be fundamentally different in their constitution. From one may come
purples, reds, and whites; from another only purples and reds; from
another purples and whites alone; whilst a fourth will breed true to
purple. Any method of investigation which fails to take account of the
radical differences of constitution which may underlie external
similarity, must necessarily be doomed to failure. Conversely, we
realize to-day that individuals identical in constitution may yet have
an entirely different ancestral history. From the cross between two
fowls with rose and pea combs, each of irreproachable pedigree for
generations, come single combs in the second generation, _and these
singles are precisely similar in their behavior to singles bred from
strains of unblemished ancestry_. In the ancestry of the one is to be
found no single over a long series of years; in the ancestry of the
other nothing but singles occurred. The creature of given constitution
may often be built up in many ways, but once formed it will behave like
others of the same constitution."[29]

[Footnote 29: Encyclopædia Britannica, Vol. XVIII, 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