The exercise of the muscles might possibly
cause such a condition of the blood that the portion of the
germ-plasm representing the muscular system of the next generation
might be especially nourished or stimulated. Thus an athletic parent
might produce more athletic children.
But let us imagine twin brothers of equal muscular development. One
from childhood on exercises the lower half of his body; the other,
the upper. Both take the same amount of exercise, and have perhaps
equal muscular development, but located in different halves of the
body. Now it is hard to conceive that it can make any difference in
the nourishing or stimulating influence of the blood, whether the
muscular activity resides in one half of the body or the other. The
children might be exactly alike.
One man drives the pen, a second plays the piano, and a third wields
a light hammer. All three use different muscles of the hand and arm.
How can this use of special muscles stamp itself upon the germ-cells
in such a way that the offspring will have these special muscles
enlarged? Granting that external influences of environment and
bodily condition may effect the germ-cells; granting even that some
of the most general effects of use and disuse might be transmitted,
what warrant have we for believing that the special acquired
characteristic can be transmitted? Weismann answers, None at all.
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