p. 119.]

IV

Vanished at last are the old theories of gradual changes in species
perpetuated and accumulated by natural selection until at last wholly
new forms have in this way been produced. True variations are now seen
to be confined within well-marked and rather narrow limits, within which
ordinary variations may occur, perhaps induced by environment. These
fluctuating variations grade off into one another on all sides, and
their differences _can_ be plotted on a frequency curve; but the very
important thing for us to remember is that these fluctuating variations
_cannot be transmitted._ Beyond these fluctuating variations come the
unit characters or factors, which are distinct from each other, or
"discontinuous," to use the technical term, and which therefore _cannot
be plotted on a frequency curve_. These factors are not modified in the
least by the environment, and their peculiarities are faithfully
transmitted in heredity with all the precision of chemical law. But even
these factors are all within the bounds of the species. There is not a
shred of scientific evidence that either natural or artificial devices
have originated a single genetic factor that was not all the time
potentially latent in the ancestry, capable of being produced at will by
the proper combination.

It is a universal law of living things that all forms left to themselves
tend to degenerate. The necessity for continuous artificial selection in
the sugar beet, in Sea Island cotton, in corn, in Jersey and Holstein
cattle, in trotting horses, proves this universal tendency to
degenerate.[30] Natural selection in a somewhat similar way tends to
postpone this degeneracy by killing off the "unfit," but selection
either artificial or natural cannot originate anything new, and its
results are here displayed merely among the small fluctuating variations
mentioned above. Even among the real genetic factors it may show itself
by allowing some to survive alone; but as no combination of diverse
factors can originate anything really new, its field for operation among
these factors is extremely limited. Among species also it is operative,
killing off some and allowing others to survive. But neither among
fluctuations, among factors, nor yet among species can selection
originate anything new.

[Footnote 30: The following represents the consensus of scientific
opinion regarding the lessons to be drawn from the phenomena of our
improved races of domes