upon a forest will filter through the tree-tops so as
to cause some of the plants beneath to grow better than others, thus
bringing about variations among individuals that may have sprung from the
myriad seeds of a single parent plant. In times of prolonged drought,
plants cannot grow at the rate which is usual and normal for their
species, and so many variations in the way of inhibited development may
arise.

Then there are the variations of a second class, more complex in nature
than the direct effects of environment,--namely, the functional results of
use and disuse. A blacksmith uses his arm muscles more constantly than do
most other men, and his prolonged exercise leads to an increase of his
muscular capacity. All of the several organic systems are capable of
considerable development by judicious exercise, as every one knows. If the
functional modifications through use were unreal, then the routine of the
gymnasium and the schoolroom would leave the body and the mind as they
were before. Furthermore, we are all familiar with the opposite effects of
disuse. Paralysis of an arm results in the cessation of its growth. When a
fall has injured the muscles and nerves of a child's limb, that structure
may fail to keep pace with the growth of the other parts of the body as a
result of its disuse. These are simple examples of a wide range of
phenomena exhibited everywhere by animals and even by the human organism,
demonstrating the plasticity of the organic mechanism and its modification
by functional primary factors of variation.

But by far the greater number of variations seem to be due to the
so-called congenital causes, which are sharply contrasted with the
influences of the first and second classes. It is quite true that the
influences of the third class cannot be surely and directly demonstrated
like the others, but however remote and vague they themselves may appear to
be, their effects are obvious and real, while at the same time their
effects are to be clearly distinguished from the products of the other two
kinds. Congenital factors reside in the physical heritage of an organism,
and their results are often evident before an individual is subjected to
environmental influences and before it begins to use its various organs.
For example, it is a matter of common observation that a child with light
hair and blue eyes may have dark-eyed and brown-haired parents. The fact
of difference is a phenomenon of variation; t