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.
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