There are abundant analogies in the inheritance
of other traits in man, lower animals and plants, that behave in exactly
the same manner.
If mental defects are inherited, then it is worth while investigating
whether mental excellencies may not also be.
3. _The persistence of like qualities regardless of difference in
environment._ Any parent with open eyes must see this in his own
children--must see that they retained the inherited traits even when
they left home and lived under entirely different surroundings. But the
histories of twins furnish the most graphic evidence. Galton, who
collected detailed histories of thirty-five pairs of twins who were
closely alike at birth, and examined their history in after years,
writes:[33] "In some cases the resemblance of body and mind had
continued unaltered up to old age, notwithstanding very different
conditions of life;" in other cases where some dissimilarity developed,
it could be traced to the influence of an illness. Making due allowance
for the influence of illness, yet "instances do exist of an apparently
thorough similarity of nature, in which such differences of external
circumstances as may be consistent with the ordinary conditions of the
same social rank and country do not create dissimilarity.
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