elinquents will vary according to the particular ward, city, county, or
state, whence the delinquents come. For this reason it is essential to
any study of the number of mental defectives in a group of juvenile
delinquents coming from a particular locality, that some idea should be
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