For
the purpose of eugenics, it is sufficient to recognize that great
differences exist between men, and women, not only in respect of
physical traits, but equally in respect of mental ability.
This conclusion might easily have been reached from a study of the facts
in Chapter I, but it seemed worth while to take time to present the fact
in a more concrete form as the result of actual measurements. The
evidence allows no doubt about the existence of considerable mental and
physical differences between men.
The question naturally arises, "What is the cause of these differences?"
The study of twins showed that the differences could not be due to
differences in training or home surroundings. If the reader will think
back over the facts set forth in the first chapter, he will see clearly
that the fundamental differences in men can not be due to anything that
happens after they are born; and the facts presented in the second
chapter showed that these differences can not be due in an important
degree to any influences acting on the child prior to birth.
CHAPTER IV
THE INHERITANCE OF MENTAL CAPACITIES
We have come to the climax of the eugenist's preliminary argument; if
the main differences between human beings are not due to anything in the
environment or training, either of this or previous generations, there
can be but one explanation for them.
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