They are difficult to record,
but are perhaps more convincing than any quantity of statistics. If one
knows well several members of a family, one is bound to see in them
likenesses with regard to mental traits, both large and small, which
may sometimes be accounted for by example on the one hand or unconscious
imitation on the other, but are often quite inexplicable on any other
theory than heredity. It is difficult to understand how the inheritance
of mental capacity can be denied by those whose eyes are open and whose
minds are open too."
Strictly speaking, it is of course true that man inherits nothing more
than the capacity of making mental acquirements. But this general
capacity is made up of many separate capacities, all of these capacities
are variable, and the variations are inherited. Such seems to us to be
the unmistakable verdict of the evidence.
Our conclusions as to the inheritance of all sorts of mental capacity
are not based on the mere presence of the same trait in parent and
child. As the psychological analysis of individual traits proceeds, it
will be possible to proceed further with the study of the inheritance of
these traits. Some work has been done on spelling, which is particularly
interesting because most people, without reflection, would take it for
granted that a child's spelling ability depends almost wholly on his
training.
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