It would be well to extend our view by measuring a whole population with one of the standard tests. If the intelligence of a thousand children picked at random from the population be measured, it will prove (as outlined in Chapter III) that some of them are feeble-minded, some are precocious or highly intelligent; and that there is every possible degree of intelligence between the two extremes. If a great number of children, all 10 years old, were tested for intelligence, it would reveal a few absolute idiots whose intelligence was no more than that of the ordinary infant, a few more who were as bright as the ordinary kindergarten child, and so up to the great bulk of normal 10-year-olds, and farther to a few prize eugenic specimens who had as much intelligence as the average college freshman. 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