If the size of a family becomes more nearly proportional to the income, instead of being inversely proportional to it as at present, and if income is even roughly a measure of the value of a family to the community--an assumption that can hardly be denied altogether, however much one may qualify it in individual cases,--then the problem of taxing family incomes will be easier. The effect of income differences will be, on the whole, eugenic. It would then seem desirable to exempt from taxation all incomes of married people below a certain critical sum, this amount being the point at which change in income may be supposed to not affect size of family. This means exemption of all incomes under $2,000, an additional $2,000 for a wife and an additional $2,000 for each child, and a steeply-graded advance above that amount, as very large incomes act to reduce the size of family by introducing a multiplicity of competing cares and interests. There is also a eugenic advantage in heavy taxes on harmful commodities and unapprovable luxuries. THE "BACK TO THE FARM" MOVEMENT One of the striking accompaniments of the development of American civilization, as of all other civilizations, is the growth of the cities. If (following the practice of the U. S. Census) all places with 2,500 or more population be classed as urban, it appears that 36.1% of the population of the United States was urban in 1890, that the percentage had risen to 40.5 in 1900, and that by 1910 not less than 46.3% 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 calculates[173] 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.