on which I must insist does not pretend to be
more than a guess but has at least the interest of being the first guess
that we have ever been in position to make as to how many factors go
towards the makeup of the germ plasm.

We have found practically no factors less than .04 of a unit apart. If our
map includes the entire length of the chromosomes and if we assume factors
are uniformly distributed along the chromosome at distances equal to the
shortest distance yet observed, viz. .04, then we can calculate roughly how
many hereditary factors there are in Drosophila. The calculation gives
about 7500 factors. The reader should be cautioned against accepting the
above assumptions as strictly true, for crossing-over values are known to
differ according to different environmental conditions (as shown by Bridges
for age), and to differ even in different parts of the chromosome as a
result of the presence of specific genetic factors (as shown by
Sturtevant). Since all the chromosomes except the X chromosomes are double
we must double our estimate to give the _total_ number of factors, but the
half number is the number of the different kinds of factors of Drosophila.

CONCLUSIONS

I have passed in review a long series of researches as to the nature of the
hereditary material. We have in consequence of this work arrived within
sight of a result that seemed a few years ago far beyond our reach. The
mechanism of heredity has, I think, been discovered--discovered not by a
flash of intuition but as the result of patient and careful study of the
evidence itself.

With the discovery of this mechanism I venture the opinion that the problem
of heredity has been solved. We know how the factors carried by the parents
are sorted out to the germ cells. The explanation does not pretend to state
how factors arise or how they influence the development of the embryo. But
these have never been an integral part of the doctrine of heredity. The
problems which they present must be worked out in their own field. So, I
repeat, the mechanism of the chromosomes offers a satisfactory solution of
the traditional problem of heredity.

* * * * *

CHAPTER IV

SELECTION AND EVOLUTION

Darwin's Theory of Natural Selection still holds today first place in every
discussion of evolution, and for this very reason the theory calls for
careful scrutiny; for it is not difficult to show that the expression
"natural selectio