This feature of memory is manifested in heredity by the way in which
offspring commonly resembles most its nearer ancestors, but
sometimes reverts to earlier ones. Brothers and sisters, each as it
were giving their own version of the same story, but in different
words, should generally resemble each other more closely than more
distant relations. And this is what actually we find.
Thirdly, the introduction of slightly new elements into a method
already established varies it beneficially; the new is soon fused
with the old, and the monotony ceases to be oppressive. But if the
new be too foreign, we cannot fuse the old and the new--nature
seeming to hate equally too wide a deviation from ordinary practice
and none at all. This fact reappears in heredity as the beneficial
effects of occasional crossing on the one hand, and on the other, in
the generally observed sterility of hybrids. If heredity be an
affair of memory, how can an embryo, say of a mule, be expected to
build up a mule on the strength of but two mule-memories? Hybridism
causes a fault in the chain of memory, and it is to this cause that
the usual sterility of hybrids must be referred.
Fourthly, it requires many repeated impressions to fix a method
firmly, but when it has been engrained into us we cease to have much
recollection of the manner in which it came to be so, or indeed of
any individual repetition, but sometimes a single impression, if
prolonged as well as profound, produces a lasting impression and is
liable to return with sudden force, and then to go on returning to
us at intervals.
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