ust be particularly slow in
assuming that such complex characters as man's mental traits are units,
in any proper genetic sense of the word. It will, for instance, require
very strong evidence to establish feeble-mindedness as a unit character.
No one who examines the collected pedigrees of families marked by
feeble-mindedness, can deny that it does appear at first sight to behave
as a unit character, inherited in the typical Mendelian fashion. The
psychologist H. H. Goddard, who started out with a strong bias against
believing that such a complex trait could even _behave_ as a unit
character, thought himself forced by the tabulation of his cases to
adopt the conclusion that it does behave as a unit character. And other
eugenists have not hesitated to affirm, mainly on the strength of Dr.
Goddard's researches, that this unit character is due to a single
determiner in the germ-plasm, which either is or is not present,--no
halfway business about it.

How were these cases of feeble-mindedness defined? The definition is
purely arbitrary. Ordinarily, any adult who tests much below 12 years by
the Binet-Simon scale is held to be feeble-minded; and the results of
this test vary a little with the skill of the person applying it and
with the edition of the scale used. Furthermore, most of the
feeble-minded cases in institutions, where the Mendelian studies have
usually been made, come from families which are themselves of a low
grade of mentality. If the whole lot of those examined were measured, it
would be difficult to draw the line between the normals and the
affected; there is not nearly so much difference between the two
classes, as one would suppose who only looks at a Mendelian chart.

[Illustration: A HUMAN FINGER-TIP

FIG. 21.--The palms of the hands and soles of the feet are
covered with little ridges or corrugations, which are supposed to be
useful in preventing the grasp from slipping; whence the name of
friction-skin has been given to these surfaces. The ridges are developed
into various patterns; the one above is a loop on the left forefinger.
The ridges are studded with the openings of the sweat glands, the
elevated position of which is supposed to prevent them from being
clogged up; further, the moisture which they secrete perhaps adds to the
friction of the skin. Friction-skin patterns are inherited in some
degree. Photograph by John Howard Payne.]

[Illustration: THE LIMITS OF HEREDITARY CONTROL

FIG. 22.