It
is then fair to suppose that the factor which produces brachydactyly
does so by affecting the pituitary gland in some way. But there must be
many other factors which also affect the pituitary and in some cases
probably favor its development, rather than hindering it. Then if the
factor for brachydactyly is depressing the pituitary, but if some other
factors are at the same time stimulating that gland, the effect shown in
the subject's fingers will be much less marked than if a group of
modifying factors were present which acted in the same direction as the
brachydactyly factor,--to perturb the action of the pituitary gland.
This illustration is largely hypothetical; but there is no room for
doubt that every factor produces more than a single effect. A white
blaze in the hair, for example, is a well-proved unit factor in man; the
factor not only produces a white streak in the hair, but affects the
pigmentation of the skin as well, usually resulting in one or more white
spots on some part of the body. It is really a factor for "piebaldism."
For the sake of clear thinking, then, the idea of a unit character due
to some unit determiner or factor in the germ-plasm must be given up,
and it must be recognized that every visible character of an individual
is the result of numerous factors, or differences in the germ-plasm.
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