available as to the probable or approximate number of mental defectives in that community. If more mental defectives are found among the population in the slum quarter of a city than in the residential quarter, it is to be expected that there will be more mental defectives in groups of juvenile delinquents from the slum quarter, because, in the first place, they constitute a larger proportion of the population, and because, secondly, of their greater proneness to social offenses. Moreover, the prevalence of the feeble-minded in certain localities may affect the attitude of the law-enforcing machinery toward the children of that community. A further result of the innate characteristics and tendencies of the feeble-minded is to be found in the effect upon them of the biological law of natural selection, resulting from the universal struggle for existence and the survival of the fittest. We need not discuss here its profound influences, economic and otherwise, upon the lives of the mentally defective in general, but it will be profitable to review briefly the effect of natural selection upon the juvenile delinquent group. Any group of delinquents is subject to this selection from the times of offenses to final commitment. It undergoes a constant sifting process whose operation is mainly determined by the natural consequences of the group members; a large proportion of the "lucky," the intelligent, or the socially favored individuals escape from the group, so that the remaining members of the group are the least fit socially and intellectually. The mentally defective delinquents constitute an undue proportion of this unfit residue, for although they may receive as many favors of chance as do their intellectually normal fellow-delinquents, they cannot, like them, by reason of intelligence or social status, escape the consequences of their delinquent acts. Furthermore, the feeble-minded offender is caught oftener than are his more clever and energetic companions of normal endowments, and after apprehension he is less likely to receive the benefits of police and court prejudices, or the advantages of family wealth and social influence. If placed on probation he is more likely to fail, because of his own weaknesses and his unfavorable environment. Hence the feeble-minded delinquent is much more likely to come before the court and also to be committed to a reformatory, jail, or industrial school than is his companion of normal mind. Therefore practically every group of juvenile delinquents which ultimately reaches commitment will have a very different aspect with regard to its proportion of mental defectives from that larger group of offenders, apprehended or non-apprehended, of which it was once a part. In fact, it is doubtful if any group of apprehended, detained, or probationed offenders can be said to be representative, or at least to be exactly representative, of the true proportion of mental defectives among all delinquents.