In other words, this trait
of general intelligence would be found distributed through the
population in accordance with that same curve of chance, which was
discussed and illustrated when we were talking about the differences
between individuals.
Now what has become of the unit character, feeble-mindedness? How can
one speak of a unit character, when the "unit" has an infinite number of
values? Is a _continuous quantity_ a _unit_?
If intelligence is due to the inheritance of a vast, but indeterminate,
number of factors of various kinds, each of which is independent,
knowledge of heredity would lead one to expect that some children would
get more of these factors than others and that, broadly speaking, no two
would get the same number. All degrees of intelligence between the idiot
and the genius would thus exist; and yet we can not doubt that a few of
these factors are more important than the others, and the presence of
even one or two of them may markedly affect the level of intelligence.
It may make the matter clearer if we return for a moment to the
physical. Height, bodily stature, offers a very good analogy for the
case we have just been discussing, because it is obvious that it must
depend on a large number of different factors, a man's size being due to
the sum total of the sizes of a great number of bones, ligaments,
tissues, etc.
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