02. The correlation for direct
heredity--that is, the resemblance between parent and offspring--it will
be remembered, is .50. As compared with this, the environmental factor
of .02 is utterly insignificant. It seems evident that whether or not
one dies from tuberculosis, under present-day urban conditions, depends
mainly on the kind of constitution one has inherited.
There is no escape, then, from the conclusion that in any individual,
death from tuberculosis is largely a matter of natural selection. But
by taking a longer view, one can actually see the change to which
natural selection is one of the contributors. The following table shows
the deaths from consumption in Massachusetts, per 10,000 population:
1851-60 39.9
1861-70 34.9
1871-80 32.7
1881-90 29.2
1891-1900 21.4
1901 17.5
1902 15.9
F. L. Hoffman further points out[60] that in Massachusetts, Rhode Island,
and Connecticut, 1872-1911, the decline in the death-rate from
tuberculosis has been about 50%. "The evidence is absolutely conclusive
that actually as well as relatively, the mortality from tuberculosis in
what is the most intensely industrial area of America has progressively
diminished during the last 40 years.
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