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