Taking the case of destitution, which includes, necessarily, most of the other evils specified, Professor Pearson measured the correlation with liability to phthisis and found it to be .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 out9 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."