es en gran medida una cuestión de selección natural. Pero al adoptar una perspectiva más amplia, uno puede ver realmente el cambio al cual la selección natural contribuye. La siguiente tabla muestra las muertes por tisis en Massachusetts, por cada 10 000 habitantes:
| 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."