sophila ampelophila_), during which time they have
originated and observed over a hundred and twenty-five new types that
breed true according to Mendel's laws. Every part of the body has been
affected by one or another of these mutations. The wings have been
shortened, or changed in shape, or made to disappear entirely. The eyes
have been changed in color or entirely eliminated. And each of these
wonderful variations was brought about not gradually, but at _a single
step_.

Professor Morgan grows justifiably sarcastic in contrasting these
demonstrated laboratory facts with the armchair theories that have so
long and so harmfully dominated biological studies. A quotation from him
will not be out of place at this point.

"I may recall in this connection that wingless flies also arose in our
cultures by a single mutation. We used to be told that wingless insects
occurred on desert islands because those insects that had the best
developed wings had been blown out to sea. Whether this is true or not,
I will not pretend to say; but at any rate wingless insects may also
arise, not through a slow process of elimination, but at a single
step.... Formerly we were taught that eyeless animals arose in caves.
This case shows that they may also arise suddenly in glass milk bottles,
by a change in a single factor."[26]

[Footnote 26: "A Critique of the Theory of Evolution," p. 67.]

We need not be particularly concerned here with the theoretical
explanations of these facts offered in terms of the microscopic or even
the infra-microscopic components of the germ cells. Morgan seems to
make out a strong case for the theory that the chromosomes found in the
nucleus are the real ultimate units that carry the hereditary factors.
But he is quite decided in the opinion that these hereditary factors are
fixed, and are not changed from generation to generation either by
environment or by selection.[27] The important thing for us in this
connection is to get a clear idea of the results following from an
application of Mendel's laws to the old, old problem of the origin of
species, incidentally noticing how the theory associated with Darwin's
name now looks in the light of these new facts.

[Footnote 27: In human beings it has been found that the effects of
alcoholism and of syphilis are indeed transmitted according to Mendelian
law, being the two solitary examples of diseased conditions that are
thus transmitted. But they are so plainly pat