nital
variations are thus a means for permanently modifying the organism, and
by their agency must we in large measure believe that evolution through
the ages has taken place.

With the acquired variations the matter stands quite differently. We can
readily understand how influences surrounding an animal may affect its
organs. The increase in the size of the muscles of the blacksmith's arm
by use we understand readily enough. But with our understanding of the
machinery of heredity we can not see how such an effect can extend to
the next generation. It is only the organ directly affected that is
modified by external conditions. Acquired variations will appear in the
part of the body influenced by the changed conditions. But the germ
plasm within the reproductive glands is not, so far as we can see,
subject to the influence of an increased use, for example, in the arm
muscles. The germ material is derived from the parents, and, if it is
simply stored in the individual, how could an acquired variation affect
it? If an individual lose a limb his offspring will not be without a
corresponding limb, for the hereditary material is in the reproductive
organs, and it is impossible to believe that the loss of the limb can
remove from the hereditary material in the reproductive glands just that
part of the germ plasm which was designed for the production of the
limb. So, too, if the germ plasm is simply stored in the individual, it
is impossible to conceive any way that it can be affected by the
conditions around the individual in such a way as to explain the
inheritance of acquired variations. If acquired variations do not affect
the germ plasm they cannot be inherited, and if the germ plasm is only a
bit of protoplasmic substance handed down from generation to generation,
we can not believe that acquired variations can influence it.

From such considerations as these have arisen two quite different views
among biologists; and, while it is not our purpose to deal with disputed
points, these views are so essential to our subject that they must be
briefly referred to. One class of biologists adhere closely to the view
already outlined, and insist for this reason that acquired variations
_can not_ under any conditions be inherited. They insist that all
inherited variations are congenital, and due therefore to direct
variations in the germ plasm, and that all instances of seeming
inheritance of acquired variations are capable of othe