We may see the real character of the restraints on fertility very well illustrated by the varying birth-rate of the upper and lower social classes belonging to the same community. If a high birth-rate were a mark of prosperity or of advanced civilization, we should expect to find it among the better social class of a community. But the reverse is the case; it is everywhere the least prosperous and the least cultured classes of a community which show the highest birth-rate. As we go from the very poor to the very rich quarters of a great city--whether Paris, Berlin, or Vienna--the average number of children to the family diminishes regularly. The difference is found in the country as well as in the towns. In Holland, for instance, whether in town or country, there are 5.19 children per marriage among the poor, and only 4.50 among the rich. In London it is notorious that the same difference appears; thus Charles Booth, the greatest authority on the social conditions of London, in the concluding volume of his vast survey, sums up the condition of things in the statement that "the lower the class the earlier the period of marriage and the greater the number of children born to each marriage." The same phenomenon is everywhere found, and it is one of great significance. The significance becomes clearer when we realize that an urban population must always be regarded as more "civilized" than a rural population, and that, in accordance with that fact, an urban population tends to be less prolific than a rural population. The town birth-rate is nearly always lower than the country birth-rate. In Germany this is very marked, and the rapidly growing urbanization of Germany is accompanied by a great fall of the birth-rate in the large cities, but not in the rural districts. In England the fall is more widespread, and though the birth-rate is much higher in the country than in the towns the decline in the rural birth-rate is now proceeding more rapidly than that in the urban birth-rate. England, which once contained a largely rural population, now possesses a mainly urban population. Every year it becomes more urban; while the town population grows, the rural population remains stationary; so that, at the present time, for every inhabitant of the country in England, there are more than three town-dwellers. As the country-dweller is more prolific than the town-dweller, this means that the rural population is constantly being poured into the towns. The larger our great cities grow, the more irresistible becomes the attraction which they exert on the children of the country, who are fascinated by them, as the birds are fascinated by the lighthouse or the moths by the candle. And the results are not altogether unlike those which this analogy suggests. At the present time, one-third of the population of London is made up of immigrants from the country. Yet, notwithstanding this immense and constant stream