Fortunately, it is very probable that the differential birth-rate is not of such ominous import in rural districts as it is in cities, although some of the tribes of degenerates which live in the country show birth-rates of four to six children per wife.19 But in the more highly civilized nations now, something like a half of the population lives in urban districts, and the startling extent to which these urban populations breed from the bottom involves a disastrous change in the balance of population within a few generations, unless it is in some way checked.
Just how great the change may be, statistically, has been emphasized by Karl Pearson, who points out that "50% of the married population provide 75% of the next generation," owing to the number of deaths before maturity, the number of celibates and the number of childless marriages. "The same rule may be expressed in another way: 50% of the next generation is produced by 25% of the married population." At this rate in a few generations the less efficient and socially valuable, with their large families, will overwhelm the more efficient and socially valuable, and their small families.
Fecundal selection is at work to-day