ry results as remote ancestry. Every thoughtful student of
children must realize that inner growth, apart from training, has
something to do with the changes which take place in a child; that he
differs from year to year because of a difference in maturity. This same
cause, then, must account to some extent for the differences between
individuals of different ages. But just how great a part it plays, what
per cent of the difference it accounts for, and what particular traits
it affects much or little, no one knows. We say in general that
nine-year-old children are more suggestible than six-year-old, and than
fourteen-year-old; that the point of view of the fifteen-year-old is
different from that of the eleven-year-old; that the power of sense
discrimination gradually increases up to about sixteen, and so on. That
these facts are true, no one can question, but how far they are due to
mere change in maturity and how far to training or to the increase in
power of some particular capacity, such as understanding directions, or
power of forced attention, is unknown. The studies which have been
undertaken along this line have failed in two particulars: first, to
distribute the actual changes found from year to year among the three
possible causes, maturity, general powers of comprehension and the like,
and training; second, to measure the same individuals from year to year.
This last error is very common in studies of human nature. It is taken
for granted that to examine ten year olds and then eleven year olds and
then twelve year olds will give what ten year olds will become in one
and two years' time respectively. To test a group of grammar grade
children and then a group of high school and then a group of college
students will not show the changes in maturity from grammar school to
college. The method is quite wrong, for it tests only the ten year olds
that stay in school long enough to become twelve year olds; it measures
only the very small per cent of the grammar school children who get to
college. In other words, it is measuring a more highly selected group
and accepting the result obtained from them as true of the entire group.
Because of these two serious errors in the investigations our knowledge
of the influence of maturity as a cause of individual differences is no
better than opinion. Two facts, however, such studies do make clear.
First, the supposition that "the increases in ability due to a given
amount of progre