In short, a knowledge of heredity shows that attempts to predict the
mode of inheritance of the important human traits (particularly mental
traits) are still uncertain in their results. The characters involved
are too complex to offer any simple sequences. If two parents have brown
eyes, it can not be said that all their children will have brown eyes;
still less can it be said that all the children of two musically gifted
parents are certain to be endowed with musical talent in any given
degree.
Prediction is possible only when uniform sequences are found. How are
such sequences to be found in heredity, if they do not appear when a
parent and his offspring are examined? Obviously it is necessary to
examine _a large number_ of parents and their offspring,--to treat the
problem by statistical methods.
But, it may be objected, a uniformity gained by such methods is
spurious. It is merely shutting the eyes to the mass of contradictions
which are concealed by an apparent statistical uniformity.
This objection would be valid, if the statistical results were used for
prediction _in individual cases_. The statistician, however, expressly
warns that his conclusions must not be used for such prediction.
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