of the total population was urban.
There are four components of this growth of urban population: (1) excess of births over deaths, (2) immigration from rural districts, (3) immigration from other countries, and (4) the extension of area by incorporation of suburbs. It is not to be supposed that the growth of the cities is wholly at the expense of the country; J. M. Gillette calculates56 that 29.8% of the actual urban gain of 11,826,000 between 1900 and 1910 was due to migration from the country, the remaining 70.2% being accounted for by the other three causes enumerated.
Thus it appears that the movement from country to city is of considerable proportions, even though it be much less than has sometimes been alleged. This movement has eugenic importance because it is generally believed, although more statistical evidence is needed, that families tend to "run out" in a few generations under city conditions; and it is generally agreed that among those who leave the rural districts to go to the cities, there are found many of the best representatives of the country families.
If superior people are going to the large cities, and if this removal leads to a smaller reproductive contribution than they would otherwise have made, then the growth of great cities is an important dysgenic factor.
This is the view taken by O. F. Cook,57 when he writes: "Statistically speaking cities are centers of population, but biologically or eugenically speaking they are centers of depopulation. They are like sink-holes or siguanas, as the Indians of Guatemala call the places where the streams of their country drop into subterranean channels and disappear. It never happens that cities develop large populations that go out and occupy the surrounding country. The movement of population is always toward the city. The currents of humanity