y. Brothers or sisters test
more alike than children taken at random from a community, and twins
test more alike than ordinary brothers and sisters. Now, as the
physical resemblance of brothers or sisters, and specially of twins,
is accepted as due to native constitution, we must logically draw the
same conclusion from their mental resemblance.

The way feeble-mindedness runs in families is a case in point. Though,
in exceptional instances, mental defect arises from brain injury at
the time of birth, or from disease (such as cerebrospinal meningitis)
during early childhood, in general it cannot be traced to such
accidents, but is inherent in the individual. Usually mental defect or
some similar condition can be found elsewhere in the family of the
mentally defective child; it is in the family stock. When both parents
are of normal intelligence and come from families with no mental
abnormality in any ancestral line, it is practically unknown that they
should have a feeble-minded {291} child; but if mental deficiency has
occurred in some of the ancestral lines, an occasional feeble-minded
child may be born even of parents who are themselves both normal. If
one parent is normal and the other feeble-minded, some of the children
are likely to be normal and others feeble-minded; but if both parents
are feeble-minded, it is said that all the children are sure to be
feeble-minded or at least dull.

These facts regarding the occurrence of feeble-mindedness cannot be
accounted for by environmental influences, especially the fact that
some children of the same family may be definitely feeble-minded and
others normal. We must remember that children of the same parents need
not have precisely similar native constitutions; they are not always
alike in physical traits such as hair color or eye color that are
certainly determined by native constitution.

The special aptitudes also run in families. You find musical families
where most of the children take readily to music, and other families
where the children respond scarcely at all to music, though their
general intelligence is good enough. You find a special liking and
gift for mathematics cropping out here and there in different
generations of the same family. No less significant is the fact that
children of the same family show ineradicable differences from one
another in such abilities. In one family were two brothers, the older
of whom showed much musical ability and came earl