chief defect lies in the fact that selection cannot originate
varieties. In all his earlier works Darwin simply accepted variations
as he found them. He was content to say that all species varied
constantly, and in every direction. He gave no theory to account for
variation. Whenever he took measurements of the dimensions of any
large series of objects of the same kind he found these measurements
to vary, apparently, in all directions. Upon the facts of these
variations, and without accounting for them, he built his own theory
of evolution. He realized his weakness, and acknowledged it in his
book. He probably did not anticipate how insistently later biologists
would demand an explanation that would account for this variation. In
his later work, responding to this criticism, Darwin originated a
theory which he called Pangenesis. He believed that when an adult
animal had responded to his environment and acquired a new character
he could transmit this character to his offspring. At that time no one
doubted this fact. The whole theory of Lamarck was based on the
assumption that this could be done. Darwin suggested that every organ
of the body threw off minute particles, which he called pangenes.
These little bodies, carried by the blood, were taken up by the egg
cells or sperm cells, and the latter cells determined the future
development. Consequently, the character of the new individual was
determined by the parental pangenes. In this way the gain acquired by
one generation could be passed on to the next. This theory was purely
speculative. He never pretended that there was the faintest
corroborating evidence visible to the microscope in the organ, in the
blood, or in the germ cell. It was not an accounting for what is, but
for what it seemed possible to him might be.

This theory of Pangenesis, in the shape in which Darwin promulgated
it, has dropped out of consideration almost entirely. DeVries of
recent years has revised it, but with distinct modifications, and most
biologists pay no attention to it.

There is a school of biologists, headed by Weissman, who have come to
be known as Neo-Darwinians. These men have insisted that Natural
Selection, if properly understood and developed, is quite sufficient
to account for the fact of evolution, including the appearance of
variations. Weissman himself is a microscopist of more than common
skill. He is thoroughly accomplished in the most modern methods of
killing, fixing,