ss toward maturity are closely alike for all children
save the so-called 'abnormally-precocious' or 'retarded' is false. The
same fraction of the total inner development, from zero to adult
ability, will produce very unequal results in different children. Inner
growth acts differently according to the original nature that is
growing. The notion that maturity is the main factor in the differences
found amongst school children, so that grading and methods of teaching
should be fitted closely to 'stage of growth,' is also false. It is by
no means very hard to find seven year olds who can do intellectual work
in which one in twenty seventeen year olds would fail."[15]

The question as to how far immediate heredity is a cause of differences
found between individuals, can only be answered by measuring how much
more alike members of the same family are in a given trait than people
picked at random, and then making allowance for similarity in their
training. The greater the likenesses between members of the same family,
and the greater the differences between members of different families,
despite similarities in training, the more can individual differences be
traced to differences in ancestry as a controlling cause. The answer to
this question has been obtained along four different lines: First,
likenesses in physical traits; second, likenesses in particular
abilities; third, likenesses in achievement along intellectual and moral
lines; fourth, greater likenesses between twins, than ordinary siblings.
In physical traits, such as eye color, hair color, cephalic index,
height, family resemblance is very strong (the coefficient of
correlation being about .5), and here training can certainly have had no
effect. In particular abilities, such as ability in spelling, the stage
reached by an individual is due primarily to his inheritance, the
ability being but little influenced by the differences in home or school
training that commonly exist. In general achievement, Galton's results
show that eminence runs in families, that one has more than three
hundred times the chance of being eminent if one has a brother, father,
or son eminent, than the individual picked at random. Wood's
investigation in royal families points to the same influence of ancestry
in determining achievement. The studies of the Edwards family on one
hand and the so-called Kallikak family on the other, point to the same
conclusion. Twins are found to be twice as much