staining, and mounting. This worker's acquaintance
with the intimate structure of the cell is probably as great as that
of any other man in the world. Weissman asserts that he has seen
inside the nucleus all the machinery necessary to explain how the
father hands over his qualities to his children. He insists, equally
strongly, that this process is such that no father can hand to his
child any qualities which he himself did not have at least in
potentiality at his birth. Everything the individual acquires during
his lifetime is his own possession, which he may use and develop to
the utmost extent, but it dies with him. His children, born after he
possesses it, can no more inherit it than those born before. Weissman
expressed this in his famous statement that "There is no inheritance
of acquired characters." The biological world has had no shock equal
to this since Darwin's time, and there are few other questions to
which scientists to-day return with such constant vigor.

If what Weissman says is true, that no variation or development which
comes to an animal during his lifetime can be transferred into his own
germ cells and handed on to his children, then it becomes evident that
we must find some cause of variation that acts within the germ cells.
This is the difficulty which Weissman meets. He says that there are
small particles in the nucleus of each cell; that these particles
which he calls determinants decide the form and the course of
development of that cell; that when that cell divides to produce
another cell it gives to this other cell one-half of each determinant.
As a result the second cell grows to be like the first. This tells us
why offspring are like their parents. There is nothing in the theory
thus far to show us why offspring are not exactly like their parents.
In other words, there is no accounting, thus far in the theory, for
variation. When the biologist studies carefully the history of an egg
while it is being formed, he sees that at one stage in its development
it throws away not one-half of each determinant, but one-half of the
determinants. When an egg does this, it deliberately casts aside
one-half of the possibilities of its own development. This throwing
away is quite as effective for all its descendants. Any ancestral
quality now lost is lost from the line forever. In the formation of
the sperm cell set free by the male a similar throwing away of
one-half the characters has taken place. The