In all large classes of things, especially living things, there are great individual differences, and in any particular case this personal variation may be so large that it obscures the real nature of the normal.

For example, three large sons may be born to two small parents; and from this case alone it might be inferred that all small parents have large sons.

Or three girls might have better memories than three boys in the same family or school, and from this it might be argued that girls are better endowed in this direction than boys.

In all such cases the proper thing to do is to get a large number of cases and combine them; then the preponderance which the first cases examined may have shown, in one direction or the other, is corrected.

This gives rise to what is called the statistical method; it is used in many practical matters, such as life insurance, but its application to the facts of life, mind, variation, evolution, etc., is only begun.