deaths among London children occurring in institutions
situated outside the County; but the proportion of such untransferred
deaths must be quite small and hardly sufficient entirely to account for the unfavourable
position comparatively of England and Wales as a whole. In the case,
then, of children, or more generally of persons under 20 years of age, the highly urbanised
environment of London does not appear to involve any higher mortality from phthisis.
It may well be that some explanation of the high London mortality is to be
sought in the nature of the changes in the London population which have been going
on for many years past. London attracts the young adults of both sexes and many
of them on marrying move out of the county, especially north of the Thames. It
is found, by use of the London life-tables, that if there had been no outward movement
among natives of London, there would have been enumerated in 1911 at the age
of 35 to 45 about 325,000 males of London birth, whereas of the total male population
of this age in 1911 the Census shows that only 171,787 were born in London, so
that 153,213 or nearly one-half of the males of London birth had moved from the
County. The proportion is about the same for females.
It is generally contended that the migrating population of young ages is mainly
drawn from the healthier young adults. On this assumption the immigrants would be
a selected population with a low mortality rate, but it is an observed fact in actuarial
experience that the initial advantage possessed by a selected population disappears
in the course of time, so that at the end of 15 or 20 years there is little difference
between the mortality of the selected population and that of the average. The
immigrant population, therefore, at about 45 years of age will largely have lost their
initial advantage and, perhaps, being unaccustomed to the London environment,
even compare unfavourably with the native-born population of the same age; but
if the emigrant London population is selected, the residuary population must have a
higher mortality than the average, and it is probable that those emigrating do not
take with them any relative who is accommodated in a London institution. It
may thus be, that London is charged with a number of paupers, mental defectives,
etc., which is out of proportion to the surviving population of later ages resident
within the County boundary upon which mortality-rates are calculated.
In this connection it is worthy of note that the proportion per thousand of
population who are inmates in lunatic asylums in London is twice as great as in
England and Wales, and so also is the proportion per thousand of population accommodated
in poor law institutions, and the majority of these inmates are of high ages.
Effect of
population
movements
on mortality