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.
The somatoplasm can only in the most general way affect the
self-perpetuating, close corporation of the germ-plasm.[A]

[Footnote A: Weismann, Essays, p. 286.]

There is thus, according to Weismann, nothing to direct variation to
certain organs, or to guide and combine the variations of these
organs along certain lines, except natural selection. To a certain
extent variation may be limited by the very structure of the animal.
But within these limits there are wide ranges where one variation is
apparently just as likely to occur as another.

Within these wide limits variation appears to be fortuitous. Natural