the rest
of the body, how is any variation possible? Different individuals of
any species have slightly different congenital tendencies. Hence in
the act of fertilization two germ-plasms of slightly different
structure and tendency are mingled. The mingling of the two produces
a germ-plasm and individual differing from both of the parents.
Thus, according to Weismann's earlier view, the origin of variation
was to be sought in sexual reproduction through the mingling of
slightly different germ-plasms.

But how did these two germ-plasms come to be different? How was the
variation started? To explain this Weismann went back to the
unicellular protozoa. These animals are undoubtedly influenced by
environment and vary under its stimuli. Here the variations were
stamped upon the germ-plasm, and the commingling of these variously
stamped germ-plasms has resulted in all the variations of higher
animals.

Of late Weismann has modified and greatly improved this portion of
his theory. He now accepts the view that external influences may act
upon the germ-plasm not only in protozoa but also in all higher
animals. Variation is thus due to the action or stimulus of
external influences, supplemented by sexual reproduction.

But the very constitution of the germ-plasm and its relation to the
body absolutely forbids the transmission of acquired somatic
characteristics and of the special effects of use and disuse.
Muscular activity promotes general health, and might thus conduce to
better-nourished germ-cells and to more vigorous and therefore
athletic descendants. 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