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It will be noted that the great increase in death from consumption in
this area began in the decade following 1840, when the large Irish
immigration began. The Irish are commonly believed to be particularly
susceptible to phthisis. Crowded together in industrial conditions, they
rapidly underwent infection, and their weak racial resistance led to a
high death-rate. The weak lines of heredity were rapidly cut off; in
other words, the intensity of natural selection was great, for a while.
The result was to leave the population of these New England states much
more resistant, on the average, than it was before; and as the Irish
immigration soon slowed down, and no new stocks with great weakness
arrived, tuberculosis naturally tended to "burn itself out." This seems
to be a partial explanation of the decline in the death-rate from
phthisis in New England during the last half century, although it is not
suggested that it represents the complete explanation: improved methods
of treatment and sanitation doubtless played their part. But that they
are the sole cause of the decline is made highly improbable by the low
correlation between phthisis and environmental factors, which was
mentioned above, and by all the other biometric study of tuberculosis,
which has proved that the results ascribed to hygiene, including
sanitorium treatment, are to some degree illusory.
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