2 average size. In large samples of the population it is found that the more skilled the occupation of the father, the fewer children there are per family. The development of this tendency to differential fertility has been accompanied by a progressive fall in the general birth rate. The result of these processes, as shown by researches, is that the part of the population from which the majority of skilled and professional workers are recruited has far too low a reproductive rate to ensure survival. The evidence of the social surveys of Lewis and Caradog Jones provided information which shows that the families which are most fertile are to be found in rural districts, where the incidence of mental defect is relatively high, or among impoverished dwellers in industrial areas. Evidence for the view that the mentally defective themselves are extremely fertile seems to be less reliable. Certainly imbeciles and idiots so rarely function as parents that they contribute to the appreciable number of offspring to the next generation. Experience shows that the feeble-minded who are well trained and cared for do not have large families either. The greatest fertility seems to be found [end]